Are images automatically filtered? Perhaps it was too big, I try again
 > 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 have > 1) with no terminator oscilloscope side: 20 mV > 2) with 50-ohm terminator: 10 mV > 3) with DA-121 no terminator: 11.4 mV > 4) 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. > > Yours > Gianni > >> 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
