Thanks for replying, I am very intrigued by this theme. See below please and tell me your opinion.
> Il giorno 5 ott 2024, alle ore 20:33, Jim Whartenby via R-390 > <r-390@mailman.qth.net> ha scritto: > > Giovanni > > I need some clarifications. > > 1) You said: "It attenuates the signal voltage of 0.56 V, i.e. 5 dB."So the > Signal Generator (SG) meter indicates that the output voltage is 0.56 volts > or are you are measuring 0.56 volts at the output of the DA-121/U when the SG > meter reads 1 volt? If so, how are you measuring this voltage? Is it peak > or peak to peak or RMS? The assumption here is that it is RMS. > I wrote wrongly; I meant that the DA-121 is a voltage divider that, > considered 125 ohm the input impedance of the receiver, multiplies the siggen > voltage x 0.56. > > 2) The DA-121/U contains two resistors, a 68 ohm resistor in parallel with > the signal generator output and a series 100 ohm resistor to the center pin > of the BNC output connector. You are then adapting the BNC output connector > of the DA-121/U to TWINAX and then connecting it to the balanced RF input > connector on the back of the R-390A, correct? Yes > > 3) What are the two resistor values in the DA-121 when you measure with your > DMM? How close are they to what is expected? I am guessing that these two > resistors are carbon composition and are a bit off in value. It is > interesting to note that carbon composition resistors will change value when > soldered into a circuit. No, it is not the original, I built it with new > components. > > 4) When you measure the BNC to TWINAX adapter, one of the TWINAX pins goes to > the center pin of the BNC connector and the other TWINAX pin goes to ground? > Yes Both read close to zero ohms? each other yes, but they are open to ground. > > 5) How old are the coax cables used in your measurements? In other words, > how lossy are they? Coax ages so the cable losses will increase and it will > have an affect on your measurements. The coax is 50 ohms? Yes, they are > normal BNC/BNC, 1 meter long, with 50 ohm cable, bought new ready to be used. > > The way I see it, 50 ohms in parallel with 68 ohms = 29 ohms. 29 ohms in > series with 100 ohms = 129 ohms which is approximately your impedance > transformation needed from 50 to 125 ohms. Because of the 68 ohms is in > parallel with the SG output, the voltage at this point should be half of what > the SG meter indicates. I am not sure it is so. The siggen indicated voltage > is in Vrms and it is true when you have a 50 ohm load. If you don’t have the > 50 ohm load, the voltage is double. I am sure of this, I tested more times > with different generators and oscilloscopes. > The second voltage divider of 100 and 125 ohms is again reducing the SG > output voltage by another half so the actual voltage applied to the receiver > is 0.5 X 0.5 or 0.25 times the SG meter reading. In other words, actual > voltage applied to the R-390A receiver is 1/4 of what the SG meter indicates > or 12 dB down. No, I am sure of 0.56. In the doubt, I built a 125 ohm > terminator and checked with the oscilloscope. Starting with 10 mVrms I read > 5.7 mVrms because the resistors are not perfect. And thus reduces the voltage > by 5 dB. Do you agree? > > So what this means to the original discussion is that the 6.5 microvolt limit > in the R-390A specification is actually 1.6 microvolts that is applied to the > R-390A balance RF input for a 10 dB S+N/N reading when all of the losses in > the test setup are accounted for. So the spec has simplified the measurement > and eliminated all of the above math. Again, spec is spec and those who > wrote it knew what they were doing. > > This back of the envelope analysis does not agree with what you have > measured. I am interested in what you find when you have a chance to take a > closer look. > > Jim > > Logic: Method used to arrive at the wrong conclusion, with confidence. > Murphy > > On Saturday, October 5, 2024 at 01:48:09 AM CDT, Ing. Giovanni Becattini > <giovanni.becatt...