Re: [time-nuts] ThunderBolt question
RIght! (about Ligado). Well, it sounds like you might be OK with your 4 way splitter and roughly 10 dB of added attenuation. Somewhat of a point can be made in favor of using attenuation to the extent you can, just so that strong local disturbances (like those famous Arizona lightning strikes) are less likely to do damage. This depends on whether or not you want the most accurate PVT information or if you can get by with less. Enjoy! Dana On Sat, Jun 6, 2020 at 1:34 PM Wes wrote: > Dana, > > I have about 45 feet of RG6 into a two-way splitter, currently feeding two > receivers, a BG7TBL GPSDO (U-blox) and a TAPR/Synergy GPS Timing Kit > (M12+). So > each device has about 7 dB loss in front of it. As a crude test if I add > a > stepped attenuator in the line to the TAPR receiver and look at the dBc > report > in LH, a 10 dB step is hardly noticed, but a 20 dB step makes a noticeable > change. I still see tracking at 30 dB but at 40, gone. > > So there is a lot of margin but I don't see any issues and the plan is to > go to > a four-way split and add a couple of devices. I'm in a rural area outside > of > Tucson so hopefully Ligado won't be an issue. We'll have to see if their > technical expertise is as good as the PR and lobby lawyers are. > > Wes > > On 6/6/2020 8:49 AM, Dana Whitlow wrote: > > Wes, it sounds like you're in pretty good shape with one thing that > > concerns me. > > Unless your transmission line is long enough to have substantial loss, > that > > 40 dB > > gain figure seems awfully high and you may find yourself overloading the > > front > > end of your receiver. > > > > Most modern receivers are sensitive enough to do just fine with no > antenna > > LNA > > at all, in the absence of line loss. I remember reading a note from > > Garmin on this > > topic years ago when they were recommending no more than about 15 dB net > > gain > > between the antenna and the receiver. > > > > With your receiver and 40 dB antenna, even if it seems to work well now, > > beware of > > Ligado! If/when they go on the air near you, you may find yourself > needing > > an > > attenuator. > > > > Dana > > > > > > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] ThunderBolt question
Dana, I have about 45 feet of RG6 into a two-way splitter, currently feeding two receivers, a BG7TBL GPSDO (U-blox) and a TAPR/Synergy GPS Timing Kit (M12+). So each device has about 7 dB loss in front of it. As a crude test if I add a stepped attenuator in the line to the TAPR receiver and look at the dBc report in LH, a 10 dB step is hardly noticed, but a 20 dB step makes a noticeable change. I still see tracking at 30 dB but at 40, gone. So there is a lot of margin but I don't see any issues and the plan is to go to a four-way split and add a couple of devices. I'm in a rural area outside of Tucson so hopefully Ligado won't be an issue. We'll have to see if their technical expertise is as good as the PR and lobby lawyers are. Wes On 6/6/2020 8:49 AM, Dana Whitlow wrote: Wes, it sounds like you're in pretty good shape with one thing that concerns me. Unless your transmission line is long enough to have substantial loss, that 40 dB gain figure seems awfully high and you may find yourself overloading the front end of your receiver. Most modern receivers are sensitive enough to do just fine with no antenna LNA at all, in the absence of line loss. I remember reading a note from Garmin on this topic years ago when they were recommending no more than about 15 dB net gain between the antenna and the receiver. With your receiver and 40 dB antenna, even if it seems to work well now, beware of Ligado! If/when they go on the air near you, you may find yourself needing an attenuator. Dana ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] ThunderBolt question
Hi Their engineering group also backs up this statement if you ask them about it face to face. The answer that comes back is: “We ran a bunch of tests with 75 and 50 ohm cable, we could see no difference in the results”. Bob > On Jun 6, 2020, at 1:30 PM, Taka Kamiya via time-nuts > wrote: > > In case of thunderbolt, the manufacturer openly recommends use of 75 ohm > cable. The manufacturer openly states not to be concerned with a mismatch. > They are in manuals. > > > --- > (Mr.) Taka Kamiya > KB4EMF / ex JF2DKG > > > > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] ThunderBolt question
In case of thunderbolt, the manufacturer openly recommends use of 75 ohm cable. The manufacturer openly states not to be concerned with a mismatch. They are in manuals. --- (Mr.) Taka Kamiya KB4EMF / ex JF2DKG ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] ThunderBolt question
Wes, it sounds like you're in pretty good shape with one thing that concerns me. Unless your transmission line is long enough to have substantial loss, that 40 dB gain figure seems awfully high and you may find yourself overloading the front end of your receiver. Most modern receivers are sensitive enough to do just fine with no antenna LNA at all, in the absence of line loss. I remember reading a note from Garmin on this topic years ago when they were recommending no more than about 15 dB net gain between the antenna and the receiver. With your receiver and 40 dB antenna, even if it seems to work well now, beware of Ligado! If/when they go on the air near you, you may find yourself needing an attenuator. Dana On Sat, Jun 6, 2020 at 9:11 AM Wes wrote: > Conventional wisdom says yes to this. I'm not so sure, if the cable is > well-made, but some of the stuff is really garbage. > > I'm using RG-6 cable purchased at DX Engineering, mainly because I had a > gift > certificate for my last order. It's quality stuff, but if you're going to > use F > connectors be sure they will work with quad-shielded. I bought some > (blue) that > were claimed to work and there was no way in hell that I could get them > on. I > had to buy another batch (purple) that do work. > > Regarding mismatches; my recently installed antenna is a PCTEL > GPSL1-TMG-SPI-40NCB. This is advertised as a "timing" antenna, so I > assume > they're worried about timing. The data sheet says the "nominal impedance" > is 50 > ohm. Butthe VSWR spec is <2.0:1. Of course this antenna has an > internal > amplifier (40 dB), whose match to the helix antenna is unknown but I'm > guessing > it's for NF optimization. So is the spec the nominal output Z (s21) of > the > internal amplifier or is it the load the amplifier needs to be presented > with to > deliver advertised gain? Who knows? Either way, my 75 ohm coax is within > limits. > > Wes N7WS > > > > > > On 6/5/2020 4:56 PM, DON MURRAY via time-nuts wrote: > > Robert... > > > > Try to find QUAD SHIELD RG-6. > > > > > > 73 > > Don > > W4WJ > > > > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] ThunderBolt question
As the chip is a message, not a time hack per se, all the delay can do is make it more difficult to decode the message. That's how these COFDM-type schemes work. It's why your digital TV signal (which in the US is not a COFDM signal, but still behaves similarly) falls off a cliff when things get bad rather than fade away into noise like analog does. With analog, you get ghosts, but in digital, you just get garbage which may or may not be decode-able into a picture and sound. Tom Holmes, N8ZM -Original Message- From: time-nuts On Behalf Of Dana Whitlow Sent: Saturday, June 06, 2020 9:54 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] ThunderBolt question I wish I could give some numbers. Sadly, I don't know how to calculate them nor do I have the equipment to do empirical measurements of that level of accuracy. But I could do an arm-waving estimate. Suppose that the delayed signal component is in phase with the direct signal, but 20 dB down, and is delayed by 100 nsec. Then I could see how the apparent centroid of a given chip could be displaced by about 10 nsec, which would be enough to cause complaint from many time-nuts. Personally I'm not into time keeping of that order of magnitude, but I'd like my GPS- disciplined oscillator to be phase stable to within 10's of ps over about a 1 minute time frame. At present I cannot seem to do this well unless I let my Rb free-run. Dana On Sat, Jun 6, 2020 at 8:43 AM Tom Holmes wrote: > No caveats required. > > Give some numbers on how loud that MP signal has to be to cause a problem. > My original example of line losses for a relatively short cable still > suggests to me that it is a minimal problem. If the S/N of a satellite is > poor, the receiver algorithm is likely to not use it if there are better > choices, and there often will be. > > Tom Holmes, N8ZM > > -Original Message- > From: time-nuts On Behalf Of Dana > Whitlow > Sent: Saturday, June 06, 2020 8:52 AM > To: Discussion