Hi We’ve had a number of posts in this thread that head off in the direction of "db is only power based" ….
Bob > On Oct 28, 2024, at 4:32 AM, Larry Haney <larry41...@gmail.com> wrote: > > Hi Bob, Since the Sensitivity and SNR of the 390s are voltage measurement > based, why is it not correct to use a voltage only scenario? The Army, Navy, > Dr Lankford and MIL-R-13947B spec 3.13.5 all use a voltage based measurement. > Granted, the actual resistance and the reactive component of impedance are > not being considered, but that is the way of the 'old' world. > > In the new world (fiber optics receivers and computer wireless receivers), > the dbm measurements were originated with them and they have those > measurements in their specs. No way to go back and change the old > documentation. We're stuck with what we have, measuring them with voltage. > > Regards, Larry > > On Sun, Oct 27, 2024 at 6:05 PM Bob kb8tq <kb...@n1k.org > <mailto:kb...@n1k.org>> wrote: >> Hi >> >> Since only voltage is being measured, it does *NOT* meet the requirement >> earlier stated to *only* use power for computations. Both the actual >> resistance and the reactive component of impedance are not being considered. >> To get power you would need data on both of those components *and* the >> voltage. Thus is is not a correct way to do things with db if power is the >> only correct approach. >> >> The impedance range mentioned is not as wide as what was shown in the >> documents posted earlier this month. >> >> Bob >> >> > On Oct 27, 2024, at 8:42 PM, Jim Whartenby <old_ra...@aol.com >> > <mailto:old_ra...@aol.com>> wrote: >> > >> > Bob >> > >> > How does the data form a few weeks ago compare to the following from some >> > 20 years ago? >> > >> > >> > Dallas Lankford on Receiver Sensitivity Measurement >> > >> > This note appeared in the R-390 email reflector on QTH. It is the most >> > cogent discussion of receiver sensitivity I have ever seen. I reproduce it >> > here (with permission) with minor typographical corrections: >> > James A. Moorer >> > >> > >> > "There has been a lot of confusion about how to measure the AM sensitivity >> > of an R-390A. Unfortunately the manuals have contributed to this >> > confusion. The 1970 Navships 0967-063-2010 manual has a sensitivity >> > measuring procedure on pages 4-2 and 4-3 which involves setting the signal >> > generator (URM-25D) to minimum output. This is equivalent to the method of >> > turning the signal generator on and off which is used at several web sites >> > to find the 10 dB S+N/N ratio. However, the Navships manual does not >> > mention a 10 dB S+N/N ratio, but rather a 10 dB rise, which it is. What >> > the Navships and web sites measure is the 10 dB S+N1/N2 where N1 is the >> > noise due to the signal and receiver, and N2 is the no-signal receiver >> > noise. Also, the 50 ohm impedance of the signal generator is not matched >> > to the 125 ohm nominal (100 - 300 ohms) antenna input impedance (through a >> > UG-636A/U and UG-971/U) of the R-390A. Consequently, the signal generator >> > reading is not the number of microvolts that appears across the R-390A >> > antenna input. The Army manual TM 11-5820-358-35 gives a Sensitivity Test, >> > not a procedure for measuring the 10 dB S+N/N ratio. The earlier Army >> > manual TM 11-856A in paragraph 166 has what it calls an AM Sensitivity >> > measurement procedure. However, there are at least two things wrong with >> > it: (1) a DA-121/U attenuator (8.9 dB) two way match (52.2 ohms to 128.8 >> > ohms) is used between the URM-25D and R-390A, and (2) the 0.8 volt noise >> > indication in step (f.) is not maximized with the antenna trimmer, nor is >> > its value checked after the signal generator is adjusted for 2.5 volts, as >> > it must be. >> > >> > Here is a correct method for measuring the AM sensitivity of an R-390A. >> > >> > I measured the real component of the R-390A antenna input impedance by >> > connecting a 250 ohm 2 watt Clarostat composition pot in the signal path, >> > and used a UG-971/U (twinax to C) and UG-636AU (C to BNC). The 10X scope >> > probe was connected across the UG-636. The URM-25D was set to some >> > convenient value that could be seen on the scope. The signal was peaked >> > (as seen on the scope) using the 390A antenna trimmer. The pot was >> > adjusted so that the scope read half the open circuit voltage (the voltage >> > from the antenna input side of the pot when disconnected from the antenna >> > input). The value of the pot was read using an accurate voltmeter, call >> > this value R1. The R-390A antenna input resistance is R = R1 + 50 at that >> > frequency. >> > >> > I may have gotten the high end numbers a little too high previously. My >> > scope method is probably not all that accurate because there is quite a >> > bit of uncertainty as to the half the open circuit voltage. A true RMS >> > voltmeter might be better. Now I am getting 180 - 220 ohms for the high >> > values. Previously I got up to 300 ohms. The low values still come in >> > around 90 - 100 ohms. Low values were found at 1.001, 1.999, and 3.999 >> > MHz. High values were found at 1.5, 4.5, and 5.5 MHz. >> > >> > >> > Dallas Lankford, 2002 >> > >> > >> > There is more to the above paper which can be found on James A. Moores >> > site: >> > https://jamminpower.org/noise.html >> > >> > >> > >> > Regards, Jim >> > >> > >> > Logic: Method used to arrive at the wrong conclusion, with confidence. >> > Murphy >> > >> > >> > On Sunday, October 27, 2024 at 04:53:11 PM CDT, Bob kb8tq <kb...@n1k.org >> > <mailto:kb...@n1k.org>> wrote: >> > >> > >> > Hi >> > >> > If you take a look at the info that was posted several weeks back: >> > >> > The input impedance goes into the “many hundreds” of ohms and well below >> > 100 ohms. That’s after using the trim cap. >> > >> > We’re not talking about +/- 10% here. We’re talking about “near open >> > circuit” down to “pretty good match for 50 ohms”. >> > >> > Bob >> > >> >> On Oct 27, 2024, at 12:59 PM, Jim Whartenby <old_ra...@aol.com >> >> <mailto:old_ra...@aol.com>> wrote: >> >> >> >> Bob >> >> >> >> I am sure that you are correct, the input impedance to the R-390 has a >> >> tolerance so the balanced input varies around 125 ohms. There should be >> >> enough of an adjustment in the "Antenna Trim" capacitor to get >> >> comfortably close to 125 ohms or even close to 50 ohms, at any frequency. >> >> >> >> >> >> >> >> But that was not the original issue. The test spec is not an assumption, >> >> it is a mandate. It says to use the DA-121 in order to comply with the >> >> test spec. So if you wanted to sell R-390s to the Signal Corps, you had >> >> to pass their test spec as written and as witnessed by the government >> >> source inspector. >> >> >> >> >> >> This discussion started with a challenge to this test spec. The RF input >> >> to the R-390, according to the spec, was higher then what the current >> >> owners experience with their receivers. All I did was to point out was >> >> that the DA-121 has an insertion loss which was not overtly accounted for >> >> in the spec. Ever since then, the insertion loss calculation has taken >> >> over the discussion. Most have stated that the DA-121 insertion loss is >> >> 5 dB which appears to be true if the impedance change from 50 to 125 ohms >> >> is not taken into account. >> >> >> >> >> >> Look at the R-390 "Pearls of Wisdom": >> >> https://www.r-390a.net/Pearls/sensitivity_alignment.pdf , check out pdf >> >> page 684. >> >> >> >> "Because