Re: [time-nuts] 75Z vs 50Z for GPS receivers
From: Brooke Clarke <[EMAIL PROTECTED]> Subject: Re: [time-nuts] 75Z vs 50Z for GPS receivers Date: Mon, 05 Feb 2007 20:20:53 -0800 Message-ID: <[EMAIL PROTECTED]> > Hi Didiere: > > There's also an app note at Linear Technology for making pulses so fast > they are used to calibrate sampling scopes. > > Some years ago I made a very simple circuit that pulses a LED with many > amps (very short time) based on a transistor going into avalanche. You can actually use a 1N4148 for below 1 ns risetimes. NBS/NIST used a 1N4148 as a 100 MHz to 700 MHz stepup in a ionic measurement setup GPS receiver design so that they fed 100 MHz up to the antenna, stepped it up to 700 MHz and used that into the mixdown step in the antenna assembly. I think their use of a 1N4148 was a twist of a culprit just to show it is the methods and not the tools which is important. :) This design is burried somewhere in the NIST archive. Cheers, Magnus ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Hi Brooke, I am familiar with the Jim Williams app note, but I felt the pulse shape was somewhat lousy (based on the pictures that came with the article) due to the very small cap on the collector, OK to calibrate or evaluate a fast scope, I guess, but too far from a square wave to be able to visually observe and analyze reflections (at least from my perspective). Due to my ignorance of the avalanche effect, I did not think of mating this with a delay line. It seems the Zetex app note with a delay line can make decent square pulses, I have to try that. I have made high power pulse generators with delay lines a long time ago (radar modulators), it's been so long I had forgotten... Didier Brooke Clarke wrote: > Hi Didiere: > > There's also an app note at Linear Technology for making pulses so fast > they are used to calibrate sampling scopes. > > Some years ago I made a very simple circuit that pulses a LED with many > amps (very short time) based on a transistor going into avalanche. > > Have Fun, > > Brooke Clarke > ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Hi Didiere: There's also an app note at Linear Technology for making pulses so fast they are used to calibrate sampling scopes. Some years ago I made a very simple circuit that pulses a LED with many amps (very short time) based on a transistor going into avalanche. Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Didier Juges wrote: >Bruce, > >I finally found the time to google for avalanche pulse generator and >found an App note from Zetex on the subject. >I was not familiar with this technology, but it seems interesting. >Amazing that you can get 50A pulses with sub-ns rise time out of a >device in an SOT-23 (or TO-92) package! > >I have uploaded the app note to >http://www.ko4bb.com/Test_Equipment/AvalanchePulser > >Now, I just need a scope fast enough to take advantage of such a short >rise time. Back to eBay again :-) > >Didier > >Dr Bruce Griffiths wrote: > > >>Didier >> >>You can always build your own 100A avalanche transistor pulser using >>several Zetex FMMT413s. >> >>Bruce >> >>Brooke Clarke wrote: >> >> >> >>>Hi Didier: >>> >>>Yes they are on eBay. The tunnel diode models have a narrower pulse >>>than the later models that use something more rugged. Probably for all >>>practical purposes one of the newer ones would work well and last >>>better. I think the military bought boat loads of these and I've heard >>>from a number of people that use them and like them a lot. >>> >>>Have Fun, >>> >>>Brooke Clarke >>> >>>w/Java http://www.PRC68.com >>>w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml >>>http://www.precisionclock.com >>> >>> >>> >>>Didier Juges wrote: >>> >>> >>> >>> >>> Going over the spec for the 1502, it seems like a nice instrument. I am worried about the fragility of the tunnel diode though. When my neighbor AD4TK fires up his amplifier, I have measured up to 1W of his signal going down my feedline (on the 80m slopper antenna)... Are these things on eBay? Didier KO4BB Brooke Clarke wrote: >Hi Didier: > >The Tek 1502 is great for doing this, especially if you have the >optional strip chart recorder. It's what it was made to do. >http://www.pacificsites.com/~brooke/Tek1502.shtml > >Have Fun, > >Brooke Clarke > > > > > > > ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>___ >>>time-nuts mailing list >>>time-nuts@febo.com >>>https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> >>> >>> >>> >>> >>___ >>time-nuts mailing list >>time-nuts@febo.com >>https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> >> >> >> > >___ >time-nuts mailing list >time-nuts@febo.com >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Didier Analog sampling scopes used avalanche transistors to drive the snap off diode shorted delay line pulse generator which in turn drove the diode sampling gate with picosecond risetime complementary pulses. Bruce Didier Juges wrote: > Bruce, > > I finally found the time to google for avalanche pulse generator and > found an App note from Zetex on the subject. > I was not familiar with this technology, but it seems interesting. > Amazing that you can get 50A pulses with sub-ns rise time out of a > device in an SOT-23 (or TO-92) package! > > I have uploaded the app note to > http://www.ko4bb.com/Test_Equipment/AvalanchePulser > > Now, I just need a scope fast enough to take advantage of such a short > rise time. Back to eBay again :-) > > Didier > > Dr Bruce Griffiths wrote: > >> Didier >> >> You can always build your own 100A avalanche transistor pulser using >> several Zetex FMMT413s. >> >> Bruce >> >> ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Bruce, I finally found the time to google for avalanche pulse generator and found an App note from Zetex on the subject. I was not familiar with this technology, but it seems interesting. Amazing that you can get 50A pulses with sub-ns rise time out of a device in an SOT-23 (or TO-92) package! I have uploaded the app note to http://www.ko4bb.com/Test_Equipment/AvalanchePulser Now, I just need a scope fast enough to take advantage of such a short rise time. Back to eBay again :-) Didier Dr Bruce Griffiths wrote: > Didier > > You can always build your own 100A avalanche transistor pulser using > several Zetex FMMT413s. > > Bruce > > Brooke Clarke wrote: > >> Hi Didier: >> >> Yes they are on eBay. The tunnel diode models have a narrower pulse >> than the later models that use something more rugged. Probably for all >> practical purposes one of the newer ones would work well and last >> better. I think the military bought boat loads of these and I've heard >> from a number of people that use them and like them a lot. >> >> Have Fun, >> >> Brooke Clarke >> >> w/Java http://www.PRC68.com >> w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml >> http://www.precisionclock.com >> >> >> >> Didier Juges wrote: >> >> >> >>> Going over the spec for the 1502, it seems like a nice instrument. I am >>> worried about the fragility of the tunnel diode though. When my neighbor >>> AD4TK fires up his amplifier, I have measured up to 1W of his signal >>> going down my feedline (on the 80m slopper antenna)... >>> >>> Are these things on eBay? >>> >>> Didier KO4BB >>> >>> Brooke Clarke wrote: >>> >>> >>> >>> Hi Didier: The Tek 1502 is great for doing this, especially if you have the optional strip chart recorder. It's what it was made to do. http://www.pacificsites.com/~brooke/Tek1502.shtml Have Fun, Brooke Clarke >>> ___ >>> time-nuts mailing list >>> time-nuts@febo.com >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> >>> >>> >>> >>> >> ___ >> time-nuts mailing list >> time-nuts@febo.com >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> >> >> > > > ___ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Hi Hal: The corner frequency is mainly becuase of the capactance at low frequencies. A transmission line at audio frequency looks like a shunt capacitor and the inductance no longer is working aginst it, so you don't get a real Zo but instaed capactance. Loss really does not come into it. In the RG-58 plot at: http://www.pacificsites.com/~brooke/Zo.shtml#RG58 the resistance is only 1.4 Ohms and is not a factor. Have Fun, Brooke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Hal Murray wrote: >>acc. my understanding, the characteristic impedance of a transmission >>line (ideally losseless) is constant and waveform independant, >> >> > >The catch is we are discussing the non-lossless case. > >The corner frequency is where the losses become significant. > >I think there is another corner where the skin effect on the center conductor >kicks in. > > > > ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
> acc. my understanding, the characteristic impedance of a transmission > line (ideally losseless) is constant and waveform independant, The catch is we are discussing the non-lossless case. The corner frequency is where the losses become significant. I think there is another corner where the skin effect on the center conductor kicks in. -- These are my opinions, not necessarily my employer's. I hate spam. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
- Original Message - From: "Bill Hawkins" <[EMAIL PROTECTED]> To: "'Discussion of precise time and frequency measurement'" Sent: Tuesday, January 30, 2007 9:34 AM Subject: Re: [time-nuts] 75Z vs 50Z for GPS receivers > Dave Brown said, in part, > > "The characteristic impedance of a transmission > line, in purely general terms, is given by the square root of R plus > jw L divided by G plus jw C, with the usual meaning for symbols > used." > > Well, I know most of those symbols, but what's gravity (G) doing in > that equation? By dimensional analysis, G must be conductance, so we > have Ohms^2 under the root, and the answer is in Ohms. > > So, what is R the resistance of, and what has conductance G? > > Bill Hawkins Bill R, L, G and C are the 'per section' basic transmission line constants. You will find these symbols used in many of the texts on transmission line theory. The whole idea of the model is to have series and shunt reactive and loss (i.e. resistive) components. So R is the series resistance, L is the series inductance, C is the shunt capacitance and G is the 'shunt' leakage conductance. G is just the reciprocal of the effective ac shunt (leakage) resistance per basic line section and accounts for dielectric loss. It 'may' also have a DC component - if the transmission line has poor insulation between conductors. As you would expect, R also is frequency dependent, although in a simple model, such as this, it is assumed constant. Regards, DaveB, NZ ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Hi Bill: The series elements are resistance and inductance. The shunt elements are conductance G (leakage resistance expressed in Mho or Siemens) and capactance For long lines the resistance of the wires is very important and for low frequencies the capactance is the dominant reactance. see the plot: http://www.pacificsites.com/~brooke/Zo.shtml#RG58 On telegraph circuits where they are sending pulses a rule of thumb was one battery for each mile of line. For more on wet cell batteries see: http://www.pacificsites.com/~brooke/Leclanche.shtml Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Bill Hawkins wrote: >Dave Brown said, in part, > >"The characteristic impedance of a transmission >line, in purely general terms, is given by the square root of R plus >jw L divided by G plus jw C, with the usual meaning for symbols used." > >Well, I know most of those symbols, but what's gravity (G) doing in >that equation? By dimensional analysis, G must be conductance, so we >have Ohms^2 under the root, and the answer is in Ohms. > >So, what is R the resistance of, and what has conductance G? > >Bill Hawkins > > >___ >time-nuts mailing list >time-nuts@febo.com >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Dave Brown said, in part, "The characteristic impedance of a transmission line, in purely general terms, is given by the square root of R plus jw L divided by G plus jw C, with the usual meaning for symbols used." Well, I know most of those symbols, but what's gravity (G) doing in that equation? By dimensional analysis, G must be conductance, so we have Ohms^2 under the root, and the answer is in Ohms. So, what is R the resistance of, and what has conductance G? Bill Hawkins ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Hi Dave: I got started looking at low frequency transmission line impedance in relation to open wire phone lines. But for short lengths it's not a transmission line but rather a lumped element. http://www.pacificsites.com/~brooke/Zo.shtml Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Dave Brown wrote: >- Original Message - >From: "Arnold Tibus" <[EMAIL PROTECTED]> >To: "Discussion of precise time and frequency measurement" > >Sent: Tuesday, January 30, 2007 4:09 AM >Subject: Re: [time-nuts] 75Z vs 50Z for GPS receivers > > > > >>Hi Brooke, >>acc. my understanding, the characteristic impedance of a >>transmission >>line (ideally losseless) is constant and waveform independant, as >>given by the relation of inductive and capacitive values (muh and >>epsilon) >>equally distributed over the line. >> >> >SNIP** > > >>Summarizing my opinion, a 50 Ohm transmission-line does have a Z of >>50 Ohms >>on a l l frequencies, the charact. impedance remain c o n s t a n >>t >>(unless the design or the material fails physically, which occur >>mainly in the >>microwave region). >> >> > >Not true. >The characteristic impedance of a transmission >line, in purely general terms, is given by the square root of R plus >jw L divided by G plus jw C, with the usual meaning for symbols used. > >In the normal high frequency case, the reactive terms predominate and >we have the usual (but strictly speaking, approximate) relationship, >with the characteristic line impedance given by the square root of L >over C. > > However, at very low frequencies, the reactive terms tend to zero and >we are left with the characteristic impedance being given >predominantly by the >ratio R over G under the square root sign. > >In practice there is a range of low frequencies where both resistive >AND reactive effects contribute to the result. > >So the CHARACTERISTIC line impedance DOES change with >frequency and in fact INCREASES at very low frequencies for most real >transmission lines. > > But the impedance SEEN looking into a transmission line is usually of >much more interest. It is a function of the characteristic impedance, >the line's electrical length AND the termination at the far end of the >line. > >Where the electrical length is short, (I prefer 0.1 wavelength, other >definitions of 'short' abound) its effects tend to zero and the >impedance seen is primarily a function of the termination and, to a >far lesser extent, a function of the characteristic impedance. As >explained above, the increase in characteristic impedance will >contribute more to the result at lower frequencies, but in most >practical cases, the termination will predominate. > >Disbelievers may now refer to their favourite transmission line text >where all will be (well... should be) revealed. Problem is, most of >'em gloss over the general case in their undignified hurry to get zed >nought being given by the square root of L over C! It certainly makes >the math easier! > >Regards >DaveB,NZ > > > > >___ >time-nuts mailing list >time-nuts@febo.com >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
- Original Message - From: "Arnold Tibus" <[EMAIL PROTECTED]> To: "Discussion of precise time and frequency measurement" Sent: Tuesday, January 30, 2007 4:09 AM Subject: Re: [time-nuts] 75Z vs 50Z for GPS receivers > Hi Brooke, > acc. my understanding, the characteristic impedance of a > transmission > line (ideally losseless) is constant and waveform independant, as > given by the relation of inductive and capacitive values (muh and > epsilon) > equally distributed over the line. SNIP** > Summarizing my opinion, a 50 Ohm transmission-line does have a Z of > 50 Ohms > on a l l frequencies, the charact. impedance remain c o n s t a n > t > (unless the design or the material fails physically, which occur > mainly in the > microwave region). Not true. The characteristic impedance of a transmission line, in purely general terms, is given by the square root of R plus jw L divided by G plus jw C, with the usual meaning for symbols used. In the normal high frequency case, the reactive terms predominate and we have the usual (but strictly speaking, approximate) relationship, with the characteristic line impedance given by the square root of L over C. However, at very low frequencies, the reactive terms tend to zero and we are left with the characteristic impedance being given predominantly by the ratio R over G under the square root sign. In practice there is a range of low frequencies where both resistive AND reactive effects contribute to the result. So the CHARACTERISTIC line impedance DOES change with frequency and in fact INCREASES at very low frequencies for most real transmission lines. But the impedance SEEN looking into a transmission line is usually of much more interest. It is a function of the characteristic impedance, the line's electrical length AND the termination at the far end of the line. Where the electrical length is short, (I prefer 0.1 wavelength, other definitions of 'short' abound) its effects tend to zero and the impedance seen is primarily a function of the termination and, to a far lesser extent, a function of the characteristic impedance. As explained above, the increase in characteristic impedance will contribute more to the result at lower frequencies, but in most practical cases, the termination will predominate. Disbelievers may now refer to their favourite transmission line text where all will be (well... should be) revealed. Problem is, most of 'em gloss over the general case in their undignified hurry to get zed nought being given by the square root of L over C! It certainly makes the math easier! Regards DaveB,NZ ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Hi Brooke, acc. my understanding, the characteristic impedance of a transmission line (ideally losseless) is constant and waveform independant, as given by the relation of inductive and capacitive values (muh and epsilon) equally distributed over the line. As long as the physical parameters of the propagation path do not vary with the frequency (due to special material behaviour or too big cable size), there cannot be a dependency on frequency of the impedance Z. Therefore the impedance Z of a transmission line can as well not be dependent on the waveform, as all not sinosoidal waveforms are just a sum of a spectrum of sinosoidal waves (spectrum of frequencies). An even very short pulse is as well just a sum of sine waves with few periods over a wide band (the reason why I think there are no real digital rf signals, just the modulation information may be called digital). The safe way to transfer a proper waveform is to apply properly matched transmission lines conforming its characteristic line impedance Z. That I cannot measure the correct line impedance under some circumstances, is another issue. If, in relation to the wavelength, the length of line is going to be short (as on low frequencies), then I am running into problems, similar as it is the case in freespace measuring fields under near field conditions. Already on lines length below a quarter of wavelength it is hard to get acceptable measurement values, and at a 100th of a wavelength or less I can practically ignore the effects of reflections, output = input. Short lines behave capacitive and at short distances the voltage is dominant in measurements! At audio frequencies I cannot talk anymore of reflections. Therefore, at very low frequencies it is not anymore of importance to use matched transmission lines for dist. of a few meters, but the lines still do have their characteristic impedance! Summarizing my opinion, a 50 Ohm transmission-line does have a Z of 50 Ohms on a l l frequencies, the charact. impedance remain c o n s t a n t (unless the design or the material fails physically, which occur mainly in the microwave region). Even if it is not of importance to use transmission lines of a specific value, it does not harm either but it may be still important to use a matched load. For signals of low frequencies as the pps with short risetimes, perhaps it would be better in effect to apply lines of hight impedance (eg. 75 --> 95 or 100 Ohm) , because the lower capacitive effects and dielectric losses, but match the end with the adequate load. There are of course other solutions, eg. when working with cmos: 'open' end at rx for a double signal level, then eliminating the reflections at the matched tx output, but that is another point and I think already discussed on this place. kind regards, Arnold On Sun, 28 Jan 2007 13:02:39 -0800, Brooke Clarke wrote: >HI Didier: >It's my understanding that the term impedance can only be applied when >sine wave signals are being used. So for pulse work you might look at >the harmonic content and try to match all those frequencies. >Long ago Bob Grove promoted the idea of using 75 Ohm TV coax for ham >antennas at 2 meters and higher frequencies because it had lower loss >than 50 Ohm coax and was much lower in cost. For ham applications the >VSWR due to the coax impedance was much smaller than the inherent match >(mismatch) of the things on either end. >It's only been in the last few years that I understood that the >impedance of a transmission line is only a constant value above some >frequency and below that is no longer a constant. So, for example, >audio signals can not be transmitted using "transmission lines" of >constant impedance. For more see: >http://www.pacificsites.com/~brooke/Zo.shtml >Have Fun, >Brooke Clarke ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Didier You can always build your own 100A avalanche transistor pulser using several Zetex FMMT413s. Bruce Brooke Clarke wrote: > Hi Didier: > > Yes they are on eBay. The tunnel diode models have a narrower pulse > than the later models that use something more rugged. Probably for all > practical purposes one of the newer ones would work well and last > better. I think the military bought boat loads of these and I've heard > from a number of people that use them and like them a lot. > > Have