of precise time and frequency measurement < > time-nuts@lists.febo.com> > Subject: Re: [time-nuts] ThunderBolt question > > Basically true, Tom, but with some caveats: > > The GPS system was originally designed, as you say, to provide > positions and velocity > information to military users in the field. Errors of a few meters were of > little consequence. > > But now, we have the time-nuts, surveyors, etc, who are decidedly pushing > the envelope > of GPS accuracy. Some users want sub-nanosecond timing accuracy, surveyors > want > mm-level position accuracy, etc, and doing these things indeed requires > extreme attention > to detail. > > As I understand it, multipath components that are delayed by about a few > chips or more > basically appear as a wee bit of added noise and have negligible > consequence unless > they are comparable to the direct signal in power. However, MP components > whose > delay is less than about a chip time have the effect of tugging the > apparent temporal > location of the direct signal to some new value. Note that this is in the > range of practical > round trip cable delays in at least some installations. At Arecibo, for > example, the one-way > cable lengths from GPS antennas on the roof to their respective receivers > was about > 200 ft. Round trip was thus 400 ft physical, hence up to about 600 ft > depending on > what the cables were using for the dielectric. That's about 0.6 chip > length for the C/A > code GPS signal. > > Another factor which will influence the error introduced will be the RF > phase relationship > between the direct and delayed signal component. > > Gee, this stuff gets complicated ... > > Dana > > > On Sat, Jun 6, 2020 at 7:30 AM Tom Holmes wrote: > > > Dana... > > The question that comes to mind is just how much effect a weak a long > > delayed reflection will have on overall system performance since it will > > only matter to SV’s with poor S/N. The modulation scene which allows all > > the SV's to transmit on the same frequency has to be pretty robust in the > > face of both widely varying signal strengths and multiple signals > arriving > > at different times. It’s a similar scheme to CDMA cell phones, which > > operate in a much more difficult environment with regard to signal > > strengths, multi-path, and number of on channel signals. And those work > > amazingly well. > > > > Further, I am led to believe that once you have enough SV’s in view to > > get a good set of ‘readings’, ionospheric effects are the limiting factor > > until you go to a multi-band receiver. > > > > Yes, to wring the last ounce of performance out of GPS takes a
Re: [time-nuts] ThunderBolt question
Hi A “cable echo” looks like any other multi-path signal. Your local environment provides *lots* of those all the time. That's regardless of how hard you try to get rid of them. If they are *stronger* than the main signal (which they can be in a “bounce off a building" case) they really mess you up. If they are weaker … not so much. 10 db is a long ways down in this case. Why? The receivers don’t do a simple average of this and that. If they did, indeed the result would be pretty awful. Instead they work very hard to pick out each “echo” that arrives and distinguish the time it arrives (and it’s amplitude) compared to the other echos. How they do this is a bit of a “that depends”. Think of it as putting signals from each time period in a bucket and then seeing which bucket is fullest. Obviously, faster processing can give you smaller buckets …. Since it is very design dependent, doing an exact calculation gets pretty deep into the vendor’s IP built into the device. Getting access to that may not be easy. Bob > On Jun 6, 2020, at 9:54 AM, Dana Whitlow wrote: > > I wish I could give some numbers. Sadly, I don't know how to calculate > them nor do I > have the equipment to do empirical measurements of that level of accuracy. > > But I could do an arm-waving estimate. Suppose that the delayed signal > component > is in phase with the direct signal, but 20 dB down, and is delayed by 100 > nsec. Then > I could see how the apparent centroid of a given chip could be displaced by > about > 10 nsec, which would be enough to cause complaint from many time-nuts. > > Personally I'm not into time keeping of that order of magnitude, but I'd > like my GPS- > disciplined oscillator to be phase stable to within 10's of ps over about a > 1 minute > time frame. At present I cannot seem to do this well unless I let my Rb > free-run. > > Dana > > > On Sat, Jun 6, 2020 at 8:43 AM Tom Holmes wrote: > >> No caveats required. >> >> Give some numbers on how loud that MP signal has to be to cause a problem. >> My original example of line losses for a relatively short cable still >> suggests to me that it is a minimal problem. If the S/N of a satellite is >> poor, the receiver algorithm is likely to not use it if there are better >> choices, and there often will be. >> >> Tom Holmes, N8ZM >> >> -Original Message- >> From: time-nuts On Behalf Of Dana >> Whitlow >> Sent: Saturday, June 06, 2020 8:52 AM >> To: Discussion of precise time and frequency measurement < >> time-nuts@lists.febo.com> >> Subject: Re: [time-nuts] ThunderBolt question >> >> Basically true, Tom, but with some caveats: >> >> The GPS system was originally designed, as you say, to provide >> positions and velocity >> information to military users in the field. Errors of a few meters were of >> little consequence. >> >> But now, we have the time-nuts, surveyors, etc, who are decidedly pushing >> the envelope >> of GPS accuracy. Some users want sub-nanosecond timing accuracy, surveyors >> want >> mm-level position accuracy, etc, and doing these things indeed requires >> extreme attention >> to detail. >> >> As I understand it, multipath components that are delayed by about a few >> chips or more >> basically appear as a wee bit of added noise and have negligible >> consequence unless >> they are comparable to the direct signal in power. However, MP components >> whose >> delay is less than about a chip time have the effect of tugging the >> apparent temporal >> location of the direct signal to some new value. Note that this is in the >> range of practical >> round trip cable delays in at least some installations. At Arecibo, for >> example, the one-way >> cable lengths from GPS antennas on the roof to their respective receivers >> was about >> 200 ft. Round trip was thus 400 ft physical, hence up to about 600 ft >> depending on >> what the cables were using for the dielectric. That's about 0.6 chip >> length for the C/A >> code GPS signal. >> >> Another factor which will influence the error introduced will be the RF >> phase relationship >> between the direct and delayed signal component. >> >> Gee, this stuff gets complicated ... >> >> Dana >> >> >> On Sat, Jun 6, 2020 at 7:30 AM Tom Holmes wrote: >> >>> Dana... >>> The question that comes to mind is just how much effect a weak a long >>> delayed reflection will have on overall system performance since it will >>> only matter to SV’s with poor S/N. The m
Re: [time-nuts] ThunderBolt question
Conventional wisdom says yes to this. I'm not so sure, if the cable is well-made, but some of the stuff is really garbage. I'm using RG-6 cable purchased at DX Engineering, mainly because I had a gift certificate for my last order. It's quality stuff, but if you're going to use F connectors be sure they will work with quad-shielded. I bought some (blue) that were claimed to work and there was no way in hell that I could get them on. I had to buy another batch (purple) that do work. Regarding mismatches; my recently installed antenna is a PCTEL GPSL1-TMG-SPI-40NCB. This is advertised as a "timing" antenna, so I assume they're worried about timing. The data sheet says the "nominal impedance" is 50 ohm. Butthe VSWR spec is <2.0:1. Of course this antenna has an internal amplifier (40 dB), whose match to the helix antenna is unknown but I'm guessing it's for NF optimization. So is the spec the nominal output Z (s21) of the internal amplifier or is it the load the amplifier needs to be presented with to deliver advertised gain? Who knows? Either way, my 75 ohm coax is within limits. Wes N7WS On 6/5/2020 4:56 PM, DON MURRAY via time-nuts wrote: Robert... Try to find QUAD SHIELD RG-6. 73 Don W4WJ ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] ThunderBolt question
