Does mailman accept HTML posts or just plain text? I ask because lately, some of my posts either don't make it back to me or are very, very delayed. I sent one around 8:00 CDT this morning and it had two words italicized in it and I still have not seen it come back. Now I'm wondering if the italics could be a problem.
Thanks, Barry - N4BUQ On Mon, Oct 7, 2024 at 11:42 AM Jim Whartenby via R-390 < r-390@mailman.qth.net> wrote: > Well, mailman not only strips photos but also any changes in typeface so > the larger type and bold letters are stripped as well. > Logic: Method used to arrive at the wrong conclusion, with confidence. > Murphy > > On Monday, October 7, 2024 at 10:53:50 AM CDT, Jim Whartenby via R-390 > <r-390@mailman.qth.net> wrote: > > GianniComments in BOLD > > On Monday, October 7, 2024 at 02:43:53 AM CDT, Ing. Giovanni Becattini > <giovanni.becatt...@icloud.com> wrote: > > Hi Jim and thanks for your patience, but I still don’t understand. > The generator sees 50 // (100+125), 50 // 225 = 52.22 ohm No, this > should be 40.9 ohms not 52.22 ohms! > R in parallel = 1/ (1/50 + 1/ 225) so 1/ (1/50 + 1/225) = 40.9 ohms or > if you prefer > > > product over sum = (50 X 225) / (50 + 225) = 11,250 / 275 = 40.9 ohms > > > The result of two resistors combined in parallel will always be a lower > value then the lowest of the two combined resistors. > Going a step further, 40.9 ohms in parallel with the Signal Generator > impedance of 50 ohms results in an impedance of 22.5 ohms so the SG now > sees about half of the expected impedance. You can think of the SG as > being a current source feeding a 50 ohm resistor. If the current source > now sees half the expected impedance, the voltage output of the SG will now > be half of the set voltage. > > Now applying the voltage divider rule to the series 100 ohm and shunt 125 > ohm resistors, the voltage across the 125 ohm resistor will be the SG > voltage X (125 / 225) = SG voltage X 0.55 We already know that the signal > generator output is half of what the SG attenuator says so 0.5 X 0.55 = > 0.27 so the output of the DA-121 is now approximately one fourth of the SG > dial setting. > I checked with the VOM using a 125 ohm terminator instead of the R-390A > and read 52.4 ohm. You must have a wiring error! The DA-121 should read > approximately 40.9 ohms at the SG terminals when the DA-121 output is > terminated with 125 ohms. This is what was calculated above. If you now > put a 50 ohm resistor across the DA-121 input, the resistance of the input > to the DA-121 should measure approximately 25 ohms. > To be sure that I was not tricked by the cables, I made the same test at > 100 kHz with 10 mV and that below is what I read, again using the 125 ohm > terminator on the oscilloscope side. > Probably I am doing something wrong, but what? > Your experimental data should closely agree with the math, it does not. > There is at least a simple wiring error or the BNC to TWINAX adapter is not > wired properly. As I mentioned in the email below, one of the TWINAX pins > should be directly connect to the BNC center conductor, the other TWINAX > pin should be directly connect to the shell of the BNC connector. There > should not be any measurable resistance, ideally a short circuit for both > ohmmeter readings. Can you verify this? > Thanks again > Gianni > Regards, > Jim > > > > > Il giorno 6 ott 2024, alle ore 17:05, Jim Whartenby <old_ra...@aol.com> > ha scritto: > Gianni > > > There is something wrong with your measurements. They do not agree with > the mathematical analysis. > > > > > > Reducing the resistances of the DA-121 with the input resistance of the > R-390 to a single resistance results in the total resistance seen by the SG > of 25 ohms. So the generator output should fall from 10 mV to 5 mV which > you confirm although there is an error of some 14% ((5.7 mV - 5 mV) / 5 > mV). But as you say, the resistors are not perfect. > > > What is apparently the problem is that your adapter from BNC to TWINAX > does not measure correctly. One TWINAX pin should connect to the BNC > center pin and the other TWINAX pin should connect to ground. If this does > not happen, the second voltage divider, the 100 ohm in series with the 125 > ohm is not connected to ground. This error would give you the voltage that > you measure. > > > There is agreement between us that when the 68 ohm resistor is connected > to the SG that the output will fall from 10 mV to about 5 mV. Putting the > two remaining resistors into the circuit results in a series 100 ohm > resistor and a parallel 125 ohm resistor. Applying voltage divider > analysis to this we have (5 mV X 125 ohms) / 225 ohms) which equals 2.28 > mV. 2.28 mV divided by 10 mV gives a ratio of 0.23 which is in agreement > with the DA-121 reducing the SG output from 10 mV to 2.5 mV or 4:1. > > > The above analysis agrees completely with figure 3, the analysis of a > T-pad, which was done in the 1950s. It changes the SG impedance of 50 ohms > to the receiver impedance of 72 ohms with a voltage loss of 4:1 which I > again enclose in this email. > > > Regards, > > Jim > Logic: Method used to arrive at the wrong conclusion, with confidence. > Murphy > > On Sunday, October 6, 2024 at 01:46:02 AM CDT, Ing. Giovanni Becattini > <giovanni.becatt...@icloud.com> wrote: > > Hi Jim,Setting rge SG to 10 mV I have1) with no terminator oscilloscope > side: 20 mV2) with 50-ohm terminator: 10 mV3) with DA-121 no terminator: > 11.4 mV4) with DA-121 and 125 ohm terminator (which simulates the > receiver): 5.7 mV > > exactly as I would expect. Now I am going to pickup another generator to > see if it behaves like the 8640. > In the afternoon I tell you the result of the test. > YoursGianni > > Il giorno 6 ott 2024, alle ore 00:00, Jim Whartenby <old_ra...@aol.com> > ha scritto: > > Giovanni > > If you measure twice the voltage with no load on the SG then the actual > voltage when the SG is properly loaded with a 50 ohm termination, what does > the meter read when you put a 25 ohm resistor on the SG output? It should > now read a third of the unterminated SG voltage. > > > > > Enclosed is page 51 of the Measurements catalog. Figure 3 shows a T pad > to match 50 ohms to 72. The resistor values are chosen to reduce the SG > output voltage by half at the input to the T pad and to 1/4 at the output > of the T pad when the T pad is terminated with a 72 ohm resistor. > > > > > The same is done with the DA-121 but the impedance transformation is now > from 50 to 125 ohms. Can you measure the voltages at the output of the SG > with an oscilloscope? It should be 2X of the SG meter reading with no load > on the SG, 1X with a 50 ohm load and 1/4X of the SG meter at the output of > the DA-121 when the DA-121 is terminated with a 125 ohm non inductive > resistor in place of the R-390A. If you do not terminate the DA-121 with a > 125 ohm load then what you report as 0.56 of the SG meter reading would be > correct. > > Regards, > > Jim > > > Logic: Method used to arrive at the wrong conclusion, with confidence. > Murphy > > On Saturday, October 5, 2024 at 03:14:58 PM CDT, Ing. Giovanni > Becattini <giovanni.becatt...@icloud.com> wrote: > > 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 > > > > > > > > > ______________________________________________________________ > 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