Fun, > > Brooke Clarke > > w/Java http://www.PRC68.com > w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml > http://www.precisionclock.com > > > > Didier Juges wrote: > > >> Going over the spec for the 1502, it seems like a nice instrument. I am >> worried about the fragility of the tunnel diode though. When my neighbor >> AD4TK fires up his amplifier, I have measured up to 1W of his signal >> going down my feedline (on the 80m slopper antenna)... >> >> Are these things on eBay? >> >> Didier KO4BB >> >> Brooke Clarke wrote: >> >> >> >>> Hi Didier: >>> >>> The Tek 1502 is great for doing this, especially if you have the >>> optional strip chart recorder. It's what it was made to do. >>> http://www.pacificsites.com/~brooke/Tek1502.shtml >>> >>> Have Fun, >>> >>> Brooke Clarke >>> >>> >>> >>> >> ___ >> time-nuts mailing list >> time-nuts@febo.com >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> >> >> >> > ___ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Certainly a vector network analyzer is the next piece of test equipment I need to get. We have several HP 8720 and 8722 where I work, and it's easy to be spoiled... Unfortunately, that's another quantum leap in cost from the spectrum analyzers and synthesizers I have, at least for those that are microwave capable. I remember a long time ago doing the reverse, taking a TDR plot (or data set) and computing the frequency domain response using FFT. Did I say that was A LONG time ago? Didier Daun Yeagley wrote: > A number of vector network analyzers have a time domain function in them. > They > use an inverse Fourier transform to get the display. > This is VERY useful in a high RF environment. A number of years ago we had a > problem on a receiver site for the DARA (Dayton) two meter repeater, and it > was > located on the WHIO TV transmitter tower. One of the guys had tried to use a > conventional TDR, but since the front end is wide open, all he could get was > gibberish. I took an HP 8753 analyzer that had the time domain option, and it > clearly showed where the fault was. It was able to do it because it uses a > narrowband receiver. > > Daun > ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
A number of vector network analyzers have a time domain function in them. They use an inverse Fourier transform to get the display. This is VERY useful in a high RF environment. A number of years ago we had a problem on a receiver site for the DARA (Dayton) two meter repeater, and it was located on the WHIO TV transmitter tower. One of the guys had tried to use a conventional TDR, but since the front end is wide open, all he could get was gibberish. I took an HP 8753 analyzer that had the time domain option, and it clearly showed where the fault was. It was able to do it because it uses a narrowband receiver. Daun -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Brooke Clarke Sent: Sunday, January 28, 2007 10:35 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] 75Z vs 50Z for GPS receivers Hi Didier: Yes they are on eBay. The tunnel diode models have a narrower pulse than the later models that use something more rugged. Probably for all practical purposes one of the newer ones would work well and last better. I think the military bought boat loads of these and I've heard from a number of people that use them and like them a lot. Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Didier Juges wrote: >Going over the spec for the 1502, it seems like a nice instrument. I am >worried about the fragility of the tunnel diode though. When my neighbor >AD4TK fires up his amplifier, I have measured up to 1W of his signal >going down my feedline (on the 80m slopper antenna)... > >Are these things on eBay? > >Didier KO4BB > >Brooke Clarke wrote: > > >>Hi Didier: >> >>The Tek 1502 is great for doing this, especially if you have the >>optional strip chart recorder. It's what it was made to do. >>http://www.pacificsites.com/~brooke/Tek1502.shtml >> >>Have Fun, >> >>Brooke Clarke >> >> >> > > >___ >time-nuts mailing list >time-nuts@febo.com >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Hi Didier: Yes they are on eBay. The tunnel diode models have a narrower pulse than the later models that use something more rugged. Probably for all practical purposes one of the newer ones would work well and last better. I think the military bought boat loads of these and I've heard from a number of people that use them and like them a lot. Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Didier Juges wrote: >Going over the spec for the 1502, it seems like a nice instrument. I am >worried about the fragility of the tunnel diode though. When my neighbor >AD4TK fires up his amplifier, I have measured up to 1W of his signal >going down my feedline (on the 80m slopper antenna)... > >Are these things on eBay? > >Didier KO4BB > >Brooke Clarke wrote: > > >>Hi Didier: >> >>The Tek 1502 is great for doing this, especially if you have the >>optional strip chart recorder. It's what it was made to do. >>http://www.pacificsites.com/~brooke/Tek1502.shtml >> >>Have Fun, >> >>Brooke Clarke >> >> >> > > >___ >time-nuts mailing list >time-nuts@febo.com >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
I think that the simplest explanation for the BIPM's recommendation that the antenna cables be matched to the antenna output impedance and the GPS receivers input impedance is as follows: If one has gone to the trouble and expense of installing an antenna that is relatively insensitive to multipath effects, then using an unmatched cable to connect the antenna to the receiver in effect reintroduces multipath signals into the receiver input, reflections on the cable due to antenna and receiver mismatch are no different in character to multipath signals. The magnitude of the effects of cable mismatch will depend on the degree of mismatch at the antenna and at the receiver as well as the cable attenuation, and the cable length For the less accurate timing receivers such as the Trimble Thunderbolt such effects may be insignificant as other the effect of other timing error sources are much larger. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Going over the spec for the 1502, it seems like a nice instrument. I am worried about the fragility of the tunnel diode though. When my neighbor AD4TK fires up his amplifier, I have measured up to 1W of his signal going down my feedline (on the 80m slopper antenna)... Are these things on eBay? Didier KO4BB Brooke Clarke wrote: > Hi Didier: > > The Tek 1502 is great for doing this, especially if you have the > optional strip chart recorder. It's what it was made to do. > http://www.pacificsites.com/~brooke/Tek1502.shtml > > Have Fun, > > Brooke Clarke > ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Didier Juges wrote: > Dr Bruce Griffiths wrote: > >> Didier Juges wrote: >> >> >>> Dr Bruce Griffiths wrote: >>> >>> >>> Didier Juges wrote: > Dr Bruce Griffiths wrote: > > > > > >> Chris >> >> >> >> >> >> >>> >>> >>> >>> >>> >>> >>> Christopher Hoover wrote: >> Most (except for Trimble,..) GPS receivers and antennas >> are designed to use 50 ohm cable. >> Trimble Bullet GPS antennas have a 50 ohm output impedance. >> Trimble literature however is ambiguous in that in the >> Resolution T receiver datasheets talk about using RG59 >> to connect to the antenna. >> >> >> >> >> >> >> >> >> > FWIW, the Thunderbolt manual says this on page 3-5: > > > Note - RG-59 is a 75 ohm coaxial cable. The ThunderBolt and > the Bullet antenna are compatible with either 50-ohm or > 75-ohm cable. Compared to most 50 ohm cable, 75 ohm cable > provides superior transmissibility for the 1.5 GHz GPS > signal and a better quality cable for the price. Mismatched > impedance is not a problem. > > > Note - The input impedance of the ThunderBolt RF input & > its antenna is 50 ohms. > > > > I would also add that BNC and N connectors come in both 50Z and 75Z > flavors. > In fact 75Z BNC connectors are pretty common, being used in > professional > video applications. (Check surplus BNC patch cords carefully.) On > the > other hand, 75Z N connectors are much less common, but were used in > CATV > plants, IIRC. > > -ch > > > > ___ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > > > > > > > >> For those of us who strive for the ultimate in performance with GPS >> timing receivers it is recommended that mismatched antenna cables not be >> used see: >> >> http://www.bipm.fr/wg/CCTF/CGGTTS/Allowed/Recommendations/CGGTTS-Guidelines.pdf >> >> Bruce >> >> >> >> >> >> > Bruce, > > They recommend that reflections be 40 dB below the main signal, so if > you have about 15 dB loss in the cable (as I do), you only need no more > than 10 dB of return loss (combined at both ends) since the reflections > will have to travel twice more through the cable than the main signal. I > believe a 1.5:1 VSWR will do that (which is obtained by matching 50 and > 75 ohms, magically :-) Of course, if the cable is shorter and has less > loss, it should be better matched. So, 50 feet of RG-59 cable will meet > the recommendation. > > I have most certainly overlooked something, but I should not be too far > off. > > Didier KO4BB > > > ___ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > > > Didier Perhaps at the subnanosecond level one may have to take into account the effect of cable mismatch on the phase and group delays? It certainly would be useful if some measurements of the effect of using mismatched cables were made. The Thunderbolt probably has inadequate performance to discern this. They also recommend low cable attenuation. However the hardest part of the spec to meet is the cable delay tempco which is 5 to 10 times better than standard coax. It may be possible to meat this spec with high velocity cables (0.95c??) which essentially have an air dielectric. I guess this is why the antenna cable temperature is regulated in some installations. Bruce >>> Bruce, >>> >>> You have done it again. Now, I have more
