[R-390] Official specs

[Ing. Giovanni Becattini]

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> Il giorno 6 ott 2024, alle ore 17:05, Jim Whartenby <old_radio at 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.becattini at 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_radio at 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.becattini at 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 at 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.becattini at 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
>