the DA-121 impedance adapter is a resistance 'L' pad and as such >> >> the output voltage is a percentage of the input voltage (45% of input, >> >> 9.1 db loss, or 4/9 of the input) for this adapter (each adapter is >> >> different). EG: if the sig gen reads 1 uV out, the voltage seen at the rx >> >> will be 0.45 uV. Of course, this is only true when the actual input >> >> impedance of the R-390A balanced antenna connection is 125 ohms. Well, as >> >> you probably know, the impedance varies from about 50 ohms to about 200 >> >> ohms, depending on the received frequency. Although I have not calculated >> >> it, I believe the variance is small enough to not make much difference. >> >> Regards, Larry" It should be pointed out that the 45% mentioned above >> >> should actually be 55% or 5/9 of the input voltage. Using the voltage >> >> divider rule, 125 ohms divided by (125 + 100 ohms) = 0.556. So the 0.45 >> >> uV mentioned above should actually be 0.55 uV. >> >> >> >> >> >> To account for the DA-121 insertion loss, one can multiply the signal >> >> generator attenuator dB reading by a little over 0.1 to get the actual >> >> power applied to the R-390 input. If the DA-121 insertion loss was 10 dB >> >> then the conversion factor would be exactly 0.1 but the DA-121 is a bit >> >> less. To convert the SG voltage reading to what the R-390 "sees" the >> >> conversion factor is 0.55 times the SG voltage. >> >> >> >> >> >> As I have tried to point out, dB is always a power measurement. Power >> >> equals the square of the voltage ratio IF and only IF the input and >> >> output resistances are the same. When they are not the same then one has >> >> to use 10 log ((Vout / Rout) / (Vin / Rin)) to find the insertion loss >> >> (or gain) of the network. >> >> >> >> >> >> I have read the above over perhaps a dozen times so hopefully any >> >> mistakes I made today have been caught. >> >> >> >> Regards, Jim >> >> >> >> >> >> Logic: Method used to arrive at the wrong conclusion, with confidence. >> >> Murphy >> >> >> >> >> >> On Sunday, October 27, 2024 at 09:09:46 AM CDT, Bob kb8tq <kb...@n1k.org >> >> <mailto:kb...@n1k.org>> wrote: >> >> >> >> >> >> Hi >> >> >> >> We know that the R390() does not supply a 125 ohm load to the antenna >> >> or the test setup. Based on doc’s shown earlier, it typically is way off >> >> from >> >> 125 ohms. >> >> >> >> This is not at all uncommon in the world of receivers. >> >> >> >> Thus the *assumption* that the radio supplies a 125 ohm load is suspect. >> >> >> >> Welcome to why “1 uV” out of any signal generator probably is not what >> >> the >> >> input to the radio actually has applied to it. >> >> >> >> Do people head off and work out what’s “really there”? You could work it >> >> out >> >> various ways. That’s not how the spec on the radio is written. If the >> >> signal >> >> generator says 1 uV that’s the correct number to use. >> >> >> >> How is this relevant? >> >> >> >> If I hook up a 50 ohm generator directly to the input of the R390(), it >> >> is running >> >> from a 50 ohm source. Based on the doc’s shown a wile back, the input to >> >> the radio is *always* higher than 50 ohms (and often by quite a bit). >> >> Loading >> >> will have a very different impact on that 50 ohm source than on a 125 ohm >> >> source. >> >> >> >> If you *do* want to work this out in the “real case” ( = radio hooked up) >> >> *and* you want to do it only based only on power : You have a whole lot >> >> of work >> >> to do. One (as yet unmentioned) part of that is the input to the radio >> >> has a reactive >> >> component. That messes a bit with power math. >> >> >> >> Bob >> >> >> >> > On Oct 27, 2024, at 9:40 AM, Larry Haney <larry41...@gmail.com >> >> > <mailto:larry41...