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Dr Bruce Griffiths wrote: > Didier Juges wrote: > >> Dr Bruce Griffiths wrote: >> >> >>> Didier Juges wrote: >>> >>> >>> Dr Bruce Griffiths wrote: > Chris > > > > > >> >> >> >> >> >> >>> Christopher Hoover wrote: >>> >>> >>> >>> >>> >>> >>> > Most (except for Trimble,..) GPS receivers and antennas > are designed to use 50 ohm cable. > Trimble Bullet GPS antennas have a 50 ohm output impedance. > Trimble literature however is ambiguous in that in the > Resolution T receiver datasheets talk about using RG59 > to connect to the antenna. > > > > > > > > FWIW, the Thunderbolt manual says this on page 3-5: Note - RG-59 is a 75 ohm coaxial cable. The ThunderBolt and the Bullet antenna are compatible with either 50-ohm or 75-ohm cable. Compared to most 50 ohm cable, 75 ohm cable provides superior transmissibility for the 1.5 GHz GPS signal and a better quality cable for the price. Mismatched impedance is not a problem. Note - The input impedance of the ThunderBolt RF input & its antenna is 50 ohms. I would also add that BNC and N connectors come in both 50Z and 75Z flavors. In fact 75Z BNC connectors are pretty common, being used in professional video applications. (Check surplus BNC patch cords carefully.) On the other hand, 75Z N connectors are much less common, but were used in CATV plants, IIRC. -ch ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > For those of us who strive for the ultimate in performance with GPS > timing receivers it is recommended that mismatched antenna cables not be > used see: > > http://www.bipm.fr/wg/CCTF/CGGTTS/Allowed/Recommendations/CGGTTS-Guidelines.pdf > > Bruce > > > > > Bruce, They recommend that reflections be 40 dB below the main signal, so if you have about 15 dB loss in the cable (as I do), you only need no more than 10 dB of return loss (combined at both ends) since the reflections will have to travel twice more through the cable than the main signal. I believe a 1.5:1 VSWR will do that (which is obtained by matching 50 and 75 ohms, magically :-) Of course, if the cable is shorter and has less loss, it should be better matched. So, 50 feet of RG-59 cable will meet the recommendation. I have most certainly overlooked something, but I should not be too far off. Didier KO4BB ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> Didier >>> >>> Perhaps at the subnanosecond level one may have to take into account the >>> effect of cable mismatch on the phase and group delays? >>> It certainly would be useful if some measurements of the effect of using >>> mismatched cables were made. >>> The Thunderbolt probably has inadequate performance to discern this. >>> >>> They also recommend low cable attenuation. >>> However the hardest part of the spec to meet is the cable delay tempco >>> which is 5 to 10 times better than standard coax. >>> It may be possible to meat this spec with high velocity cables (0.95c??) >>> which essentially have an air dielectric. >>> I guess this is why the antenna cable temperature is regulated in some >>> installations. >>> >>> Bruce >>> >>> >>> >> Bruce, >> >> You have done it again. Now, I have more ideas for interesting >> experimentation and still not more time :-) >> >> I have 3 coax runs going from my ham shack to the top of my tower to >> feed the HF (14 to 30 MHz) and two VHF antennas (6m and 2m, or 50 MHz >> and 144 MHz). One cable (HF) is regular RG-213, another (6m) is RG-214 >> (essentially like RG-213 with double shield), the last one (2m) is >> Ultra-flex air dielectric (more like 9913). All 3
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Hi Didier: The Tek 1502 is great for doing this, especially if you have the optional strip chart recorder. It's what it was made to do. http://www.pacificsites.com/~brooke/Tek1502.shtml Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com . . . . > >I have 3 coax runs going from my ham shack to the top of my tower to >feed the HF (14 to 30 MHz) and two VHF antennas (6m and 2m, or 50 MHz >and 144 MHz). One cable (HF) is regular RG-213, another (6m) is RG-214 >(essentially like RG-213 with double shield), the last one (2m) is >Ultra-flex air dielectric (more like 9913). All 3 runs are close in >length, about 135 feet. I have fed them with the 1 PPS signal and looked >at reflections with the storage scope (that's how I know the length). It >is interesting to see the big reflection when the signal gets to the >antennas. Of course, the antennas are a poor match for the 1 PPS signal >(fortunately), so they are essentially a short circuit. > >I need to take pictures, now that it is relatively cold here (everything >being relative) and take the same pictures in the summer and look for >differences between the 3 coax lines. Out of the 135 feet, 60 feet are >in the air going up the tower, 50 feet are in the ground (in a 4" PVC >pipe), the rest is in the garage and the attic, so temperature is not >well controlled or constant, but it should all vary in the same direction. > >Didier > > > ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Didier Juges wrote: > > Bruce, > > You have done it again. Now, I have more ideas for interesting > experimentation and still not more time :-) > > I have 3 coax runs going from my ham shack to the top of my tower to > feed the HF (14 to 30 MHz) and two VHF antennas (6m and 2m, or 50 MHz > and 144 MHz). One cable (HF) is regular RG-213, another (6m) is RG-214 > (essentially like RG-213 with double shield), the last one (2m) is > Ultra-flex air dielectric (more like 9913). All 3 runs are close in > length, about 135 feet. I have fed them with the 1 PPS signal and looked > at reflections with the storage scope (that's how I know the length). It > is interesting to see the big reflection when the signal gets to the > antennas. Of course, the antennas are a poor match for the 1 PPS signal > (fortunately), so they are essentially a short circuit. > > I need to take pictures, now that it is relatively cold here (everything > being relative) and take the same pictures in the summer and look for > differences between the 3 coax lines. Out of the 135 feet, 60 feet are > in the air going up the tower, 50 feet are in the ground (in a 4" PVC > pipe), the rest is in the garage and the attic, so temperature is not > well controlled or constant, but it should all vary in the same direction. > > Didier > > The antenna TDR pictures are at the usual place http://www.ko4bb.com/Test_Equipment/Thunderbolt/ I have put a 10dB 50 ohm attenuator at the output of the Thunderbolt to normalize the output impedance. Not perfect (measures 66 ohms I think), but better than feeding directly. I will redo it with a good 50 ohm series resistor. From the 10dB pad, I have a 4 foot coax cable going to a T, with the T plugged into the scope (1 Mohm input) and the other end of the T going to the antenna. The plot of the 2m antenna puzzled me at first, because it seems well matched over a broad range of frequencies. Then I realized that I have a mast mounted preamplifier that is powered through the coax, so the choke and filter for the supply voltage is providing a match at lower frequencies for the feed line. I am still surprised that the delay to 1st reflection is only a little shorter for the 2m antenna than for the 6m antenna (444nS vs. 488nS). The velocity factors are supposed to be 95% and 66% respectively, so I must have a lot more coax on the 2m antenna. Anyway, I have those for reference now, and I will take the data again when it's warmer. Didier KO4BB ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Didier Juges wrote: > Dr Bruce Griffiths wrote: > >> Didier Juges wrote: >> >> >>> Dr Bruce Griffiths wrote: >>> >>> >>> Chris > > > > > >> Christopher Hoover wrote: >> >> >> >> >> >> Most (except for Trimble,..) GPS receivers and antennas are designed to use 50 ohm cable. Trimble Bullet GPS antennas have a 50 ohm output impedance. Trimble literature however is ambiguous in that in the Resolution T receiver datasheets talk about using RG59 to connect to the antenna. >>> FWIW, the Thunderbolt manual says this on page 3-5: >>> >>> >>> Note - RG-59 is a 75 ohm coaxial cable. The ThunderBolt and >>> the Bullet antenna are compatible with either 50-ohm or >>> 75-ohm cable. Compared to most 50 ohm cable, 75 ohm cable >>> provides superior transmissibility for the 1.5 GHz GPS >>> signal and a better quality cable for the price. Mismatched >>> impedance is not a problem. >>> >>> >>> Note - The input impedance of the ThunderBolt RF input & >>> its antenna is 50 ohms. >>> >>> >>> >>> I would also add that BNC and N connectors come in both 50Z and 75Z >>> flavors. >>> In fact 75Z BNC connectors are pretty common, being used in professional >>> video applications. (Check surplus BNC patch cords carefully.) On the >>> other hand, 75Z N connectors are much less common, but were used in CATV >>> plants, IIRC. >>> >>> -ch >>> >>> >>> >>> ___ >>> time-nuts mailing list >>> time-nuts@febo.com >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> >>> >>> >>> >>> >>> >>> >>> For those of us who strive for the ultimate in performance with GPS timing receivers it is recommended that mismatched antenna cables not be used see: http://www.bipm.fr/wg/CCTF/CGGTTS/Allowed/Recommendations/CGGTTS-Guidelines.pdf Bruce >>> Bruce, >>> >>> They recommend that reflections be 40 dB below the main signal, so if >>> you have about 15 dB loss in the cable (as I do), you only need no more >>> than 10 dB of return loss (combined at both ends) since the reflections >>> will have to travel twice more through the cable than the main signal. I >>> believe a 1.5:1 VSWR will do that (which is obtained by matching 50 and >>> 75 ohms, magically :-) Of course, if the cable is shorter and has less >>> loss, it should be better matched. So, 50 feet of RG-59 cable will meet >>> the recommendation. >>> >>> I have most certainly overlooked something, but I should not be too far off. >>> >>> Didier KO4BB >>> >>> >>> ___ >>> time-nuts mailing list >>> time-nuts@febo.com >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> >>> >>> >>> >> Didier >> >> Perhaps at the subnanosecond level one may have to take into account the >> effect of cable mismatch on the phase and group delays? >> It certainly would be useful if some measurements of the effect of using >> mismatched cables were made. >> The Thunderbolt probably has inadequate performance to discern this. >> >> They also recommend low cable attenuation. >> However the hardest part of the spec to meet is the cable delay tempco >> which is 5 to 10 times better than standard coax. >> It may be possible to meat this spec with high velocity cables (0.95c??) >> which essentially have an air dielectric. >> I guess this is why the antenna cable temperature is regulated in some >> installations. >> >> Bruce >> >> > Bruce, > > You have done it again. Now, I have more ideas for interesting > experimentation and still not more time :-) > > I have 3 coax runs going from my ham shack to the top of my tower to > feed the HF (14 to 30 MHz) and two VHF antennas (6m and 2m, or 50 MHz > and 144 MHz). One cable (HF) is regular RG-213, another (6m) is RG-214 > (essentially like RG-213 with double shield), the last one (2m) is > Ultra-flex air dielectric (more like 9913). All 3 runs are close in > length, about 135 feet. I have fed them with the 1 PPS signal and looked > at reflections with the storage scope (that's how I know the length). It > is interesting to see the big reflection when the signal gets to the > antennas. Of course, the antennas are a poor match for the 1 PPS signal > (fortunately), so they are essentially a short circuit. > > I need to t
