Hi Jacques, Thanks, I followed you perfectly. The “3 dB” was a typo by me, I thought 5 and wrote 3 (well 4.9). It is exactly what I was interested to demonstrate (and what the experience says).
Is it correct to say that the DA-121 reduced the signal power by 4.9 dB [20 log (V2/V1)] and the signal voltage by 2.45 dB [10 log (V2/V1)]? The last doubt I have is why should I change the 68 ohm to 50 in the DA-121? Very kind from you all! > Il giorno 8 ott 2024, alle ore 17:30, Jacques Fortin <jacque...@videotron.ca> > ha scritto: > > Hello Gianni, > PLEASE READ MY COMMENTS IMBEDDED IN YOUR TEXT BELOW. > I USE THE UPPERCASE TEXT BECAUSE EVERY OTHER FONT ATTRIBUTE IS CLEARED UP BY > THE FORUM SERVER. > > 73, Jacques, VE2JFE in Montreal > > Hi Friends, forgive the savage who tries to understand how the clock works :) > What you Bob say is exactly what I would like to understand, i.e., if the > incredibly high sensitivities I heard are real or fantasy. > Let's imagine that we are in a perfect word and neglect the frequency effect. > All the discussion started, from my side, to determine the attenuation factor > of the DA-121. So I try to better focus my questions: > > 1) if I have a siggen with 50 ohm output and an rx with 50 ohm input, is it > correct to say that the voltage at the receiver’s input is that pointed out > by the generator’s knob? > > EXACT: MOST GENERATORS INDICATES THE OUTPUT VOLTAGE WHEN THE EXTERNAL LOAD IS > EQUAL TO THE INTERNAL OUTPUT IMPEDANCE. > (MAXIMUM POWER TRANSFER CASE) WHICH MEANS 50 OHMS FOR MOST OF THE AMERICAN > STUFF BUT 75 OHMS FOR THE OLD BRITISH ONES. > SOME HAVE ALSO (LIKE MY OLD TRUSTY HP 8657A) A SETTING THAT INDICATES THE > "UNLOADED" OUTPUT VOLTAGE AS THE EMF (ELECTROMOTIVE FORCE) VALUE. > TO BE CLEAR, WITH A "NORMAL" RF GENERATOR SET TO DELIVER 10mV IN A 50 OHMS > LOAD, THE INTERNAL "EQUIVALENT" GENERATOR IS A 20mV SOURCE IN SERIES WITH A > 50 OHMS RESISTOR (THE THEVENIN EQUIVALENT OF THE SOURCE, IF YOU LIKE). > > 2) in this specific case, I have a black box, constituted by the DA-121 > (68/100 ohm) and the R-390. It seems that from the generator’s point of view > it is equivalent to a rx with 50 ohm input impedance. So, if I set the > generator for 10 mV output, I have 10 mVrms at the 68 ohm resistor. Right? > > NOT EXACTLY: IF THE OUTPUT OF THE DA-121 IS UNLOADED, 11.525 mV WILL APPEAR > ACROSS THE 68 OHMS RESISTOR AND THE THEVENIN EQUIVALENT OF THE DA-121 OUTPUT > WILL BE A 11.525 mV SOURCE IN SERIES WITH A 128.81 OHMS RESISTOR. > WITH THE DA-121 OUTPUT LOADED BY A 125 OHMS RESISTOR, THE VOLTAGE ACROSS IT > WILL BE 5.676 mV. > COMPARED TO THE GENERATOR "SET" VALUE OF 10mV, THIS IS 4.919 dB lower. > > 3) if above is right, the effective voltage at the receiver’s input (125 ohm) > is the voltage at the 68 ohm reduced by the divider made by 100+125 ohm. > Right? > > RIGHT, IN ALL CASES. > > If so, the loss factor of the DA-121 is 3 dB ( Vin x .56). > 20 log (.56) = - 5 dB, not 3 dB.... > > This can be practically tested, and the voltage at the 125 ohm resistor I am > using instead of the R-390 for simplicity is exactly 0.56 Vin (this is true > also for DC). > > PRETTY CLOSE TO THE THEORICAL VALUE, RIGHT. > > This above is what I can suppose by myself but I understand you are > explaining to me that it is