[R-390] Official specs

Barry Scott 72volkswagon at gmail.com
Mon Oct 7 12:50:02 EDT 2024 

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 at 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 at mailman.qth.net> wrote:
>
>  GianniComments in BOLD
>
>     On Monday, October 7, 2024 at 02:43:53 AM CDT, Ing. Giovanni Becattini
> <giovanni.becattini at 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_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 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_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
>
>
>
>
>
>
>
>
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