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Didier Juges wrote: > It is true is that the impedance of a transmission line is not constant > with frequency, particularly at the low end (audio). > > At the higher end, a lot of things happen, such as impedance, > attenuation and velocity factor all change (a little) with frequency. > Also, at the higher end, leakage takes place. The center conductor can > no longer be considered to be completely enclosed by the shield and some > of the signal traveling in the cable is radiated out, increasing > insertion loss (and some outside signals can get in the cable as well). > Semi rigid cable (where the outer conductor is a solid piece of copper > tubing) takes care of the leakage issue pretty well, at least far away > from the connectors. > > However, most well designed coax cable exhibit a wide frequency range > where most of these characteristics are stable enough for practical > purposes, and the only thing you have to worry about is the increased > attenuation (or insertion loss) with increased frequency. So from HF to > VHF (and probably UHF), most coax cable can be considered as having a > constant impedance. Now, the 1 PPS signal has frequency components as > low as 1Hz of course, and going well into the higher MHz region, > depending on the rise and fall times, so you would think the lower > frequency components must be affected by the change of impedance of the > cable at the low end. > > The characteristic impedance of coax only means anything when the line > is long enough compared to the signal wavelength (or frequency). With a > 50 feet length of cable, that is about 1 MHz or 2. Below that frequency, > the characteristic impedance means nothing (that means the line is short > enough that you can consider the voltage and current to be the same all > along the line) and the cable is equivalent to a capacitor. > > So, unless you deal with very very long transmission lines (like the > phone companies do), you can ignore the fact that the characteristic > impedance changes at the low end. > > This can be illustrated by the waveform > http://www.ko4bb.com/Test_Equipment/Thunderbolt/PPS_into_50feet_75ohm_cable_1Mohm_load.jpg > > You can see the initial reflections are nice and square, and as the > signal keeps bouncing, the waveform rounds up. This is due to the higher > order harmonics being attenuated more than the lower harmonics. > Eventually, the signal looks more and more like a sinewave before it is > completely attenuated. > > I uploaded a couple more pictures: > http://www.ko4bb.com/Test_Equipment/Thunderbolt/2_PPS_into_long_75ohm_cable_thru_500ohm.jpg > and > http://www.ko4bb.com/Test_Equipment/Thunderbolt/2_PPS_into_short_50ohm_cable_thru_500ohm.jpg > > These show the waveform at the end of the long (respectively short) coax > cable when the cable is fed through a approx 500 ohm resistor. The scope > was set to 1 Mohm input impedance. > > You can see that there is no ringing any more, with either cable. The > dominant feature of the waveform is the attenuation of the leading edge > due to the capacitance of the cable. The longer the cable (which is > still very short compared to 1 Hz), the more capacitance. (Please note > that 75 ohm cable has less capacitance per foot than 50 ohm cable.) Yet, > when driven by a driver having matched impedance, the leading edge is > very square, with about 20nS rise time. > > Didier KO4BB > Didier The effect of reflections from the source and load are still evident particularly if the timebase speed is increased. However the transitions are monotonic albeit with small step treads evident for the duration of the cable round trip delay. If the load is a time interval counter or similar device, setting the trigger threshold about halfway up the first step ensures that the reflections have little or no effect on measurement accuracy. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Dr Bruce Griffiths wrote: > Didier Juges wrote: > >> Dr Bruce Griffiths wrote: >> >> >>> Chris >>> >>> >>> > Christopher Hoover wrote: > > > > > >>> Most (except for Trimble,..) GPS receivers and antennas >>> are designed to use 50 ohm cable. >>> Trimble Bullet GPS antennas have a 50 ohm output impedance. >>> Trimble literature however is ambiguous in that in the >>> Resolution T receiver datasheets talk about using RG59 >>> to connect to the antenna. >>> >>> >>> >>> >>> >>> >> FWIW, the Thunderbolt manual says this on page 3-5: >> >> >> Note - RG-59 is a 75 ohm coaxial cable. The ThunderBolt and >> the Bullet antenna are compatible with either 50-ohm or >> 75-ohm cable. Compared to most 50 ohm cable, 75 ohm cable >> provides superior transmissibility for the 1.5 GHz GPS >> signal and a better quality cable for the price. Mismatched >> impedance is not a problem. >> >> >> Note - The input impedance of the ThunderBolt RF input & >> its antenna is 50 ohms. >> >> >> >> I would also add that BNC and N connectors come in both 50Z and 75Z >> flavors. >> In fact 75Z BNC connectors are pretty common, being used in professional >> video applications. (Check surplus BNC patch cords carefully.) On the >> other hand, 75Z N connectors are much less common, but were used in CATV >> plants, IIRC. >> >> -ch >> >> >> >> ___ >> time-nuts mailing list >> time-nuts@febo.com >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> >> >> >> >> >> >> >>> For those of us who strive for the ultimate in performance with GPS >>> timing receivers it is recommended that mismatched antenna cables not be >>> used see: >>> >>> http://www.bipm.fr/wg/CCTF/CGGTTS/Allowed/Recommendations/CGGTTS-Guidelines.pdf >>> >>> Bruce >>> >>> >>> >> Bruce, >> >> They recommend that reflections be 40 dB below the main signal, so if >> you have about 15 dB loss in the cable (as I do), you only need no more >> than 10 dB of return loss (combined at both ends) since the reflections >> will have to travel twice more through the cable than the main signal. I >> believe a 1.5:1 VSWR will do that (which is obtained by matching 50 and >> 75 ohms, magically :-) Of course, if the cable is shorter and has less >> loss, it should be better matched. So, 50 feet of RG-59 cable will meet >> the recommendation. >> >> I have most certainly overlooked something, but I should not be too far off. >> >> Didier KO4BB >> >> >> ___ >> time-nuts mailing list >> time-nuts@febo.com >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> >> >> > Didier > > Perhaps at the subnanosecond level one may have to take into account the > effect of cable mismatch on the phase and group delays? > It certainly would be useful if some measurements of the effect of using > mismatched cables were made. > The Thunderbolt probably has inadequate performance to discern this. > > They also recommend low cable attenuation. > However the hardest part of the spec to meet is the cable delay tempco > which is 5 to 10 times better than standard coax. > It may be possible to meat this spec with high velocity cables (0.95c??) > which essentially have an air dielectric. > I guess this is why the antenna cable temperature is regulated in some > installations. > > Bruce > Bruce, You have done it again. Now, I have more ideas for interesting experimentation and still not more time :-) I have 3 coax runs going from my ham shack to the top of my tower to feed the HF (14 to 30 MHz) and two VHF antennas (6m and 2m, or 50 MHz and 144 MHz). One cable (HF) is regular RG-213, another (6m) is RG-214 (essentially like RG-213 with double shield), the last one (2m) is Ultra-flex air dielectric (more like 9913). All 3 runs are close in length, about 135 feet. I have fed them with the 1 PPS signal and looked at reflections with the storage scope (that's how I know the length). It is interesting to see the big reflection when the signal gets to the antennas. Of course, the antennas are a poor match for the 1 PPS signal (fortunately), so they are essentially a short circuit. I need to take pictures, now that it is relatively cold here (everything being relative) and take the same pictures in the summer and look for differences between the 3 coax lines. Out of the 135 feet, 60 feet are in the air going up the tower, 50 feet are in the ground (in a 4" PVC pipe), the rest is in the garage and the attic, so te
Re: [time-nuts] 75Z vs 50Z for GPS receivers
It is true is that the impedance of a transmission line is not constant with frequency, particularly at the low end (audio). At the higher end, a lot of things happen, such as impedance, attenuation and velocity factor all change (a little) with frequency. Also, at the higher end, leakage takes place. The center conductor can no longer be considered to be completely enclosed by the shield and some of the signal traveling in the cable is radiated out, increasing insertion loss (and some outside signals can get in the cable as well). Semi rigid cable (where the outer conductor is a solid piece of copper tubing) takes care of the leakage issue pretty well, at least far away from the connectors. However, most well designed coax cable exhibit a wide frequency range where most of these characteristics are stable enough for practical purposes, and the only thing you have to worry about is the increased attenuation (or insertion loss) with increased frequency. So from HF to VHF (and probably UHF), most coax cable can be considered as having a constant impedance. Now, the 1 PPS signal has frequency components as low as 1Hz of course, and going well into the higher MHz region, depending on the rise and fall times, so you would think the lower frequency components must be affected by the change of impedance of the cable at the low end. The characteristic impedance of coax only means anything when the line is long enough compared to the signal wavelength (or frequency). With a 50 feet length of cable, that is about 1 MHz or 2. Below that frequency, the characteristic impedance means nothing (that means the line is short enough that you can consider the voltage and current to be the same all along the line) and the cable is equivalent to a capacitor. So, unless you deal with very very long transmission lines (like the phone companies do), you can ignore the fact that the characteristic impedance changes at the low end. This can be illustrated by the waveform http://www.ko4bb.com/Test_Equipment/Thunderbolt/PPS_into_50feet_75ohm_cable_1Mohm_load.jpg You can see the initial reflections are nice and square, and as the signal keeps bouncing, the waveform rounds up. This is due to the higher order harmonics being attenuated more than the lower harmonics. Eventually, the signal looks more and more like a sinewave before it is completely attenuated. I uploaded a couple more pictures: http://www.ko4bb.com/Test_Equipment/Thunderbolt/2_PPS_into_long_75ohm_cable_thru_500ohm.jpg and http://www.ko4bb.com/Test_Equipment/Thunderbolt/2_PPS_into_short_50ohm_cable_thru_500ohm.jpg These show the waveform at the end of the long (respectively short) coax cable when the cable is fed through a approx 500 ohm resistor. The scope was set to 1 Mohm input impedance. You can see that there is no ringing any more, with either cable. The dominant feature of the waveform is the attenuation of the leading edge due to the capacitance of the cable. The longer the cable (which is still very short compared to 1 Hz), the more capacitance. (Please note that 75 ohm cable has less capacitance per foot than 50 ohm cable.) Yet, when driven by a driver having matched impedance, the leading edge is very square, with about 20nS rise time. Didier KO4BB Brooke Clarke wrote: > HI Didier: > > It's my understanding that the term impedance can only be applied when > sine wave signals are being used. So for pulse work you might look at > the harmonic content and try to match all those frequencies. > > Long ago Bob Grove promoted the idea of using 75 Ohm TV coax for ham > antennas at 2 meters and higher frequencies because it had lower loss > than 50 Ohm coax and was much lower in cost. For ham applications the > VSWR due to the coax impedance was much smaller than the inherent match > (mismatch) of the things on either end. > > It's only been in the last few years that I understood that the > impedance of a transmission line is only a constant value above some > frequency and below that is no longer a constant. So, for example, > audio signals can not be transmitted using "transmission lines" of > constant impedance. For more see: > http://www.pacificsites.com/~brooke/Zo.shtml > > Have Fun, > > Brooke Clarke > ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Poul-Henning Kamp wrote: > In message <[EMAIL PROTECTED]>, Dr Bruce Griffiths writes: > > >> I can see the difference between the short 50 ohm cable terminated in 50 >> ohms and the short 50 ohm cable terminated in 75 ohms. >> It would be instructive to repeat this with a short length of 75 ohm >> cable terminated in50 and 75 ohms. >> > > The technique for visualizing these kinds of things is called Time Domain > Reflectometry and you can do it with a good pulse generator and an > oscilloscope. > > A HP5359A is perfect for the job, but less will do it. > > Read here how-to: > > http://www.hparchive.com/Application_Notes/HP-AppNote-62.pdf > > Poul-Henning > > Poul The Tektronix booklet: / Time-Domain Reflectometry Measurements by James A Strickland /from their measurement concept series is also instructive, at 102 pages it has far more detail and examples. Unfortunately a quick search failed to find a scanned version of this on the web. I have a copy so if there were sufficient demand I could scan it. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