wrong. > > But If you are right, the DA-121 should have a 9 dB loss factor, and this > seems to be completely denied from the practical experience. Again, forget > the frequency effect until this point. > > SAY 4.9dB LOSS. > BUT IF THE DA-121 68 OHMS RESISTOR IS REPLACED BY A 50 OHMS ONE, THE GLOBAL > ATTENUATION (REAL OUTPUT AT THE 125 OHMS LOAD VS THE SG SETTING) WILL BE 6 dB > LOWER (0.5) AND THE DA-121 OUTPUT IMPEDANCE WILL BE 125 OHMS ALSO (MAXIMUM > POWER TRANSFER CASE). > > Thanks for your patience :) > > Gianni > > > > >> Il giorno 8 ott 2024, alle ore 14:58, Bob Camp <kb...@n1k.org> ha scritto: >> >> Hi >> >> Another wrinkle in this: >> >> If your generator is set to 1 uV, that should mean you get 1 uv across a 50 >> ohm load on the generator. The impedance at that point is 25 ohms (50 ohms >> for the load in parallel with 50 ohms from the generator). >> >> If your receiver happens to be a high impedance input (as many are ….) you >> get 2 uV at the input to the radio and the impedance is 50 ohms. >> >> Maybe take out some random load resistor and you just doubled the >> sensitivity. >> >> Why would you do this? (though maybe not at HF …) >> >> When you go from 25 ohms to 50 ohms, the thermal noise from the resistor(s) >> goes up by 3 db. Your signal went up by 6 db. You are now 3 db further above >> the thermal noise floor. >> >> Not a big deal on the typical HF setup. It is a big deal as you go up in >> frequency. Yes this makes some other things you do up there “fun” …. Even at >> HF, folks racing to get that 0.001 uV sensitivity number (or some equally >> absurd number) probably are doing this as well. >> >> So: counting on this or that radio to supply a matched input …. maybe not a >> good idea without testing out the specific radio at the operating frequency. >> >> Bob >> >> >> >>> On Oct 8, 2024, at 3:31 AM, Ing. Giovanni Becattini via R-390 >>> <r-390@mailman.qth.net> wrote: >>> >>> Hi Jim, >>> >>> I am not sure I can follow you. As I told, I am not using the R-390A but >>> just a 125 ohms resistor as a terminator. >>> >>> So: >>> 68 // 225 = 52.21 (seen from the generator) >>> >>> The voltage on the rx is then the siggen’s output voltage divided by >>> 100+125, i.e. Vin x 0.556. In dB, 3 dB, that is what I wanted to know. >>> >>> I tested that also in DC, with the same results. Luckily for this time >>> practice and theory are completely in accord. >>> >>> Things could be different increasing the frequency, obviously, and using >>> the real R-390A, but in my case, @ 7.5 MHz, they are rather similar. >>> >>> Am I wrong? >>> >>>> Il giorno 7 ott 2024, alle ore 22:28, Jacques Fortin >>>> <jacque...@videotron.ca> ha scritto: >>>> >>>> Jim, I do not know if I can follow you correctly in your explanations >>>> below. >>>> How does this DA-121 adaptor is made ?? >>>> >>>> 73, Jacques, VE2JFE in Montreal >>>> >>>> -----Message d'origine----- >>>> De : r-390-boun...@mailman.qth.net <r-390-boun...@mailman.qth.net> >>>> De la part de Jim Whartenby via R-390 Envoyé : 7 octobre 2024 12:42 >>>> Cc : r-390@mailman.qth.net Objet : Re: [R-390] Official specs >>>> >>>> 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 >> > > ______________________________________________________________ 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
