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