In message <[EMAIL PROTECTED]>, Dr Bruce Griffiths writes: >I can see the difference between the short 50 ohm cable terminated in 50 >ohms and the short 50 ohm cable terminated in 75 ohms. >It would be instructive to repeat this with a short length of 75 ohm >cable terminated in50 and 75 ohms. The technique for visualizing these kinds of things is called Time Domain Reflectometry and you can do it with a good pulse generator and an oscilloscope. A HP5359A is perfect for the job, but less will do it. Read here how-to: http://www.hparchive.com/Application_Notes/HP-AppNote-62.pdf Poul-Henning -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Didier Juges wrote: > Dr Bruce Griffiths wrote: > >> Chris >> >> >>> >>> >>> Christopher Hoover wrote: >> Most (except for Trimble,..) GPS receivers and antennas >> are designed to use 50 ohm cable. >> Trimble Bullet GPS antennas have a 50 ohm output impedance. >> Trimble literature however is ambiguous in that in the >> Resolution T receiver datasheets talk about using RG59 >> to connect to the antenna. >> >> >> >> >> > FWIW, the Thunderbolt manual says this on page 3-5: > > > Note - RG-59 is a 75 ohm coaxial cable. The ThunderBolt and > the Bullet antenna are compatible with either 50-ohm or > 75-ohm cable. Compared to most 50 ohm cable, 75 ohm cable > provides superior transmissibility for the 1.5 GHz GPS > signal and a better quality cable for the price. Mismatched > impedance is not a problem. > > > Note - The input impedance of the ThunderBolt RF input & > its antenna is 50 ohms. > > > > I would also add that BNC and N connectors come in both 50Z and 75Z > flavors. > In fact 75Z BNC connectors are pretty common, being used in professional > video applications. (Check surplus BNC patch cords carefully.) On the > other hand, 75Z N connectors are much less common, but were used in CATV > plants, IIRC. > > -ch > > > > ___ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > > > >> For those of us who strive for the ultimate in performance with GPS >> timing receivers it is recommended that mismatched antenna cables not be >> used see: >> >> http://www.bipm.fr/wg/CCTF/CGGTTS/Allowed/Recommendations/CGGTTS-Guidelines.pdf >> >> Bruce >> >> > Bruce, > > They recommend that reflections be 40 dB below the main signal, so if > you have about 15 dB loss in the cable (as I do), you only need no more > than 10 dB of return loss (combined at both ends) since the reflections > will have to travel twice more through the cable than the main signal. I > believe a 1.5:1 VSWR will do that (which is obtained by matching 50 and > 75 ohms, magically :-) Of course, if the cable is shorter and has less > loss, it should be better matched. So, 50 feet of RG-59 cable will meet > the recommendation. > > I have most certainly overlooked something, but I should not be too far off. > > Didier KO4BB > > > ___ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > Didier Perhaps at the subnanosecond level one may have to take into account the effect of cable mismatch on the phase and group delays? It certainly would be useful if some measurements of the effect of using mismatched cables were made. The Thunderbolt probably has inadequate performance to discern this. They also recommend low cable attenuation. However the hardest part of the spec to meet is the cable delay tempco which is 5 to 10 times better than standard coax. It may be possible to meat this spec with high velocity cables (0.95c??) which essentially have an air dielectric. I guess this is why the antenna cable temperature is regulated in some installations. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Didier Juges wrote: > Dr Bruce Griffiths wrote: > >> Didier Juges wrote: >> >> >>> Dr Bruce Griffiths wrote: >>> >>> >>> Christopher Hoover wrote: >> Most (except for Trimble,..) GPS receivers and antennas >> are designed to use 50 ohm cable. >> Trimble Bullet GPS antennas have a 50 ohm output impedance. >> Trimble literature however is ambiguous in that in the >> Resolution T receiver datasheets talk about using RG59 >> to connect to the antenna. >> >> >> >> >> > FWIW, the Thunderbolt manual says this on page 3-5: > > > Note - RG-59 is a 75 ohm coaxial cable. The ThunderBolt and > the Bullet antenna are compatible with either 50-ohm or > 75-ohm cable. Compared to most 50 ohm cable, 75 ohm cable > provides superior transmissibility for the 1.5 GHz GPS > signal and a better quality cable for the price. Mismatched > impedance is not a problem. > > > Note - The input impedance of the ThunderBolt RF input & > its antenna is 50 ohms. > > > > I would also add that BNC and N connectors come in both 50Z and 75Z > flavors. > In fact 75Z BNC connectors are pretty common, being used in professional > video applications. (Check surplus BNC patch cords carefully.) On the > other hand, 75Z N connectors are much less common, but were used in CATV > plants, IIRC. > > -ch > > > > ___ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > > > Impedance mismatch can be more problematic when connecting the PPS output of a GPS receiver to a time interval counter. RCA to BNC cables can catch one out if one doesn't notice that these use 75 ohm coax. 93 ohm RG62 cables with BNC connectors are not unknown, they were used in some nuclear instrumentation. I have a few of these lying around. Bruce >>> I noticed that when my counter insisted the PPS output of the >>> Thunderbolt was 5 Hz, even though it looked fine on the scope at >>> 200mS/div, until I set the scope's sweep fast enough (20nS/div or so) >>> and then I saw the reflections in the ~6 foot cable. In my case, it was >>> not an issue of 50 vs 75 ohm cable impedance, simply I had to terminate >>> the cable by setting the input impedance of the 5370A to 50 ohm instead >>> of the default 1 Mohm. I tried both 50 and 75 ohm terminations >>> (externally) and it did not make much difference to the counter, even >>> though the signal was definitely cleaner with 50 ohm (that cable was 50 >>> ohm). >>> >>> >>> Didier KO4BB >>> >>> ___ >>> time-nuts mailing list >>> time-nuts@febo.com >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> >>> >>> >>> >> Didier >> >> I found, as expected when using a 75 ohm cable to connect a 220 ohm GPS >> PPS output to an HP5370A terminated in 50 ohms, the threshold setting is >> a bit more critical than when using a 50 ohm cable. >> Its advisable to trigger on the incident wave and not on a subsequent >> reflection. This is less of a problem if the PPS source has an output >> impedance closer to the cable impedance. >> >> Bruce >> >> > Bruce, > > Trimble does not specify the output impedance of the PPS output in the > Thunderbolt manual, but they say it will provide TTL levels into 50 ohms. > Is 220 ohms impedance a standard value for PPS outputs? > What kind of cable is one supposed to use that has 220 ohms impedance? > > Considering the ringing in the Thunderbolt PPS output, the output > impedance is definitely not 50 ohms. If it were 50 ohms, I should read > 10V into an open circuit, but it only puts out 5V into an open, so the > output impedance must be very low. I posted 3 pictures of the PPS output > into an open (1 M ohm scope input impedance), 75 ohms (home-made > termination for the scope) and 50 ohms. There is no visible difference > in the ringing between 50 and 75 ohms. The cable was about 4 feet of 50 > ohm good quality cable. The scope is a TDS210 (60 MHz bandwidth). I do > have faster scopes (150 MHz) but non storage, and sweeping a 1 PPS > signal at 50nS/div on a non-storage scope does not make good pictures. > > The pictures are at http://www.ko4bb.com/Test_Equipment/Thunderbolt > > Sorry they have not been cropped. > > Then, I switched to one of the "good" 50 feet roll of 75 ohms cable I > bought on eBay and made more pictures. > The ringing without termination looks very pretty :-) > > In my experience when using long cables with fast signals, it is > import
Re: [time-nuts] 75Z vs 50Z for GPS receivers
On Sun, 28 Jan 2007 13:02:39 -0800, Brooke Clarke <[EMAIL PROTECTED]> wrote: >Long ago Bob Grove promoted the idea of using 75 Ohm TV coax for ham >antennas at 2 meters and higher frequencies because it had lower loss >than 50 Ohm coax and was much lower in cost. For ham applications the >VSWR due to the coax impedance was much smaller than the inherent match >(mismatch) of the things on either end. yes. I started doing that in the late 70s when a large quantity of CATV hard line fell into my possession. I did some loss measurements on a 500 ft roll and was pleasantly surprised to find the loss much less than a roll of 1/2" 50 ohm hard line that I also had on hand. >From then on I used CATV coax for all my ham activities on 2 meters and above. On 2 meters and 440 I was surprised to find the SWR significantly lower than theory predicted, measured with a Bird wattmeter. I never had the time and curiosity at the same time to investigate so I just tucked that one away in the long term storage area of my brain. John --- John De Armond See my website for my current email address http://www.neon-john.com Cleveland, Occupied TN Don't let your schooling interfere with your education-Mark Twain ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