@icloud.com> wrote: > > Hi Jim and thanks for your reply. I read the very interesting document you > pointed out. I did not understand everything, but for my practical interest > it confirms that the impedance matching is mandatory. > I am using an HP8640B as a signal generator. Let’s suppose it is ideally > calibrated. I use also the DA-121/U impedance adapter which shows 50 ohm to > the siggen and 125 to the receiver. It is the fourth type of pad of figure 4 > of the article. > My practical question is how to take in account the DA-121? > It attenuates the signal voltage of 0.56 V, i.e. 5 dB. So, > - in volts: the voltage value for the 10 dB S/N I read on the generator’s > scale should be multiplied by 0.56. > - in dBm: the dBm value for the 10 dB S/N I read on the generator’s scale > should be reduced by 5 dBm. > > Is this correct? > Thanks > > > Il giorno 5 ott 2024, alle ore 05:13, Jim Whartenby via R-390 > <r-390@mailman.qth.net> ha scritto: > BobI don't know why you are speculating "Since you can use any signal > generator" when MIL-R-13947B(SigC) on page 20 specifies the Measurements Corp > Model 82 signal Generator, or it's equal. Your concern with matched > impedances between the generator and the R-390 may appear to be valid but > this seems to have been accounted for in the R-390 spec with higher input > voltages to account for the mismatch losses. Collins and the Signal Corps > specified the 125 ohm input impedance of the R-390 and they were surely aware > that the standard impedance of high end signal generators was normally 50 > ohms. > > Check out http://hparchive.com/Boonton/BRC-The-Notebook-03.pdf which is a > 1954 Boonton Radio explanation for the use of Dummy Antennas. Everything you > need is in the first four pages. It explains why the mismatch between 50 > ohms and the receiver input impedance reduces the loading on the receiver > input tuned circuits and recovers the Q of the receiver RF input tuned > circuits. This also affects favorably the S+N/N measurement. There is a > reason for the apparent madness. > Jim > > Logic: Method used to arrive at the wrong conclusion, with confidence. > Murphy > > On Friday, October 4, 2024 at 07:23:24 PM CDT, Bob Camp <kb...@n1k.org> > wrote: > > Hi > > Based on a *lot* of interaction with DOD source inspectors over the years … > it’s very much a “that depends” sort of thing. The guy you get this month may > be *very* different than than the guy who comes in next month. > > Since you can use any signal generator, there is room for “trouble” with > minimal specs. Typically the way this works out is a request to clarify > things. Unfortunately that stuff does not get into the official specs. Yes, > some of us have pointed that out as a problem …. never got addressed AFIK. > > Bob > > > > On Oct 4, 2024, at 5:19 PM, Jim Whartenby via R-390 <r-390@mailman.qth.net> > wrote: > > Bob > You are over thinking the testing. The generator is terminated but not in > it's characteristic impedance. Collins and the agency letting the contract > agreed on a test method and wrote it down in the test procedure. Anything > that would affect the testing like VSWR has already been considered and > accounted for. That Hams don't use the proper termination for the signal > generator, according to the spec, which affects the perceived sensitivity is > another issue. Spec is spec as they say. > > Not all Government Source Inspectors are knowable about the equipment that > they are reviewing and putting their inspection stamp on. Not all Signal > Corps equipment is inspected by Army GSIs. Every once in a while, Army > equipment will have an anchor stamp on it and vice-versa. Sometimes certain > specs are wavered if not considered critical, most often they are not. > Again, spec is spec. > Regards, > Jim > > Logic: Method used to arrive at the wrong conclusion, with confidence. > Murphy > > On Friday, October 4, 2024 at 02:43:36 PM CDT, Bob Camp <kb...