@gmail.com> <mailto:larry41...@gmail.com >> >> > <mailto:larry41...@gmail.com>>> wrote: >> >> > >> >> > Jim, I agree with this posting of yours *except* for the 1st and last >> >> > statements. >> >> > >> >> > 1. First you said: 'What has been overlooked is that there is an >> >> > impedance >> >> > transformation from 50 to 125 ohms.' We are all very aware of this >> >> > fact. >> >> > >> >> > 2. Lastly you said: 'To convert the* SG voltage output* into the >> >> > voltage >> >> > actually seen by the R-390, multiply the SG reading by *0.1235* or >> >> > divide >> >> > the SG reading by 8.097, either way works.' That is *not right* at all. >> >> > You just went through a nice step by step explanation about how to >> >> > determine the power loss, then you use that power loss ratio (0.1235) to >> >> > determine the voltage seen by the 390. *Wrong, wrong, wrong.* The >> >> > last 3 >> >> > steps in your procedure are: 1. dB = 10 Log ^ (.00247watts / 0.02 >> >> > watts), >> >> > 2. dB = 10 Log ^ 0.1235, 3. dB = -9.083. *No real disagreement there*. >> >> > The input watts to the da-121 = 0.02 watts, the output watts from the >> >> > da-121 = .00247 watts, that's a 12.35% loss of *power* in watts, not >> >> > voltage. You *can not* use the 0.1235 *power loss* relationship to >> >> > directly calculate the *voltage loss* relationship of the da-121 as you >> >> > are >> >> > doing in your last statement. >> >> > >> >> > One way to correctly calculate the voltage coming out of the da-121 >> >> > (Vout), >> >> > would be to use the formula: >> >> > >> >> > Vout = Sqr rt (Pout (watts) x impedance (ohms)) >> >> > >> >> > Where Pout is the power coming out of the da-121 (in this case, 0.00247 >> >> > watts) and impedance is the da-121 load impedance provided by the 390, >> >> > 125 >> >> > ohms. >> >> > >> >> > Vout = Sqr rt (.00247 x 125) = 0.5556 Volts >> >> > >> >> > .00247 x 125 = 0.30875 >> >> > Sqr rt 0.30875 = 0.5556 >> >> > Vout = 0.5556 volts >> >> > >> >> > Vout is what's going into the 390 (in this scenario). >> >> > >> >> > Regards, Larry >> >> > >> >> > >> >> > -------------------------------------------------------------------------------------------------------------------------------------------------- >> >> > On Wed, Oct 23, 2024 at 9:35 AM Jim Whartenby <old_ra...@aol.com >> >> > <mailto:old_ra...@aol.com> <mailto:old_ra...@aol.com >> >> > <mailto:old_ra...@aol.com>>> wrote: >> >> > >> >> >> What has been overlooked is that there is an impedance transformation >> >> >> from >> >> >> 50 to 125 ohms. Any calculation that ignores this transformation is in >> >> >> error. The only solution that accounts for different impedances is by >> >> >> looking at the respective powers at both input and output. >> >> >> >> >> >> >> >> >> 1 volt into the DA-121 gives 0.556 volts out. Looking at the power-in >> >> >> verses power-out using the respective impedances: >> >> >> >> >> >> >> >> >> Power = voltage squared / resistance >> >> >> >> >> >> Pin = 1 volt ^2 / 50 ohms = 0.02 watts >> >> >> >> >> >> Pout = 0.556 volt ^2 / 125 ohms = .00247 watts >> >> >> >> >> >> dB = 10 Log ^ (Pout / Pin) >> >> >> >> >> >> dB = 10 Log ^ (.00247watts / 0.02 watts) >> >> >> >> >> >> dB = 10 Log ^ 0.1235 >> >> >> >> >> >> dB = -9.083 >> >> >> >> >> >> >> >> >> To convert the SG voltage output into the voltage actually seen by the >> >> >> R-390, multiply the SG reading by 0.1235 or divide the SG reading by >> >> >> 8.097, >> >> >> either way works. >> >> >> >> >> >> >> >> >> Regards, 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 <mailto:R-390@mailman.qth.net> >> >> > >> >> > This list hosted by: http://www.qsl.net <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 <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