I wish I could give some numbers. Sadly, I don't know how to calculate them nor do I have the equipment to do empirical measurements of that level of accuracy. But I could do an arm-waving estimate. Suppose that the delayed signal component is in phase with the direct signal, but 20 dB down, and is delayed by 100 nsec. Then I could see how the apparent centroid of a given chip could be displaced by about 10 nsec, which would be enough to cause complaint from many time-nuts. Personally I'm not into time keeping of that order of magnitude, but I'd like my GPS- disciplined oscillator to be phase stable to within 10's of ps over about a 1 minute time frame. At present I cannot seem to do this well unless I let my Rb free-run. Dana On Sat, Jun 6, 2020 at 8:43 AM Tom Holmes wrote: > No caveats required. > > Give some numbers on how loud that MP signal has to be to cause a problem. > My original example of line losses for a relatively short cable still > suggests to me that it is a minimal problem. If the S/N of a satellite is > poor, the receiver algorithm is likely to not use it if there are better > choices, and there often will be. > > Tom Holmes, N8ZM > > -Original Message- > From: time-nuts On Behalf Of Dana > Whitlow > Sent: Saturday, June 06, 2020 8:52 AM > To: Discussion of precise time and frequency measurement < > time-nuts@lists.febo.com> > Subject: Re: [time-nuts] ThunderBolt question > > Basically true, Tom, but with some caveats: > > The GPS system was originally designed, as you say, to provide > positions and velocity > information to military users in the field. Errors of a few meters were of > little consequence. > > But now, we have the time-nuts, surveyors, etc, who are decidedly pushing > the envelope > of GPS accuracy. Some users want sub-nanosecond timing accuracy, surveyors > want > mm-level position accuracy, etc, and doing these things indeed requires > extreme attention > to detail. > > As I understand it, multipath components that are delayed by about a few > chips or more > basically appear as a wee bit of added noise and have negligible > consequence unless > they are comparable to the direct signal in power. However, MP components > whose > delay is less than about a chip time have the effect of tugging the > apparent temporal > location of the direct signal to some new value. Note that this is in the > range of practical > round trip cable delays in at least some installations. At Arecibo, for > example, the one-way > cable lengths from GPS antennas on the roof to their respective receivers > was about > 200 ft. Round trip was thus 400 ft physical, hence up to about 600 ft > depending on > what the cables were using for the dielectric. That's about 0.6 chip > length for the C/A > code GPS signal. > > Another factor which will influence the error introduced will be the RF > phase relationship > between the direct and delayed signal component. > > Gee, this stuff gets complicated ... > > Dana > > > On Sat, Jun 6, 2020 at 7:30 AM Tom Holmes wrote: > > > Dana... > > The question that comes to mind is just how much effect a weak a long > > delayed reflection will have on overall system performance since it will > > only matter to SV’s with poor S/N. The modulation scene which allows all > > the SV's to transmit on the same frequency has to be pretty robust in the > > face of both widely varying signal strengths and multiple signals > arriving > > at different times. It’s a similar scheme to CDMA cell phones, which > > operate in a much more difficult environment with regard to signal > > strengths, multi-path, and number of on channel signals. And those work > > amazingly well. > > > > Further, I am led to believe that once you have enough SV’s in view to > > get a good set of ‘readings’, ionospheric effects are the limiting factor > > until you go to a multi-band receiver. > > > > Yes, to wring the last ounce of performance out of GPS takes attention to > > the details, but don’t lose sight of how it was designed to work for > users > > in less than optimum (military field operations) in the first place. > > From Tom Holmes, N8ZM > > > > > On Jun 6, 2020, at 7:14 AM, Dana Whitlow > wrote: > > > > > > It's one thing to maintain lock in a multipath environment, quite > > another > > > thing > > > to get "full" accuracy of GPS measurements of PVT. > > > > > > An interesting difference between my scenario of poorly matched > > impedances > > > and "ordinary" multipath is this: In the poor matching scenario, all > the >
Re: [time-nuts] ThunderBolt question
No caveats required. Give some numbers on how loud that MP signal has to be to cause a problem. My original example of line losses for a relatively short cable still suggests to me that it is a minimal problem. If the S/N of a satellite is poor, the receiver algorithm is likely to not use it if there are better choices, and there often will be. Tom Holmes, N8ZM -Original Message- From: time-nuts On Behalf Of Dana Whitlow Sent: Saturday, June 06, 2020 8:52 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] ThunderBolt question Basically true, Tom, but with some caveats: The GPS system was originally designed, as you say, to provide positions and velocity information to military users in the field. Errors of a few meters were of little consequence. But now, we have the time-nuts, surveyors, etc, who are decidedly pushing the envelope of GPS accuracy. Some users want sub-nanosecond timing accuracy, surveyors want mm-level position accuracy, etc, and doing these things indeed requires extreme attention to detail. As I understand it, multipath components that are delayed by about a few chips or more basically appear as a wee bit of added noise and have negligible consequence unless they are comparable to the direct signal in power. However, MP components whose delay is less than about a chip time have the effect of tugging the apparent temporal location of the direct signal to some new value. Note that this is in the range of practical round trip cable delays in at least some installations. At Arecibo, for example, the one-way cable lengths from GPS antennas on the roof to their respective receivers was about 200 ft. Round trip was thus 400 ft physical, hence up to about 600 ft depending on what the cables were using for the dielectric. That's about 0.6 chip length for the C/A code GPS signal. Another factor which will influence the error introduced will be the RF phase relationship between the direct and delayed signal component. Gee, this stuff gets complicated ... Dana On Sat, Jun 6, 2020 at 7:30 AM Tom Holmes wrote: > Dana... > The question that comes to mind is just how much effect a weak a long > delayed reflection will have on overall system performance since it will > only matter to SV’s with poor S/N. The modulation scene which allows all > the SV's to transmit on the same frequency has to be pretty robust in the > face of both widely varying signal strengths and multiple signals arriving > at different times. It’s a similar scheme to CDMA cell phones, which > operate in a much more difficult environment with regard to signal > strengths, multi-path, and number of on channel signals. And those work > amazingly well. > > Further, I am led to believe that once you have enough SV’s in view to > get a good set of ‘readings’, ionospheric effects are the limiting factor > until you go to a multi-band receiver. > > Yes, to wring the last ounce of performance out of GPS takes attention to > the details, but don’t lose sight of how it was designed to work for users > in less than optimum (military field operations) in the first place. > From Tom Holmes, N8ZM > > > On Jun 6, 2020, at 7:14 AM, Dana Whitlow wrote: > > > > It's one thing to maintain lock in a multipath environment, quite > another > > thing > > to get "full" accuracy of GPS measurements of PVT. > > > > An interesting difference between my scenario of poorly matched > impedances > > and "ordinary" multipath is this: In the poor matching scenario, all the > > received > > signals will be impaired identically, while in the ordinary multipath > > scenario, > > signals from different satellites will suffer different (and > time-varying) > > multipath > > impairments. I'm not at all sure what effect this difference will have > on > > final > > outcome, but my gut feel is that the case where all signals are impaired > > identically > > could lead to worse effects. > > > > Dana > > > > > >> On Fri, Jun 5, 2020 at 8:43 PM Tom Holmes wrote: > >> > >> Dana... > >> > >> I think that you are neglecting two important mitigating factors. > >> > >> 1. the cable loss at 1575MHz, even for a 25' run of RG-6, reduces those > >> reflections quite a lot from one end to the other. It amounts to 2 > - 3 > >> dB in 25', depending on cable quality. > >> > >> 2. a 1.5:1 SWR is not a very big reflection to begin with, on the order > of > >> 20% of the incident power, about 7 dB. I am rounding a lot here just to > >> keep the math easy...for me. > >> > >> By the time a reflection has made the round trip from the receiver back > to &g
Re: [time-nuts] ThunderBolt question
