GR-1001A. Even funner. Thanks, Barry - N4BUQ
On Wed, Oct 23, 2024 at 4:46 PM Jacques Fortin <jacque...@videotron.ca> wrote: > > Bob, I see your point, but we know everything about that too ! > That's bring me back to the end of the '70s when my workbench signal > generator was a Marconi Instruments TF955/5. > Output calibrated in Volts EMF (open circuit voltage) and 75 ohms of output > impedance ! > Using it, you had to compute what was the real load voltage you got all the > time... > > 73, Jacques, VE2JFE in Montreal > > -----Message d'origine----- > De : Bob kb8tq <kb...@n1k.org> > Envoyé : 23 octobre 2024 16:49 > À : Jacques Fortin <jacque...@videotron.ca> > Cc : Ing. Giovanni Becattini <giovanni.becatt...@icloud.com>; > r-390@mailman.qth.net; Larry Haney <larry41...@gmail.com> > Objet : Re: [R-390] Official specs > > Hi > > If you start with a Spice model and put in a 2V source with some series > resistance. You loose 6 db when you get to 1.0 V. That’s how Spice looks at > things. > > If you start with the signal generator convention you have a “spice” 2V > source and a 50 ohm resistor “inside the box”. Put on a 50 ohm load and you > have 1V. That’s your zero db point with the signal generator. > > Start one way and you are 6 db down. > > Start the other way and you are at zero db. > > Bob > > > On Oct 23, 2024, at 1:03 PM, Jacques Fortin <jacque...@videotron.ca> wrote: > > > > Hello Bob, > > > > I 100% agree that the convention on a RF source is that the displayed > > output voltage is valid only when the SG is loaded with the correct > > termination impedance. > > I disagree however that the outcome "depends" on what can be used to > > provide the answer. > > If anyone is ignorant enough to use a Spice model that not include a source > > output impedance, it is sure that the result of such "simulation" will be > > different from what is obtained with properly set up test equipment. > > A properly used simulation software results will not be different that what > > can be obtained with "real" instruments. > > If ever this is the case, the inputs to the simulation program are faulty. > > > > Bob Pease (RIP) once fell in that trap: he took revenge by throwing his > > computer from the top of the building into the parking lot below. > > > > But, at the end of the day, nothing is more practical than a good theory. > > > > 73, Jacques, VE2JFE in Montreal > > > > -----Message d'origine----- > > De : r-390-boun...@mailman.qth.net <r-390-boun...@mailman.qth.net> De > > la part de Bob Camp Envoyé : 23 octobre 2024 12:26 À : Ing. Giovanni > > Becattini <giovanni.becatt...@icloud.com> Cc : r-390@mailman.qth.net; > > Larry Haney <larry41...@gmail.com> Objet : Re: [R-390] Official specs > > > > Hi > > > > The very basic issue here turns that into a “that depends” answer: > > > > If you are playing with a Spice model, and starting from the voltage on > > your ideal source, you get one answer. > > > > If you are working at RF and using a signal generator (and doing things > > properly) you get a very different answer. > > > > The convention on an RF source is that the “starting voltage” is measured > > with the correct termination in place. > > > > Since we’re talking about RF …. that’s how it would be done. > > > > Bob > > > >> On Oct 23, 2024, at 10:39 AM, Ing. Giovanni Becattini via R-390 > >> <r-390@mailman.qth.net> wrote: > >> > >> Hi, > >> > >> I find this topic very intriguing, so I cannot help but return to this > >> discussion. > >> > >> As I told you, I don't consider myself an RF expert or simply a 390 > >> expert, but I have had to solve complex engineering problems many times in > >> my life. So I think we should first create a mathematical model that is as > >> simple as possible, i.e. without taking into account the frequency effect. > >> Once the model works, we can try to make it more real with the right > >> corrections. > >> > >> So I would like to ask a question to see if we are on the same page: > >> Do you agree that if the R-390A were a perfect 125 ohm resistor and we > >> were working at 1 kHz, the DA-121 would attenuate 5 dB in voltage and 8.98 > >> in power? > >> > >> Greetings > >> > >> Gianni > >> > >>> Il giorno 23 ott 2024, alle ore 16:15, Larry Haney <larry41...