HI Didier: It's my understanding that the term impedance can only be applied when sine wave signals are being used. So for pulse work you might look at the harmonic content and try to match all those frequencies. Long ago Bob Grove promoted the idea of using 75 Ohm TV coax for ham antennas at 2 meters and higher frequencies because it had lower loss than 50 Ohm coax and was much lower in cost. For ham applications the VSWR due to the coax impedance was much smaller than the inherent match (mismatch) of the things on either end. It's only been in the last few years that I understood that the impedance of a transmission line is only a constant value above some frequency and below that is no longer a constant. So, for example, audio signals can not be transmitted using "transmission lines" of constant impedance. For more see: http://www.pacificsites.com/~brooke/Zo.shtml Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Didier Juges wrote: >Dr Bruce Griffiths wrote: > > >>Didier Juges wrote: >> >> >> >>>Dr Bruce Griffiths wrote: >>> >>> >>> >>> Christopher Hoover wrote: >>Most (except for Trimble,..) GPS receivers and antennas >>are designed to use 50 ohm cable. >>Trimble Bullet GPS antennas have a 50 ohm output impedance. >>Trimble literature however is ambiguous in that in the >>Resolution T receiver datasheets talk about using RG59 >>to connect to the antenna. >> >> >> >> >> >> >FWIW, the Thunderbolt manual says this on page 3-5: > > >Note - RG-59 is a 75 ohm coaxial cable. The ThunderBolt and >the Bullet antenna are compatible with either 50-ohm or >75-ohm cable. Compared to most 50 ohm cable, 75 ohm cable >provides superior transmissibility for the 1.5 GHz GPS >signal and a better quality cable for the price. Mismatched >impedance is not a problem. > > >Note - The input impedance of the ThunderBolt RF input & >its antenna is 50 ohms. > > > >I would also add that BNC and N connectors come in both 50Z and 75Z >flavors. >In fact 75Z BNC connectors are pretty common, being used in professional >video applications. (Check surplus BNC patch cords carefully.) On the >other hand, 75Z N connectors are much less common, but were used in CATV >plants, IIRC. > >-ch > > > >___ >time-nuts mailing list >time-nuts@febo.com >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > > > > Impedance mismatch can be more problematic when connecting the PPS output of a GPS receiver to a time interval counter. RCA to BNC cables can catch one out if one doesn't notice that these use 75 ohm coax. 93 ohm RG62 cables with BNC connectors are not unknown, they were used in some nuclear instrumentation. I have a few of these lying around. Bruce >>>I noticed that when my counter insisted the PPS output of the >>>Thunderbolt was 5 Hz, even though it looked fine on the scope at >>>200mS/div, until I set the scope's sweep fast enough (20nS/div or so) >>>and then I saw the reflections in the ~6 foot cable. In my case, it was >>>not an issue of 50 vs 75 ohm cable impedance, simply I had to terminate >>>the cable by setting the input impedance of the 5370A to 50 ohm instead >>>of the default 1 Mohm. I tried both 50 and 75 ohm terminations >>>(externally) and it did not make much difference to the counter, even >>>though the signal was definitely cleaner with 50 ohm (that cable was 50 >>>ohm). >>> >>> >>>Didier KO4BB >>> >>>___ >>>time-nuts mailing list >>>time-nuts@febo.com >>>https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> >>> >>> >>> >>> >>Didier >> >>I found, as expected when using a 75 ohm cable to connect a 220 ohm GPS >>PPS output to an HP5370A terminated in 50 ohms, the threshold setting is >>a bit more critical than when using a 50 ohm cable. >>Its advisable to trigger on the incident wave and not on a subsequent >>reflection. This is less of a problem if the PPS source has an output >>impedance closer to the cable impedance. >> >>Bruce >> >> >> >Bruce, > >Trimble does not specify the output impedance of the PPS output in the >Thunderbolt manual, but they say it will provide TTL levels into 50 ohms. >Is 220 ohms impedance a standard value for PPS outputs? >What kind of cable is one supposed to use that has 220 ohms impedance? > >Considering the ringing in the Thunderbolt PPS output, the output >impedance is definitely not 50
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Dr Bruce Griffiths wrote: > Chris > >> >> >>> Christopher Hoover wrote: >>> >>> >>> > Most (except for Trimble,..) GPS receivers and antennas > are designed to use 50 ohm cable. > Trimble Bullet GPS antennas have a 50 ohm output impedance. > Trimble literature however is ambiguous in that in the > Resolution T receiver datasheets talk about using RG59 > to connect to the antenna. > > > > FWIW, the Thunderbolt manual says this on page 3-5: Note - RG-59 is a 75 ohm coaxial cable. The ThunderBolt and the Bullet antenna are compatible with either 50-ohm or 75-ohm cable. Compared to most 50 ohm cable, 75 ohm cable provides superior transmissibility for the 1.5 GHz GPS signal and a better quality cable for the price. Mismatched impedance is not a problem. Note - The input impedance of the ThunderBolt RF input & its antenna is 50 ohms. I would also add that BNC and N connectors come in both 50Z and 75Z flavors. In fact 75Z BNC connectors are pretty common, being used in professional video applications. (Check surplus BNC patch cords carefully.) On the other hand, 75Z N connectors are much less common, but were used in CATV plants, IIRC. -ch ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > For those of us who strive for the ultimate in performance with GPS > timing receivers it is recommended that mismatched antenna cables not be > used see: > > http://www.bipm.fr/wg/CCTF/CGGTTS/Allowed/Recommendations/CGGTTS-Guidelines.pdf > > Bruce > Bruce, They recommend that reflections be 40 dB below the main signal, so if you have about 15 dB loss in the cable (as I do), you only need no more than 10 dB of return loss (combined at both ends) since the reflections will have to travel twice more through the cable than the main signal. I believe a 1.5:1 VSWR will do that (which is obtained by matching 50 and 75 ohms, magically :-) Of course, if the cable is shorter and has less loss, it should be better matched. So, 50 feet of RG-59 cable will meet the recommendation. I have most certainly overlooked something, but I should not be too far off. Didier KO4BB ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Dr Bruce Griffiths wrote: > Didier Juges wrote: > >> Dr Bruce Griffiths wrote: >> >> >>> Christopher Hoover wrote: >>> >>> >>> > Most (except for Trimble,..) GPS receivers and antennas > are designed to use 50 ohm cable. > Trimble Bullet GPS antennas have a 50 ohm output impedance. > Trimble literature however is ambiguous in that in the > Resolution T receiver datasheets talk about using RG59 > to connect to the antenna. > > > > FWIW, the Thunderbolt manual says this on page 3-5: Note - RG-59 is a 75 ohm coaxial cable. The ThunderBolt and the Bullet antenna are compatible with either 50-ohm or 75-ohm cable. Compared to most 50 ohm cable, 75 ohm cable provides superior transmissibility for the 1.5 GHz GPS signal and a better quality cable for the price. Mismatched impedance is not a problem. Note - The input impedance of the ThunderBolt RF input & its antenna is 50 ohms. I would also add that BNC and N connectors come in both 50Z and 75Z flavors. In fact 75Z BNC connectors are pretty common, being used in professional video applications. (Check surplus BNC patch cords carefully.) On the other hand, 75Z N connectors are much less common, but were used in CATV plants, IIRC. -ch ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> Impedance mismatch can be more problematic when connecting the PPS >>> output of a GPS receiver to a time interval counter. >>> RCA to BNC cables can catch one out if one doesn't notice that these use >>> 75 ohm coax. >>> >>> 93 ohm RG62 cables with BNC connectors are not unknown, they were used >>> in some nuclear instrumentation. >>> I have a few of these lying around. >>> >>> Bruce >>> >>> >>> >> I noticed that when my counter insisted the PPS output of the >> Thunderbolt was 5 Hz, even though it looked fine on the scope at >> 200mS/div, until I set the scope's sweep fast enough (20nS/div or so) >> and then I saw the reflections in the ~6 foot cable. In my case, it was >> not an issue of 50 vs 75 ohm cable impedance, simply I had to terminate >> the cable by setting the input impedance of the 5370A to 50 ohm instead >> of the default 1 Mohm. I tried both 50 and 75 ohm terminations >> (externally) and it did not make much difference to the counter, even >> though the signal was definitely cleaner with 50 ohm (that cable was 50 >> ohm). >> >> >> Didier KO4BB >> >> ___ >> time-nuts mailing list >> time-nuts@febo.com >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> >> >> > Didier > > I found, as expected when using a 75 ohm cable to connect a 220 ohm GPS > PPS output to an HP5370A terminated in 50 ohms, the threshold setting is > a bit more critical than when using a 50 ohm cable. > Its advisable to trigger on the incident wave and not on a subsequent > reflection. This is less of a problem if the PPS source has an output > impedance closer to the cable impedance. > > Bruce > Bruce, Trimble does not specify the output impedance of the PPS output in the Thunderbolt manual, but they say it will provide TTL levels into 50 ohms. Is 220 ohms impedance a standard value for PPS outputs? What kind of cable is one supposed to use that has 220 ohms impedance? Considering the ringing in the Thunderbolt PPS output, the output impedance is definitely not 50 ohms. If it were 50 ohms, I should read 10V into an open circuit, but it only puts out 5V into an open, so the output impedance must be very low. I posted 3 pictures of the PPS output into an open (1 M ohm scope input impedance), 75 ohms (home-made termination for the scope) and 50 ohms. There is no visible difference in the ringing between 50 and 75 ohms. The cable was about 4 feet of 50 ohm good quality cable. The scope is a TDS210 (60 MHz bandwidth). I do have faster scopes (150 MHz) but non storage, and sweeping a 1 PPS signal at 50nS/div on a non-storage scope does not make good pictures. The pictures are at http://www.ko4bb.com/Test_Equipment/Thunderbolt Sorry they have not been cropped. Then, I switched to one of the "good" 50 feet roll of 75 ohms cable I bought on eBay and made more pictures. The ringing without termination looks very pretty :-) In my experience when using long cables with fast signals, it is important to have at least one end properly terminated to eliminate reflections and ringing. As long as one end is properly terminated, reflections will not bounce back and forth. Matching both ends optimizes power transfer (and frequency response) which, for timing signals, is not