big reflection to begin with, on the order >> of >>>> 20% of the incident power, about 7 dB. I am rounding a lot here just to >>>> keep the math easy...for me. >>>> >>>> By the time a reflection has made the round trip from the receiver back >> to >>>> the antenna and them back to the receiver, which is how the delay would >>>> have >>>> to manifest itself, it will be down at least 15 dB from its original >> self, >>>> and probably more. Given the coding of GPS signals which allows several >>>> satellites to share a common frequency band, that is not going to be >> much >>>> of >>>> a problem. And if only one end of the path actually is 75 ohms, then >> there >>>> won't be a delayed signal. >>>> >>>> Tom Holmes, N8ZM >>>> >>>> -Original Message- >>>> From: time-nuts On Behalf Of Dana >>>> Whitlow >>>> Sent: Friday, June 05, 2020 9:01 PM >>>> To: Taka Kamiya ; Discussion of precise time and >>>> frequency measurement >>>> Subject: Re: [time-nuts] ThunderBolt question >>>> >>>> I'd like to point out that mismatches at the ends of an antenna cable >> *can* >>>> cause trouble. When both ends are mismatched, each bit of detail in the >>>> signal >>>> gets partially reflected back and forth, each time delayed by the round >>>> trip propagation >>>> delay in the cable, and so you have something like multipath going on. >>>> Fortunately the >>>> successive reflections get weaker with time, generally quite rapidly. >>>> Since many >>>> GPS users seem very concerned about multipath resulting from poor >> antenna >>>> placement, >>>> I think this factor should be considered as well and not just get swept >>>> under the rug. >>>> >>>> The amplitude of the "multipath" resulting from cable mismatches >> depends on >>>> the product >>>> of the voltage reflection coefficients at the two ends of the cable. If >>>> either end is perfectly >>>> matched, then the quality of the match at the other end is not >> significant >>>> vis-a-vis apparent >>>> multipath problems and only affects transmission loss. >>>> >>>> But when there is a mismatch on both ends, then the length of the cable >>>> comes into play >>>> as well. A longer cable means more delay between successive >> reflections, >>>> which is just >>>> like multipath involving longer delays between the direct and the >> reflected >>>> signals. >>>> >>>> Cheers, >>>> >>>> Dana (K8YUM) >>>> >>>> On Fri, Jun 5, 2020 at 7:13 PM Taka Kamiya via time-nuts < >>>> time-nuts@lists.febo.com> wrote: >>>> >>>>> 50 ohm / 75 ohm question is really irrelevant in this kind of thing. >>>>> Trmble itself says in manual, not to be concerned with this apparent >>>>> mismatch. >>>>> In my particular case, I have a home lab standard and existing >> system. I >>>>> have an antenna and network of distribution amplifiers. They are all >> 50 >>>>> ohms and N connectors. Some ports have BNC adapters attached. I have >>>>> pretty much standardized everything to SMA, N, or BNC. >>>>> >>>>> I boxed a power supply, T-bolt, and buffer amp in a metal case. I >> bought >>>>> a short cable (RG58) that goes from F to BNC. On back of the case, I >>>> have >>>>> BNC to N adapter. I also have a few adapters that goes from F to BNC >> for >>>>> the test bench. It really doesn't matter what you use, as long as it >>>> makes >>>>> a solid connection. >>>>> >>>>> Advantage of F connectors and RG6 are, cheap, abundant, and low loss >> for >>>>> the size. Advantage of having house standard is, less adapters and >> less >>>>> headache. >>>>> >>>>> --- >>>>> (Mr.) Taka Kamiya >>>>> KB4EMF / ex JF2DKG >>>>> >>>>> >>>>> On Friday, June 5, 2020, 7:22:33 PM EDT, Robert DiRosario < >>>>> ka3...@comcast.net> wrote: >>&g
Re: [time-nuts] ThunderBolt question
Basically true, Tom, but with some caveats: The GPS system was originally designed, as you say, to provide positions and velocity information to military users in the field. Errors of a few meters were of little consequence. But now, we have the time-nuts, surveyors, etc, who are decidedly pushing the envelope of GPS accuracy. Some users want sub-nanosecond timing accuracy, surveyors want mm-level position accuracy, etc, and doing these things indeed requires extreme attention to detail. As I understand it, multipath components that are delayed by about a few chips or more basically appear as a wee bit of added noise and have negligible consequence unless they are comparable to the direct signal in power. However, MP components whose delay is less than about a chip time have the effect of tugging the apparent temporal location of the direct signal to some new value. Note that this is in the range of practical round trip cable delays in at least some installations. At Arecibo, for example, the one-way cable lengths from GPS antennas on the roof to their respective receivers was about 200 ft. Round trip was thus 400 ft physical, hence up to about 600 ft depending on what the cables were using for the dielectric. That's about 0.6 chip length for the C/A code GPS signal. Another factor which will influence the error introduced will be the RF phase relationship between the direct and delayed signal component. Gee, this stuff gets complicated ... Dana On Sat, Jun 6, 2020 at 7:30 AM Tom Holmes wrote: > Dana... > The question that comes to mind is just how much effect a weak a long > delayed reflection will have on overall system performance since it will > only matter to SV’s with poor S/N. The modulation scene which allows all > the SV's to transmit on the same frequency has to be pretty robust in the > face of both widely varying signal strengths and multiple signals arriving > at different times. It’s a similar scheme to CDMA cell phones, which > operate in a much more difficult environment with regard to signal > strengths, multi-path, and number of on channel signals. And those work > amazingly well. > > Further, I am led to believe that once you have enough SV’s in view to > get a good set of ‘readings’, ionospheric effects are the limiting factor > until you go to a multi-band receiver. > > Yes, to wring the last ounce of performance out of GPS takes attention to > the details, but don’t lose sight of how it was designed to work for users > in less than optimum (military field operations) in the first place. > From Tom Holmes, N8ZM > > > On Jun 6, 2020, at 7:14 AM, Dana Whitlow wrote: > > > > It's one thing to maintain lock in a multipath environment, quite > another > > thing > > to get "full" accuracy of GPS measurements of PVT. > > > > An interesting difference between my scenario of poorly matched > impedances > > and "ordinary" multipath is this: In the poor matching scenario, all the > > received > > signals will be impaired identically, while in the ordinary multipath > > scenario, > > signals from different satellites will suffer different (and > time-varying) > > multipath > > impairments. I'm not at all sure what effect this difference will have > on > > final > > outcome, but my gut feel is that the case where all signals are impaired > > identically > > could lead to worse effects. > > > > Dana > > > > > >> On Fri, Jun 5, 2020 at 8:43 PM Tom Holmes wrote: > >> > >> Dana... > >> > >> I think that you are neglecting two important mitigating factors. > >> > >> 1. the cable loss at 1575MHz, even for a 25' run of RG-6, reduces those > >> reflections quite a lot from one end to the other. It amounts to 2 > - 3 > >> dB in 25', depending on cable quality. > >> > >> 2. a 1.5:1 SWR is not a very big reflection to begin with, on the order > of > >> 20% of the incident power, about 7 dB. I am rounding a lot here just to > >> keep the math easy...for me. > >> > >> By the time a reflection has made the round trip from the receiver back > to > >> the antenna and them back to the receiver, which is how the delay would > >> have > >> to manifest itself, it will be down at least 15 dB from its original > self, > >> and probably more. Given the coding of GPS signals which allows several > >> satellites to share a common frequency band, that is not going to be > much > >> of > >> a problem. And if only one end of the path actually is 75 ohms, then > there > >> won't be a delayed signal. > >> > >> Tom Holmes, N8ZM > >
Re: [time-nuts] ThunderBolt question
