Hi That’s sort of the point to this. Truth or not, there is a procedure:
Put in this resistor based gizmo. Set the signal generator to 3 uV (or whatever). Is the sensitivity in spec? If so …. move on. Bob > On Oct 27, 2024, at 11:24 AM, Ing. Giovanni Becattini > <giovanni.becatt...@icloud.com> wrote: > > Hi, > > From what I read, the problem seems to have no practical solution, and the > Truth is far away. > > But, more practically, why not to agree on a test set-up and procedure for > R-390A sensitivity measurement? It will not tell us… the Truth, but at least > we could have a common ground to check what I wanted to know when I opened > this post, i.e., if I tuned up correctly my receiver or if I have to insist. > > We will not talk about microvolts but at least of qthmicrovolts…😁 > > Gianni > > > >> Il giorno 27 ott 2024, alle ore 15:09, Bob kb8tq <kb...@n1k.org> ha scritto: >> >> 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> 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> 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 >>> >>> 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