@gmail.com> > >>> ha scritto: > >>> > >>> Jim, I read your referenced post a few times looking for the answer > >>> we're all looking for, what the microvolt level is that is going > >>> into the 390 for a given level going into the DA-121, but I couldn't > >>> find it. All I read was a bunch of db numbers that don't make any > >>> difference. We need to know about the uV levels. You can talk > >>> about insertion losses all you want, but that does not tell us what the > >>> uV level is that is going into the 390. > >>> > >>> I used my URM-25D to generate a 1 MegaHertz 50 uV signal into the > >>> DA-121 and got 28 uV going into the 390 (that's a 44% reduction of > >>> signal from the 25D as measured with my HP 400FL RMS RF AC > >>> voltmeter). Nothing else matters. The calculation is very simple: > >>> 50 - 28 = 22, 22 / 50 = .4444 or 44.44%. That means that 55.55% of the > >>> signal from the SG is getting to the > >>> 390. The accuracy of my 400FL is +/- 1%. All my signal measurements > >>> were > >>> in RF RMS volts measured with my HP 400FL. > >>> > >>> The ONLY DC measurements I made were to measure the resistances in > >>> the > >>> DA-121 and mine are a 70 ohm shunt and a 100 ohm in series. These > >>> are close to the documented values of 68 ohms and 100 ohms. > >>> > >>> So, what uV level of signal do you MEASURE (not calculated or > >>> theorized) going into the DA-121 and going into the 390. Let's keep > >>> it simple and stick to *MICROVOLTS* because that is what the > >>> sensitivity and signal to noise ratio measurements use, NOT db. > >>> > >>> By the way, since this test is all about the DA-121, you should be > >>> using the documented resistor values in it for testing (68 and 100 ohms). > >>> > >>> And contrary to what you said, my DC circuit calculations > >>> (resistance and estimated signal loss) do agree with my RF > >>> measurements. The resistance calculation is: 100 ohms / (100 + 125 ohms) > >>> = .4444. That's a 44% loss. > >>> To get the signal level at the 390, multiply the SG output by 56%. > >>> And I did not calculate any db loss, the 5 db loss is what my 400FL says > >>> it is. > >>> > >>> For anyone wanting to make their own DA-121, use what's documented > >>> in it, a > >>> 68 ohm shunt and 100 ohm series resistor. Otherwise you will get a > >>> different answer from those that use a real DA-121. > >>> > >>> Regards, Larry > >>> > >>> On Tue, Oct 22, 2024 at 3:36 PM Jim Whartenby <old_ra...@aol.com> wrote: > >>> > >>>> Larry > >>>> > >>>> I built a test fixture that is essentially two DA-121's connected > >>>> back to back. Photos and drawing are enclosed. This does the > >>>> conversion from 50 ohms to 125 ohms and then back to 50 ohms. I > >>>> used 1% resistors to make the attenuator circuit with the values close > >>>> to those found here: > >>>> > >>>> https://k7mem.com/Res_Attenuator.html > >>>> > >>>> > >>>> The closest I could come to the 64.18 ohms result from the > >>>> attenuator calculator was 63.9 ohms. This is from the parallel > >>>> combination of 3 each > >>>> 237 ohm in parallel with a 1k, in parallel with a 499 ohm resistor. > >>>> Five resistors in parallel, all 1% resistors. The result was 63.85 > >>>> ohms, a 0.5% error. The sub for the 96.83 ohm resistor is a 100 > >>>> ohm 1% resistor (3% > >>>> error) and the sub for the R-390's 125 ohm impedance was a 121 ohm > >>>> 1% resistor (3% error). This is still much better then the 5% > >>>> resistors used in the original DA-121. > >>>> > >>>> > >>>> For a test oscillator I used a Helper SM-1000 signal generator and > >>>> measured the insertion loss with a Stoddart NM-25T frequency > >>>> selective voltmeter. The insertion loss was measured at 10 MHz > >>>> using two 4 foot BNC > >>>> RG-58 coax cables from Pomona Electric. 4 foot of coax from the > >>>> SM-1000 to the test fixture and another 4 feet from the test fixture to > >>>> the NM-25T. > >>>> > >>>> > >>>> The SG was set for a reading of 30 dB on the NM-25T signal strength > >>>> meter when measuring a BNC through connection and then measured 11 dB > >>>> when the > >>>> test fixture was installed in place of the BNC through. The insertion > >>>> loss for the test fixture is 19 dB. Dividing this by two since > >>>> there are essentially two DA-121s back to back gives an insertion > >>>> loss of about 9.5 dB for a single DA-121. This closely agrees with > >>>> the attenuator calculator findings. > >>>> > >>>> > >>>> So it seems that your DC circuit calculation do not agree with the > >>>> RF measurements. Transmission lines behave differently then DC > >>>> circuits. You calculate a 5 dB insertion loss, I measure a 9.5 dB > >>>> insertion loss. > >>>> > >>>> > >>>> Here