Dana... The question that comes to mind is just how much effect a weak a long delayed reflection will have on overall system performance since it will only matter to SV’s with poor S/N. The modulation scene which allows all the SV's to transmit on the same frequency has to be pretty robust in the face of both widely varying signal strengths and multiple signals arriving at different times. It’s a similar scheme to CDMA cell phones, which operate in a much more difficult environment with regard to signal strengths, multi-path, and number of on channel signals. And those work amazingly well. Further, I am led to believe that once you have enough SV’s in view to get a good set of ‘readings’, ionospheric effects are the limiting factor until you go to a multi-band receiver. Yes, to wring the last ounce of performance out of GPS takes attention to the details, but don’t lose sight of how it was designed to work for users in less than optimum (military field operations) in the first place. From Tom Holmes, N8ZM > On Jun 6, 2020, at 7:14 AM, Dana Whitlow wrote: > > It's one thing to maintain lock in a multipath environment, quite another > thing > to get "full" accuracy of GPS measurements of PVT. > > An interesting difference between my scenario of poorly matched impedances > and "ordinary" multipath is this: In the poor matching scenario, all the > received > signals will be impaired identically, while in the ordinary multipath > scenario, > signals from different satellites will suffer different (and time-varying) > multipath > impairments. I'm not at all sure what effect this difference will have on > final > outcome, but my gut feel is that the case where all signals are impaired > identically > could lead to worse effects. > > Dana > > >> On Fri, Jun 5, 2020 at 8:43 PM Tom Holmes wrote: >> >> Dana... >> >> I think that you are neglecting two important mitigating factors. >> >> 1. the cable loss at 1575MHz, even for a 25' run of RG-6, reduces those >> reflections quite a lot from one end to the other. It amounts to 2 - 3 >> dB in 25', depending on cable quality. >> >> 2. a 1.5:1 SWR is not a very big reflection to begin with, on the order of >> 20% of the incident power, about 7 dB. I am rounding a lot here just to >> keep the math easy...for me. >> >> By the time a reflection has made the round trip from the receiver back to >> the antenna and them back to the receiver, which is how the delay would >> have >> to manifest itself, it will be down at least 15 dB from its original self, >> and probably more. Given the coding of GPS signals which allows several >> satellites to share a common frequency band, that is not going to be much >> of >> a problem. And if only one end of the path actually is 75 ohms, then there >> won't be a delayed signal. >> >> Tom Holmes, N8ZM >> >> -Original Message- >> From: time-nuts On Behalf Of Dana >> Whitlow >> Sent: Friday, June 05, 2020 9:01 PM >> To: Taka Kamiya ; Discussion of precise time and >> frequency measurement >> Subject: Re: [time-nuts] ThunderBolt question >> >> I'd like to point out that mismatches at the ends of an antenna cable *can* >> cause trouble. When both ends are mismatched, each bit of detail in the >> signal >> gets partially reflected back and forth, each time delayed by the round >> trip propagation >> delay in the cable, and so you have something like multipath going on. >> Fortunately the >> successive reflections get weaker with time, generally quite rapidly. >> Since many >> GPS users seem very concerned about multipath resulting from poor antenna >> placement, >> I think this factor should be considered as well and not just get swept >> under the rug. >> >> The amplitude of the "multipath" resulting from cable mismatches depends on >> the product >> of the voltage reflection coefficients at the two ends of the cable. If >> either end is perfectly >> matched, then the quality of the match at the other end is not significant >> vis-a-vis apparent >> multipath problems and only affects transmission loss. >> >> But when there is a mismatch on both ends, then the length of the cable >> comes into play >> as well. A longer cable means more delay between successive reflections, >> which is just >> like multipath involving longer delays between the direct and the reflected >> signals. >> >> Cheers, >> >> Dana (K8YUM) >> >> On Fri, Jun 5, 2020 at 7:13 PM Taka Kamiya via time-nuts < >&g
Re: [time-nuts] ThunderBolt question
I must take issue with John's statement that "impedance matching is easy". It's easy only in the case of very limited bandwidth and if you are willing to ignore such issues as getting good IMD performance, good noise performance, and good broadband AC stability. The last item is critically important because in many situations an amplifier is driving (or being driven from) a filter whose out-of-band port impedance is nothing remotely like the intended value within the passband, being highly reflective out of band. And the idea of just adding a matched termination across the input of a high- Z buffer automatically adds noise, and is impractical at high frequencies because at such frequencies there is no such thing as a high input impedance except perhaps over a very small fractional bandwidth. These factors (along with others) help to explain why good RF amplifier designers (which I do *not* claim to be) get paid the big bucks. And why, despite their best efforts, good input and output impedance matching of amplifiers remains an elusive goal. Dana On Sat, Jun 6, 2020 at 2:12 AM John Moran, Scawby Design < j...@scawbydesign.co.uk> wrote: > Everyone ... > > I must admit to being amazed at the cavalier attitude to impedance > matching. I dread to think what a state we would be in if the original > telecoms networks were designed with such disregard. > > OK, my background is in the old telecoms - land-line stuff where we had a > variety of impedances to work with, balanced and unbalanced, but mainly 600 > ohms for the audio and line side and 50 or 75 ohms for the internal stuff. > But whatever we were working with, designing to match the impedance closely > was a critical parameter, and not difficult at all. Regarding connectors, > you could mix and match types as long as everything you used was designed > to match the same impedance and terminate the cable properly. > > When you are designing amplifiers to be flat within 0.1dB over a wide > bandwidth, impedance matching matters both for steady-state amplitude > settings and ringing caused by the reflections. > > There is a whole discipline around transmission lines going back nearly > 200 years, for a reason. > > OK, TimeNuts tend to be piping single frequencies around the place, but I > thought this was a place looking for precision, and playing with low-level > signals, and hunting down esoteric artefacts and anomalies. Wading > roughshod through transmission theory is at odds with that for me, sorry. > > It's not as though designing stuff to have the right input and output > impedances is difficult. Nowadays with integrated amplifiers you can just > use the brute-force method of hanging a 50 ohm resistor across a > 'high-impedance' input and another 50 ohm resistor in series with a 'zero > ohm' output impedance. Back in the day we designed them to inherently have > the right input and output impedance and so saved throwing lots of signal > away. > > My humble apologies for the rant ... but I just couldn't believe what I > was reading this morning when I opened the mail. > > I guess I'll get thrown out for this ... > > John > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] ThunderBolt question
It's one thing to maintain lock in a multipath environment, quite another thing to get "full" accuracy of GPS measurements of PVT. An interesting difference between my scenario of poorly matched impedances and "ordinary" multipath is this: In the poor matching scenario, all the received signals will be impaired identically, while in the ordinary multipath scenario, signals from different satellites will suffer different (and time-varying) multipath impairments. I'm not at all sure what effect this difference will have on final outcome, but my gut feel is that the case where all signals are impaired identically could lead to worse effects. Dana On Fri, Jun 5, 2020 at 8:43 PM Tom Holmes wrote: > Dana... > > I think that you are neglecting two important mitigating factors. > > 1. the cable loss at 1575MHz, even for a 25' run of RG-6, reduces those > reflections quite a lot from one end to the other. It amounts to 2 - 3 > dB in 25', depending on cable quality. > > 2. a 1.5:1 SWR is not a very big reflection to begin with, on the order of > 20% of the incident power, about 7 dB. I am rounding a lot here just to > keep the math easy...for me. > > By the time a reflection has made the round trip from the receiver back to > the antenna and them back to the receiver, which is how the delay would > have > to manifest itself, it will be down at least 15 dB from its original self, > and probably more. Given the coding of GPS signals which allows several > satellites to share a common frequency band, that is not going to be much > of > a problem. And if only one end of the path actually is 75 ohms, then there > won't be a delayed signal. > > Tom Holmes, N8ZM > > -Original Message- > From: time-nuts On Behalf Of Dana > Whitlow > Sent: Friday, June 05, 2020 9:01 PM > To: Taka Kamiya ; Discussion of precise time and > frequency measurement > Subject: Re: [time-nuts] ThunderBolt question > > I'd like to point out that mismatches at the ends of an antenna cable *can* > cause trouble. When both ends are mismatched, each bit of detail in the > signal > gets partially reflected back and forth, each time delayed by the round > trip propagation > delay in the cable, and so you have something like multipath going on. > Fortunately the > successive reflections get weaker with time, generally quite rapidly. > Since many > GPS users seem very concerned about multipath resulting from poor antenna > placement, > I think this factor should be considered as well and not just get swept > under the rug. > > The amplitude of the "multipath" resulting from cable mismatches depends on > the product > of the voltage reflection coefficients at the two ends of the cable. If > either end is perfectly > matched, then the quality of the match at the other end is not significant > vis-a-vis apparent > multipath problems and only affects transmission loss. > > But when there is a mismatch on both ends, then the length of the cable > comes into play > as well. A longer cable means more delay between successive reflections, > which is just > like multipath involving longer delays between the direct and the reflected > signals. > > Cheers, > > Dana (K8YUM) > > On Fri, Jun 5, 2020 at 7:13 PM Taka Kamiya via time-nuts < > time-nuts@lists.febo.com> wrote: > > > 50 ohm / 75 ohm question is really irrelevant in this kind of thing. > > Trmble itself says in manual, not to be concerned with this apparent > > mismatch. > > In my particular case, I have a home lab standard and existing system. I > > have an antenna and network of distribution amplifiers. They are all 50 > > ohms and N connectors. Some ports have BNC adapters attached. I have > > pretty much standardized everything to SMA, N, or BNC. > > > > I boxed a power supply, T-bolt, and buffer amp in a metal case. I bought > > a short cable (RG58) that goes from F to BNC. On back of the case, I > have > > BNC to N adapter. I also have a few adapters that goes from F to BNC for > > the test bench. It really doesn't matter what you use, as long as it > makes > > a solid connection. > > > > Advantage of F connectors and RG6 are, cheap, abundant, and low loss for > > the size. Advantage of having house standard is, less adapters and less > > headache. > > > > --- > > (Mr.) Taka Kamiya > > KB4EMF / ex JF2DKG > > > > > > On Friday, June 5, 2020, 7:22:33 PM EDT, Robert DiRosario < > > ka3...@comcast.net> wrote: > > > > I have a Trimble ThunderBolt GPSDO that I just received. It has an F > > connector for the antenn
Re: [time-nuts] ThunderBolt question
The original question was need for impedance matching between gps antenna and Tbolt. The responses were accurate and appropriate. 50 Ohm vs 75 Ohm is a nonissue. On Sat, 6 Jun 2020 07:11:49 +, John Moran, Scawby Design wrote: >Everyone ... >I must admit to being amazed at the cavalier attitude to impedance matching. I >dread to think what a state we would be in if the original telecoms networks were designed with such disregard. >OK, my background is in the old telecoms - land-line stuff where we had a >variety of impedances to work with, balanced and unbalanced, but mainly 600 ohms for the audio and line side and 50 or 75 ohms for the internal stuff. But whatever we were working with, designing to match the impedance closely was a critical parameter, and not difficult at all. Regarding connectors, you could mix and match types as long as everything you used was designed to match the same impedance and terminate the cable properly. >When you are designing amplifiers to be flat within 0.1dB over a wide >bandwidth, impedance matching matters both for steady-state amplitude settings and ringing caused by the reflections. >There is a whole discipline around transmission lines going back nearly 200 >years, for a reason. 200 years predates invention of the telegraph and the Edison recorder. I am sure that impedance matching was not an issue in that era. >OK, TimeNuts tend to be piping single frequencies around the place, but I >thought this was a place looking for precision, and playing with low-level signals, and hunting down esoteric artefacts and anomalies. Wading roughshod through transmission theory is at odds with that for me, sorry. >It's not as though designing stuff to have the right input and output >impedances is difficult. Nowadays with integrated amplifiers you can just use the brute-force method of hanging a 50 ohm resistor across a 'high-impedance' input and another 50 ohm resistor in series with a 'zero ohm' output impedance. Back in the day we designed them to inherently have the right input and output impedance and so saved throwing lots of signal away. >My humble apologies for the rant ... but I just couldn't believe what I was >reading this morning when I opened the mail. >I guess I'll get thrown out for this ... >John Regards, Bill Beam NL7F ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] ThunderBolt question
Everyone ... I must admit to being amazed at the cavalier attitude to impedance matching. I dread to think what a state we would be in if the original telecoms networks were designed with such disregard. OK, my background is in the old telecoms - land-line stuff where we had a variety of impedances to work with, balanced and unbalanced, but mainly 600 ohms for the audio and line side and 50 or 75 ohms for the internal stuff. But whatever we were working with, designing to match the impedance closely was a critical parameter, and not difficult at all. Regarding connectors, you could mix and match types as long as everything you used was designed to match the same impedance and terminate the cable properly. When you are designing amplifiers to be flat within 0.1dB over a wide bandwidth, impedance matching matters both for steady-state amplitude settings and ringing caused by the reflections. There is a whole discipline around transmission lines going back nearly 200 years, for a reason. OK, TimeNuts tend to be piping single frequencies around the place, but I thought this was a place looking for precision, and playing with low-level signals, and hunting down esoteric artefacts and anomalies. Wading roughshod through transmission theory is at odds with that for me, sorry. It's not as though designing stuff to have the right input and output impedances is difficult. Nowadays with integrated amplifiers you can just use the brute-force method of hanging a 50 ohm resistor across a 'high-impedance' input and another 50 ohm resistor in series with a 'zero ohm' output impedance. Back in the day we designed them to inherently have the right input and output impedance and so saved throwing lots of signal away. My humble apologies for the rant ... but I just couldn't believe what I was reading this morning when I opened the mail. I guess I'll get thrown out for this ... John ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] ThunderBolt question
You will need a “GPS antenna” with an LNA in it for decent (perhaps at all) reception. F connector is/was common for GPS antenna connections. I doubt the receiver is anything close to 50 ohms, or 75 ohms 73, K6TD > On Jun 5, 2020, at 4:24 PM, Robert DiRosario wrote: > > I have a Trimble ThunderBolt GPSDO that I just received. It has an F > connector for the antenna input, and BNC connectors for the 1 pps and 10 MHz > outputs. Is the receiver input impedance really 75 Ohms, or is it 50 Ohms and > they just used the F connector to distinguish it from the others? What do > people do, just use a 50 Ohm antenna? > > Thanks > > Robert DiRosario > > KA3ZYX > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] ThunderBolt question