@n1k.org> > wrote: > > Hi > > Ok …. but ….. > > If it is simply a resistor, then the generator is unterminated. It’s designed > and calibrated to run into a 50 ohm load. If all that’s there is a series > resistor the generator is not properly set up (and thus not calibrated ….). > Yes some generators deal with this better than others. > > My guess is that there’s more to the load circuit than just that resistor. > Without a schematic …. who knows …. > > Bob > > > > On Oct 4, 2024, at 12:53 PM, Jim Whartenby via R-390 <r-390@mailman.qth.net> > wrote: > > Bob > But we do know what the interface is between the signal generator and antenna > input. On document page 20, the balanced RF input is in series with a 125 > ohm non-inductive resistor and the unbalanced RF input is in series with a 50 > pF capacitor. Both interfaces do not include the generator's output > impedance. > > As for the sensitivity not being what the R-390/URR or the R-390A/URR is > capable of, well there are perhaps 100's of tests that the receiver must > successfully pass before it is accepted. Tightening any of the specs to > exactly what the receiver may be capable of passing will guarantee that no > one receiver will ever pass all of the acceptance tests. Then there is the > added problem of your test equipment's error tolerance and being traceable > back to the NIST standards. Is your 10 microvolts from the signal generator > really 10 microvolts? > > Unless otherwise stated, the specification calls out a value that the > receiver must do better than. A sensitivity of just under 6.5 microvolts for > a 10 dB S+N/N with an audio power output of 10 milliwatts seems reasonable > over the range of 2 to 32 MHz for either balanced or unbalanced RF inputs. > This is perhaps typical for just about all HF receivers built for the > military, at least for what I am aware of, but I am sure that there will be > the rare exception. > > Regards, > Jim > Logic: Method used to arrive at the wrong conclusion, with confidence. > Murphy > > On Friday, October 4, 2024 at 09:39:26 AM CDT, Bob Camp <kb...@n1k.org> > wrote: > > Hi > > The gotcah here is that we really don’t know what the interface between the > signal generator and radio looked like for these official tests. Despite the > document going into a lot of detail, they did not include a schematic or a > part number. (or at least not one that I could find). There are a *lot* of > ways they might have been doing things ….. > > Bob > > > On Oct 4, 2024, at 10:19 AM, Ing. Giovanni Becattini via R-390 > <r-390@mailman.qth.net> wrote: > > But when we perform the classical sensitivity test with a modulated signal, > do we get the AM or CW value? > > Following the instructions I got a value of -104 dBm, i.e. 1.41 uV. Because I > used the DA-121, the real value should be even better, far from 5 uV. And > surely my R-390A is not as good as it could be . > > Is my reasoning correct? > > > Il giorno 4 ott 2024, alle ore 16:13, Barry <n4...@knology.net> ha scritto: > > I don’t think of 1uV as “bad” but most of these radios will beat that. > > Thanks, > Barry > > > On Oct 4, 2024, at 8:55 AM, Ing. Giovanni Becattini via R-390 > <r-390@mailman.qth.net> wrote: > > Wow, thanks! > > By the way, do you know why the sensitivity values were so bad? Are we the > ones who are taking measures that are too lenient or are they the ones who > were too conservative? > > Thanks again > > Gianni > > > Il giorno 4 ott 2024, alle ore 15:37, Larry Haney <larry41...@gmail.com> ha > scritto: > > Hi Gianni and Barry, Tom Marcotte and Al Tirevold (SK) obtained and cleaned > up the specs for the 390A and put it on our website. Here's the link: > mil-r-13947b– (r-390a.net) <https://www.r-390a.net/mil-r-13947b.pdf>. > Unfortunately, the entry in References page indicates it is for the R390(), > but it is also the the 390A. I will be changing that shortly. > > Regards, Larry > > > On Fri, Oct 4, 2024 at 4:49 AM Ing. Giovanni Becattini via R-390 > <r-390@mailman.qth.net <mailto:r-390@mailman.qth.net>> wrote: > Have ever been released official specs for the 390A? > > The manuals report few data and neither too correct, for what I can > understand (e.g., sensitivity) > > Thanks > ______________________________________________________________ > 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 <mailto:R-390@mailman.qth.net> > > This list hosted by: http://www.qsl.net <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 > > > > ______________________________________________________________ > 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
