Well, this one came right back into my other email account (I'm subscribed from two accounts). I now wonder if it doesn't like the HTML. I know one list I've been on doesn't like them and perhaps it was this one.
I'll resend my earlier post in plain text and see how that goes. Thanks, Barry - N4BUQ > 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 ______________________________________________________________ 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