I have used these "Hockey Puck" GPS antennas with 100' (thats feet) of RG-174. No problem locking up. Granted, the antenna needs an unobstructed view, but the antennas today work a lot better. I have a Laptop in the shack, near an inner wall, with one of these GPS/GLONASS USB units. I consistently see 8 or more satellites and get a 3D fix. That's on my desktop with an SB-220 looming over the antenna. Try it, you will be surprised. 73, Dick, W1KSZ On Fri, Jun 5, 2020 at 6:43 PM Tom Holmes wrote: > Dana... > > I think that you are neglecting two important mitigating factors. > > 1. the cable loss at 1575MHz, even for a 25' run of RG-6, reduces those > reflections quite a lot from one end to the other. It amounts to 2 - 3 > dB in 25', depending on cable quality. > > 2. a 1.5:1 SWR is not a very big reflection to begin with, on the order of > 20% of the incident power, about 7 dB. I am rounding a lot here just to > keep the math easy...for me. > > By the time a reflection has made the round trip from the receiver back to > the antenna and them back to the receiver, which is how the delay would > have > to manifest itself, it will be down at least 15 dB from its original self, > and probably more. Given the coding of GPS signals which allows several > satellites to share a common frequency band, that is not going to be much > of > a problem. And if only one end of the path actually is 75 ohms, then there > won't be a delayed signal. > > Tom Holmes, N8ZM > > -Original Message- > From: time-nuts On Behalf Of Dana > Whitlow > Sent: Friday, June 05, 2020 9:01 PM > To: Taka Kamiya ; Discussion of precise time and > frequency measurement > Subject: Re: [time-nuts] ThunderBolt question > > I'd like to point out that mismatches at the ends of an antenna cable *can* > cause trouble. When both ends are mismatched, each bit of detail in the > signal > gets partially reflected back and forth, each time delayed by the round > trip propagation > delay in the cable, and so you have something like multipath going on. > Fortunately the > successive reflections get weaker with time, generally quite rapidly. > Since many > GPS users seem very concerned about multipath resulting from poor antenna > placement, > I think this factor should be considered as well and not just get swept > under the rug. > > The amplitude of the "multipath" resulting from cable mismatches depends on > the product > of the voltage reflection coefficients at the two ends of the cable. If > either end is perfectly > matched, then the quality of the match at the other end is not significant > vis-a-vis apparent > multipath problems and only affects transmission loss. > > But when there is a mismatch on both ends, then the length of the cable > comes into play > as well. A longer cable means more delay between successive reflections, > which is just > like multipath involving longer delays between the direct and the reflected > signals. > > Cheers, > > Dana (K8YUM) > > On Fri, Jun 5, 2020 at 7:13 PM Taka Kamiya via time-nuts < > time-nuts@lists.febo.com> wrote: > > > 50 ohm / 75 ohm question is really irrelevant in this kind of thing. > > Trmble itself says in manual, not to be concerned with this apparent > > mismatch. > > In my particular case, I have a home lab standard and existing system. I > > have an antenna and network of distribution amplifiers. They are all 50 > > ohms and N connectors. Some ports have BNC adapters attached. I have > > pretty much standardized everything to SMA, N, or BNC. > > > > I boxed a power supply, T-bolt, and buffer amp in a metal case. I bought > > a short cable (RG58) that goes from F to BNC. On back of the case, I > have > > BNC to N adapter. I also have a few adapters that goes from F to BNC for > > the test bench. It really doesn't matter what you use, as long as it > makes > > a solid connection. > > > > Advantage of F connectors and RG6 are, cheap, abundant, and low loss for > > the size. Advantage of having house standard is, less adapters and less > > headache. > > > > --- > > (Mr.) Taka Kamiya > > KB4EMF / ex JF2DKG > > > > > > On Friday, June 5, 2020, 7:22:33 PM EDT, Robert DiRosario < > > ka3...@comcast.net> wrote: > > > > I have a Trimble ThunderBolt GPSDO that I just received. It has an F > > connector for the antenna input, and BNC connectors for the 1 pps and 10 > > MHz outputs. Is the receiver input impedance really 75 Ohms, or is it 50 > > Ohms and they just used the F connector to distinguish it from the >
Re: [time-nuts] ThunderBolt question
A 1.5:1 SWR = ~14 dB return loss. On 6/5/2020 6:42 PM, Tom Holmes wrote: Dana... I think that you are neglecting two important mitigating factors. 1. the cable loss at 1575MHz, even for a 25' run of RG-6, reduces those reflections quite a lot from one end to the other. It amounts to 2 - 3 dB in 25', depending on cable quality. 2. a 1.5:1 SWR is not a very big reflection to begin with, on the order of 20% of the incident power, about 7 dB. I am rounding a lot here just to keep the math easy...for me. By the time a reflection has made the round trip from the receiver back to the antenna and them back to the receiver, which is how the delay would have to manifest itself, it will be down at least 15 dB from its original self, and probably more. Given the coding of GPS signals which allows several satellites to share a common frequency band, that is not going to be much of a problem. And if only one end of the path actually is 75 ohms, then there won't be a delayed signal. Tom Holmes, N8ZM ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] ThunderBolt question
Dana... I think that you are neglecting two important mitigating factors. 1. the cable loss at 1575MHz, even for a 25' run of RG-6, reduces those reflections quite a lot from one end to the other. It amounts to 2 - 3 dB in 25', depending on cable quality. 2. a 1.5:1 SWR is not a very big reflection to begin with, on the order of 20% of the incident power, about 7 dB. I am rounding a lot here just to keep the math easy...for me. By the time a reflection has made the round trip from the receiver back to the antenna and them back to the receiver, which is how the delay would have to manifest itself, it will be down at least 15 dB from its original self, and probably more. Given the coding of GPS signals which allows several satellites to share a common frequency band, that is not going to be much of a problem. And if only one end of the path actually is 75 ohms, then there won't be a delayed signal. Tom Holmes, N8ZM -Original Message- From: time-nuts On Behalf Of Dana Whitlow Sent: Friday, June 05, 2020 9:01 PM To: Taka Kamiya ; Discussion of precise time and frequency measurement Subject: Re: [time-nuts] ThunderBolt question I'd like to point out that mismatches at the ends of an antenna cable *can* cause trouble. When both ends are mismatched, each bit of detail in the signal gets partially reflected back and forth, each time delayed by the round trip propagation delay in the cable, and so you have something like multipath going on. Fortunately the successive reflections get weaker with time, generally quite rapidly. Since many GPS users seem very concerned about multipath resulting from poor antenna placement, I think this factor should be considered as well and not just get swept under the rug. The amplitude of the "multipath" resulting from cable mismatches depends on the product of the voltage reflection coefficients at the two ends of the cable. If either end is perfectly matched, then the quality of the match at the other end is not significant vis-a-vis apparent multipath problems and only affects transmission loss. But when there is a mismatch on both ends, then the length of the cable comes into play as well. A longer cable means more delay between successive reflections, which is just like multipath involving longer delays between the direct and the reflected signals. Cheers, Dana (K8YUM) On Fri, Jun 5, 2020 at 7:13 PM Taka Kamiya via time-nuts < time-nuts@lists.febo.com> wrote: > 50 ohm / 75 ohm question is really irrelevant in this kind of thing. > Trmble itself says in manual, not to be concerned with this apparent > mismatch. > In my particular case, I have a home lab standard and existing system. I > have an antenna and network of distribution amplifiers. They are all 50 > ohms and N connectors. Some ports have BNC adapters attached. I have > pretty much standardized everything to SMA, N, or BNC. > > I boxed a power supply, T-bolt, and buffer amp in a metal case. I bought > a short cable (RG58) that goes from F to BNC. On back of the case, I have > BNC to N adapter. I also have a few adapters that goes from F to BNC for > the test bench. It really doesn't matter what you use, as long as it makes > a solid connection. > > Advantage of F connectors and RG6 are, cheap, abundant, and low loss for > the size. Advantage of having house standard is, less adapters and less > headache. > > --- > (Mr.) Taka Kamiya > KB4EMF / ex JF2DKG > > > On Friday, June 5, 2020, 7:22:33 PM EDT, Robert DiRosario < > ka3...@comcast.net> wrote: > > I have a Trimble ThunderBolt GPSDO that I just received. It has an F > connector for the antenna input, and BNC connectors for the 1 pps and 10 > MHz outputs. Is the receiver input impedance really 75 Ohms, or is it 50 > Ohms and they just used the F connector to distinguish it from the > others? What do people do, just use a 50 Ohm antenna? > > Thanks > > Robert DiRosario > > KA3ZYX > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] ThunderBolt question