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Chris > >> Christopher Hoover wrote: >> >> Most (except for Trimble,..) GPS receivers and antennas are designed to use 50 ohm cable. Trimble Bullet GPS antennas have a 50 ohm output impedance. Trimble literature however is ambiguous in that in the Resolution T receiver datasheets talk about using RG59 to connect to the antenna. >>> FWIW, the Thunderbolt manual says this on page 3-5: >>> >>> >>> Note - RG-59 is a 75 ohm coaxial cable. The ThunderBolt and >>> the Bullet antenna are compatible with either 50-ohm or >>> 75-ohm cable. Compared to most 50 ohm cable, 75 ohm cable >>> provides superior transmissibility for the 1.5 GHz GPS >>> signal and a better quality cable for the price. Mismatched >>> impedance is not a problem. >>> >>> >>> Note - The input impedance of the ThunderBolt RF input & >>> its antenna is 50 ohms. >>> >>> >>> >>> I would also add that BNC and N connectors come in both 50Z and 75Z flavors. >>> In fact 75Z BNC connectors are pretty common, being used in professional >>> video applications. (Check surplus BNC patch cords carefully.) On the >>> other hand, 75Z N connectors are much less common, but were used in CATV >>> plants, IIRC. >>> >>> -ch >>> >>> >>> >>> ___ >>> time-nuts mailing list >>> time-nuts@febo.com >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> >>> >>> >>> For those of us who strive for the ultimate in performance with GPS timing receivers it is recommended that mismatched antenna cables not be used see: http://www.bipm.fr/wg/CCTF/CGGTTS/Allowed/Recommendations/CGGTTS-Guidelines.pdf Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Didier Juges wrote: > Dr Bruce Griffiths wrote: > >> Christopher Hoover wrote: >> >> Most (except for Trimble,..) GPS receivers and antennas are designed to use 50 ohm cable. Trimble Bullet GPS antennas have a 50 ohm output impedance. Trimble literature however is ambiguous in that in the Resolution T receiver datasheets talk about using RG59 to connect to the antenna. >>> FWIW, the Thunderbolt manual says this on page 3-5: >>> >>> >>> Note - RG-59 is a 75 ohm coaxial cable. The ThunderBolt and >>> the Bullet antenna are compatible with either 50-ohm or >>> 75-ohm cable. Compared to most 50 ohm cable, 75 ohm cable >>> provides superior transmissibility for the 1.5 GHz GPS >>> signal and a better quality cable for the price. Mismatched >>> impedance is not a problem. >>> >>> >>> Note - The input impedance of the ThunderBolt RF input & >>> its antenna is 50 ohms. >>> >>> >>> >>> I would also add that BNC and N connectors come in both 50Z and 75Z flavors. >>> In fact 75Z BNC connectors are pretty common, being used in professional >>> video applications. (Check surplus BNC patch cords carefully.) On the >>> other hand, 75Z N connectors are much less common, but were used in CATV >>> plants, IIRC. >>> >>> -ch >>> >>> >>> >>> ___ >>> time-nuts mailing list >>> time-nuts@febo.com >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> >>> >>> >>> >> Impedance mismatch can be more problematic when connecting the PPS >> output of a GPS receiver to a time interval counter. >> RCA to BNC cables can catch one out if one doesn't notice that these use >> 75 ohm coax. >> >> 93 ohm RG62 cables with BNC connectors are not unknown, they were used >> in some nuclear instrumentation. >> I have a few of these lying around. >> >> Bruce >> >> > I noticed that when my counter insisted the PPS output of the > Thunderbolt was 5 Hz, even though it looked fine on the scope at > 200mS/div, until I set the scope's sweep fast enough (20nS/div or so) > and then I saw the reflections in the ~6 foot cable. In my case, it was > not an issue of 50 vs 75 ohm cable impedance, simply I had to terminate > the cable by setting the input impedance of the 5370A to 50 ohm instead > of the default 1 Mohm. I tried both 50 and 75 ohm terminations > (externally) and it did not make much difference to the counter, even > though the signal was definitely cleaner with 50 ohm (that cable was 50 > ohm). > > > Didier KO4BB > > ___ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > Didier I found, as expected when using a 75 ohm cable to connect a 220 ohm GPS PPS output to an HP5370A terminated in 50 ohms, the threshold setting is a bit more critical than when using a 50 ohm cable. Its advisable to trigger on the incident wave and not on a subsequent reflection. This is less of a problem if the PPS source has an output impedance closer to the cable impedance. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers
Dr Bruce Griffiths wrote: > Christopher Hoover wrote: > >>> Most (except for Trimble,..) GPS receivers and antennas >>> are designed to use 50 ohm cable. >>> Trimble Bullet GPS antennas have a 50 ohm output impedance. >>> Trimble literature however is ambiguous in that in the >>> Resolution T receiver datasheets talk about using RG59 >>> to connect to the antenna. >>> >>> >> FWIW, the Thunderbolt manual says this on page 3-5: >> >> >> Note - RG-59 is a 75 ohm coaxial cable. The ThunderBolt and >> the Bullet antenna are compatible with either 50-ohm or >> 75-ohm cable. Compared to most 50 ohm cable, 75 ohm cable >> provides superior transmissibility for the 1.5 GHz GPS >> signal and a better quality cable for the price. Mismatched >> impedance is not a problem. >> >> >> Note - The input impedance of the ThunderBolt RF input & >> its antenna is 50 ohms. >> >> >> >> I would also add that BNC and N connectors come in both 50Z and 75Z flavors. >> In fact 75Z BNC connectors are pretty common, being used in professional >> video applications. (Check surplus BNC patch cords carefully.) On the >> other hand, 75Z N connectors are much less common, but were used in CATV >> plants, IIRC. >> >> -ch >> >> >> >> ___ >> time-nuts mailing list >> time-nuts@febo.com >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> >> >> > Impedance mismatch can be more problematic when connecting the PPS > output of a GPS receiver to a time interval counter. > RCA to BNC cables can catch one out if one doesn't notice that these use > 75 ohm coax. > > 93 ohm RG62 cables with BNC connectors are not unknown, they were used > in some nuclear instrumentation. > I have a few of these lying around. > > Bruce > I noticed that when my counter insisted the PPS output of the Thunderbolt was 5 Hz, even though it looked fine on the scope at 200mS/div, until I set the scope's sweep fast enough (20nS/div or so) and then I saw the reflections in the ~6 foot cable. In my case, it was not an issue of 50 vs 75 ohm cable impedance, simply I had to terminate the cable by setting the input impedance of the 5370A to 50 ohm instead of the default 1 Mohm. I tried both 50 and 75 ohm terminations (externally) and it did not make much difference to the counter, even though the signal was definitely cleaner with 50 ohm (that cable was 50 ohm). Didier KO4BB ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers (was Re: ACE-III GPSreceivers (Dr Bruce Griffiths))
RG62 was also used for the interconnection between the Heathkit SB303 receiver and SB401 transmitter. I thought that was a strange choice of cables but at the time there were miles of the stuff being used for IBM 3270 terminals as noted below. -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Poul-Henning Kamp Sent: Sunday, January 28, 2007 6:20 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] 75Z vs 50Z for GPS receivers (was Re: ACE-III GPSreceivers (Dr Bruce Griffiths)) In message <[EMAIL PROTECTED]>, Dr Bruce Griffiths writes: >93 ohm RG62 cables with BNC connectors are not unknown, they were used >in some nuclear instrumentation. >I have a few of these lying around. Other uses: 93 Ohm: IBM 3270 terminal cabling 75 Ohm: Practically all telecoms 50 Ohm: Thin ethernet. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers (was Re: ACE-III GPS receivers (Dr Bruce Griffiths))
Christopher Hoover said the following on 01/28/2007 04:48 AM: > I would also add that BNC and N connectors come in both 50Z and 75Z flavors. > In fact 75Z BNC connectors are pretty common, being used in professional > video applications. (Check surplus BNC patch cords carefully.) On the > other hand, 75Z N connectors are much less common, but were used in CATV > plants, IIRC. Just as a further note, 50 and 75 ohm BNCs will interconnect (the metal parts are the same, with the impedance difference coming from the dielectric used) but 50 and 75 ohm N connectors don't -- the center pin of the 50 ohm version is thicker. If you mate a 50 ohm male with a 75 ohm female, you will damage the female. If you mate a 75 ohm male with a 50 ohm female, the connection will range from shaky to nonexistent depending on how well centered the pin is. John ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers (was Re: ACE-III GPS receivers (Dr Bruce Griffiths))
In message <[EMAIL PROTECTED]>, Dr Bruce Griffiths writes: >93 ohm RG62 cables with BNC connectors are not unknown, they were used >in some nuclear instrumentation. >I have a few of these lying around. Other uses: 93 Ohm: IBM 3270 terminal cabling 75 Ohm: Practically all telecoms 50 Ohm: Thin ethernet. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers (was Re: ACE-III GPS receivers (Dr Bruce Griffiths))
Christopher Hoover wrote: >> Most (except for Trimble,..) GPS receivers and antennas >> are designed to use 50 ohm cable. >> Trimble Bullet GPS antennas have a 50 ohm output impedance. >> Trimble literature however is ambiguous in that in the >> Resolution T receiver datasheets talk about using RG59 >> to connect to the antenna. >> > > FWIW, the Thunderbolt manual says this on page 3-5: > > > Note - RG-59 is a 75 ohm coaxial cable. The ThunderBolt and > the Bullet antenna are compatible with either 50-ohm or > 75-ohm cable. Compared to most 50 ohm cable, 75 ohm cable > provides superior transmissibility for the 1.5 GHz GPS > signal and a better quality cable for the price. Mismatched > impedance is not a problem. > > > Note - The input impedance of the ThunderBolt RF input & > its antenna is 50 ohms. > > > > I would also add that BNC and N connectors come in both 50Z and 75Z flavors. > In fact 75Z BNC connectors are pretty common, being used in professional > video applications. (Check surplus BNC patch cords carefully.) On the > other hand, 75Z N connectors are much less common, but were used in CATV > plants, IIRC. > > -ch > > > > ___ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > Impedance mismatch can be more problematic when connecting the PPS output of a GPS receiver to a time interval counter. RCA to BNC cables can catch one out if one doesn't notice that these use 75 ohm coax. 93 ohm RG62 cables with BNC connectors are not unknown, they were used in some nuclear instrumentation. I have a few of these lying around. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] 75Z vs 50Z for GPS receivers (was Re: ACE-III GPS receivers (Dr Bruce Griffiths))
> Most (except for Trimble,..) GPS receivers and antennas > are designed to use 50 ohm cable. > Trimble Bullet GPS antennas have a 50 ohm output impedance. > Trimble literature however is ambiguous in that in the > Resolution T receiver datasheets talk about using RG59 > to connect to the antenna. FWIW, the Thunderbolt manual says this on page 3-5: Note - RG-59 is a 75 ohm coaxial cable. The ThunderBolt and the Bullet antenna are compatible with either 50-ohm or 75-ohm cable. Compared to most 50 ohm cable, 75 ohm cable provides superior transmissibility for the 1.5 GHz GPS signal and a better quality cable for the price. Mismatched impedance is not a problem. Note - The input impedance of the ThunderBolt RF input & its antenna is 50 ohms. I would also add that BNC and N connectors come in both 50Z and 75Z flavors. In fact 75Z BNC connectors are pretty common, being used in professional video applications. (Check surplus BNC patch cords carefully.) On the other hand, 75Z N connectors are much less common, but were used in CATV plants, IIRC. -ch ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