is an experiment that you can try. Insert a 50 ohm resistor > >>>> in parallel with the 50 ohm coax. What do you think will happen? > >>>> Perhaps nothing since the coax is 50 ohms and the resistor is also > >>>> 50 ohms? In reality, the coax has reactive elements, parallel > >>>> capacitance and series inductance that make up the coax impedance. > >>>> Neither of which will dissipate the signal carried on the coax. > >>>> The only losses are from the resistance of the conductors that make > >>>> up the coax. Adding a parallel resistor will attenuate the signal to > >>>> the receiver by 3 dB. > >>>> > >>>> > >>>> If anyone on this list wants to make their own version of the > >>>> DA-121, I can supply the resistor values I used for a token $2 plus > >>>> postage. Just DM me with your address and if you want one or two > >>>> resistor sets. > >>>> > >>>> > >>>> Regards, Jim > >>>> > >>>> Logic: Method used to arrive at the wrong conclusion, with confidence. > >>>> Murphy > >>>> > >>>> > >>>> On Friday, October 18, 2024 at 05:36:08 AM CDT, Larry Haney < > >>>> larry41...@gmail.com> wrote: > >>>> > >>>> > >>>> Hi Jim, I just checked and I only have 1 da-121. As for insertion > >>>> loss, my coax is very short and the connections are very good so > >>>> the loss there would not be possible for me to measure. Now for > >>>> the insertion loss due to impedance mismatch (due to resistance > >>>> variations) would also not be possible for me to measure, as I > >>>> don't have the equipment required for that. But, because the 3 > >>>> resistors in the circuit are very close to the required values for > >>>> a perfect > >>>> 50 ohm match to the sig gen, I am sure that the insertion loss due > >>>> to that very slight impedance mismatch is extremely small. I have > >>>> no way to measure that loss as I don't have the 3 exact value > >>>> resistors to compare it to. I could calculate it, but I believe that > >>>> would be a waste of time without being able to measure it. > >>>> > >>>> After all the input you have given me and the research just done, > >>>> I'm satisfied with my current measurements and calculations (IE: > >>>> the output voltage of the da-121 is 56% of the input voltage when > >>>> the load is > >>>> 125 ohms). > >>>> > >>>> My biggest concern about making snr measurements is for those folks > >>>> that don't have a recently calibrated sig gen or calibrated rms AC > >>>> voltmeter to verify their readings with. > >>>> > >>>> Regards, Larry > >>>> > >>>> On Thu, Oct 17, 2024 at 1:55 PM Jim Whartenby <old_ra...@aol.com> wrote: > >>>> > >>>> Larry > >>>> No, just one SG and one 125 ohm load. You should be able to > >>>> determine the total loss through two DA-121 attenuators connected > >>>> back to back with an o'scope and then divide the loss by two to solve > >>>> for the insertion loss. > >>>> Jim > >>>> Logic: Method used to arrive at the wrong conclusion, with confidence. > >>>> Murphy > >>>> > >>>> > >>> ______________________________________________________________ > >>> R-390 mailing list > >>> Home: http://mailman.qth.net/mailman/listinfo/r-390 > >>> Help: http://mailman.qth.net/mmfaq.htm > >>> Post: mailto:R-390@mailman.qth.net > >>> > >>> This list hosted by: http://www.qsl.net Please help support this > >>> email list: http://www.qsl.net/donate.html > >> > >> ______________________________________________________________ > >> R-390 mailing list > >> Home: http://mailman.qth.net/mailman/listinfo/r-390 > >> Help: http://mailman.qth.net/mmfaq.htm > >> Post: mailto:R-390@mailman.qth.net > >> > >> This list hosted by: http://www.qsl.net Please help support this > >> email > >> list: http://www.qsl.net/donate.html > > > > ______________________________________________________________ > > R-390 mailing list > > Home: http://mailman.qth.net/mailman/listinfo/r-390 > > Help: http://mailman.qth.net/mmfaq.htm > > Post: mailto:R-390@mailman.qth.net > > > > This list hosted by: http://www.qsl.net Please help support this email > > list: http://www.qsl.net/donate.html > > > > ______________________________________________________________ > R-390 mailing list > Home: http://mailman.qth.net/mailman/listinfo/r-390 > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:R-390@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ R-390 mailing list Home: http://mailman.qth.net/mailman/listinfo/r-390 Help: http://mailman.qth.net/mmfaq.htm Post: mailto:R-390@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