Even more so when the cable is long, as it is likely to be with a GPS receiver. The cable attenuation does wonders attenuating the effects of VSWR... Didier KO4BB On Fri, Jun 5, 2020 at 6:30 PM Tim Shoppa wrote: > Many of us use F connectors and 75 ohm CATV RG-6 coax for GPS antennas when > both antenna and receiver are specified for 50 ohms. > > Don't sweat the difference between 75 and 50 ohms. > > Tim N3QE > > On Fri, Jun 5, 2020 at 7:22 PM Robert DiRosario > wrote: > > > I have a Trimble ThunderBolt GPSDO that I just received. It has an F > > connector for the antenna input, and BNC connectors for the 1 pps and 10 > > MHz outputs. Is the receiver input impedance really 75 Ohms, or is it 50 > > Ohms and they just used the F connector to distinguish it from the > > others? What do people do, just use a 50 Ohm antenna? > > > > Thanks > > > > Robert DiRosario > > > > KA3ZYX > > > > ___ > > time-nuts mailing list -- time-nuts@lists.febo.com > > To unsubscribe, go to > > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > > and follow the instructions there. > > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] ThunderBolt question
I'd like to point out that mismatches at the ends of an antenna cable *can* cause trouble. When both ends are mismatched, each bit of detail in the signal gets partially reflected back and forth, each time delayed by the round trip propagation delay in the cable, and so you have something like multipath going on. Fortunately the successive reflections get weaker with time, generally quite rapidly. Since many GPS users seem very concerned about multipath resulting from poor antenna placement, I think this factor should be considered as well and not just get swept under the rug. The amplitude of the "multipath" resulting from cable mismatches depends on the product of the voltage reflection coefficients at the two ends of the cable. If either end is perfectly matched, then the quality of the match at the other end is not significant vis-a-vis apparent multipath problems and only affects transmission loss. But when there is a mismatch on both ends, then the length of the cable comes into play as well. A longer cable means more delay between successive reflections, which is just like multipath involving longer delays between the direct and the reflected signals. Cheers, Dana (K8YUM) On Fri, Jun 5, 2020 at 7:13 PM Taka Kamiya via time-nuts < time-nuts@lists.febo.com> wrote: > 50 ohm / 75 ohm question is really irrelevant in this kind of thing. > Trmble itself says in manual, not to be concerned with this apparent > mismatch. > In my particular case, I have a home lab standard and existing system. I > have an antenna and network of distribution amplifiers. They are all 50 > ohms and N connectors. Some ports have BNC adapters attached. I have > pretty much standardized everything to SMA, N, or BNC. > > I boxed a power supply, T-bolt, and buffer amp in a metal case. I bought > a short cable (RG58) that goes from F to BNC. On back of the case, I have > BNC to N adapter. I also have a few adapters that goes from F to BNC for > the test bench. It really doesn't matter what you use, as long as it makes > a solid connection. > > Advantage of F connectors and RG6 are, cheap, abundant, and low loss for > the size. Advantage of having house standard is, less adapters and less > headache. > > --- > (Mr.) Taka Kamiya > KB4EMF / ex JF2DKG > > > On Friday, June 5, 2020, 7:22:33 PM EDT, Robert DiRosario < > ka3...@comcast.net> wrote: > > I have a Trimble ThunderBolt GPSDO that I just received. It has an F > connector for the antenna input, and BNC connectors for the 1 pps and 10 > MHz outputs. Is the receiver input impedance really 75 Ohms, or is it 50 > Ohms and they just used the F connector to distinguish it from the > others? What do people do, just use a 50 Ohm antenna? > > Thanks > > Robert DiRosario > > KA3ZYX > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] ThunderBolt question
50 ohm / 75 ohm question is really irrelevant in this kind of thing. Trmble itself says in manual, not to be concerned with this apparent mismatch. In my particular case, I have a home lab standard and existing system. I have an antenna and network of distribution amplifiers. They are all 50 ohms and N connectors. Some ports have BNC adapters attached. I have pretty much standardized everything to SMA, N, or BNC. I boxed a power supply, T-bolt, and buffer amp in a metal case. I bought a short cable (RG58) that goes from F to BNC. On back of the case, I have BNC to N adapter. I also have a few adapters that goes from F to BNC for the test bench. It really doesn't matter what you use, as long as it makes a solid connection. Advantage of F connectors and RG6 are, cheap, abundant, and low loss for the size. Advantage of having house standard is, less adapters and less headache. --- (Mr.) Taka Kamiya KB4EMF / ex JF2DKG On Friday, June 5, 2020, 7:22:33 PM EDT, Robert DiRosario wrote: I have a Trimble ThunderBolt GPSDO that I just received. It has an F connector for the antenna input, and BNC connectors for the 1 pps and 10 MHz outputs. Is the receiver input impedance really 75 Ohms, or is it 50 Ohms and they just used the F connector to distinguish it from the others? What do people do, just use a 50 Ohm antenna? Thanks Robert DiRosario KA3ZYX ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] ThunderBolt question
Robert... Try to find QUAD SHIELD RG-6. 73 Don W4WJ On Friday, June 5, 2020 Dana Whitlow wrote: Robert, One does not know what the RF input impedance of the unit is unless one measures it (very carefully at low levels) with a VNA or similar instrument. Just 'cause some manufacturer says that it's 50 ohms does not mean very much- it's not uncommon to see actual impedances off by a factor of 1,5:1 or even 2:1. I've heard of people using 75 ohm line with F connectors at both ends to connect an antenna to a GPS receiver and claim that it causes no problems. The nice thing about 75 ohm line with F connectors is that it's readily available in various lengths at your friendly local brick and mortar stores. I'd just try it and see how it works. Then look deeper only if you come to suspect that the GPS unit's performance is not up to snuff. Dana (K8YUM) On Fri, Jun 5, 2020 at 6:22 PM Robert DiRosario wrote: > I have a Trimble ThunderBolt GPSDO that I just received. It has an F > connector for the antenna input, and BNC connectors for the 1 pps and 10 > MHz outputs. Is the receiver input impedance really 75 Ohms, or is it 50 > Ohms and they just used the F connector to distinguish it from the > others? What do people do, just use a 50 Ohm antenna? > > Thanks > > Robert DiRosario > > KA3ZYX > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] ThunderBolt question
Robert, One does not know what the RF input impedance of the unit is unless one measures it (very carefully at low levels) with a VNA or similar instrument. Just 'cause some manufacturer says that it's 50 ohms does not mean very much- it's not uncommon to see actual impedances off by a factor of 1,5:1 or even 2:1. I've heard of people using 75 ohm line with F connectors at both ends to connect an antenna to a GPS receiver and claim that it causes no problems. The nice thing about 75 ohm line with F connectors is that it's readily available in various lengths at your friendly local brick and mortar stores. I'd just try it and see how it works. Then look deeper only if you come to suspect that the GPS unit's performance is not up to snuff. Dana (K8YUM) On Fri, Jun 5, 2020 at 6:22 PM Robert DiRosario wrote: > I have a Trimble ThunderBolt GPSDO that I just received. It has an F > connector for the antenna input, and BNC connectors for the 1 pps and 10 > MHz outputs. Is the receiver input impedance really 75 Ohms, or is it 50 > Ohms and they just used the F connector to distinguish it from the > others? What do people do, just use a 50 Ohm antenna? > > Thanks > > Robert DiRosario > > KA3ZYX > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] ThunderBolt question
Many of us use F connectors and 75 ohm CATV RG-6 coax for GPS antennas when both antenna and receiver are specified for 50 ohms. Don't sweat the difference between 75 and 50 ohms. Tim N3QE On Fri, Jun 5, 2020 at 7:22 PM Robert DiRosario wrote: > I have a Trimble ThunderBolt GPSDO that I just received. It has an F > connector for the antenna input, and BNC connectors for the 1 pps and 10 > MHz outputs. Is the receiver input impedance really 75 Ohms, or is it 50 > Ohms and they just used the F connector to distinguish it from the > others? What do people do, just use a 50 Ohm antenna? > > Thanks > > Robert DiRosario > > KA3ZYX > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
[time-nuts] ThunderBolt question
I have a Trimble ThunderBolt GPSDO that I just received. It has an F connector for the antenna input, and BNC connectors for the 1 pps and 10 MHz outputs. Is the receiver input impedance really 75 Ohms, or is it 50 Ohms and they just used the F connector to distinguish it from the others? What do people do, just use a 50 Ohm antenna? Thanks Robert DiRosario KA3ZYX ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
