[R-390] Official specs

Wed Oct 9 15:29:03 EDT 2024

Hi Gianni,

When I used the 4.9dB figure, it was just to indicate by how much dB (if the SG output adjustment is on dBs only) the output signal should be raised on the SG to “match” the desired output level (10mV in the example previously used).

However, this is an improper use of the dB “rule” in this case because the DA-121 input and output impedances do not match, and that 4.9dB figure cannot be translated as a power ratio (10 log Pout/Pin).

The real power ratio using a DA-121 following the SG output is (calculated) -8.897dB (output power to the 125 ohms load being 0.1289 of the power delivered to the DA-121 input).

For the DA-121, it seems that it was designed to use standard resistor values only and be close to match both the 125 ohms load and the SG output, but not precisely.

If the 68 ohms in the DA-121 is changed to 50 ohms, the “equivalent” generator feeding the 125 ohms load will be (from the previously used example again) 10mV EMF with a 125 ohms output impedance, so a perfect match on the load side (5mV loaded: maximum power transfer) but not on the SG side.

As Jim already pointed out, the DA-121 input will behave as a 40.91 ohms load.

I can already hear some readers yelling at me to have created a 1.222 to 1 SWR on the cable between the SG and the modified DA-121, but I believe that the SG will don’t care…

But, OK, a properly designed matching pad should present the right values of impedance on both sides by attenuating the output vs input signal by a specified amount (50 ohms input, 125 ohms output, -6dB).

There is two solutions for this (see attached .pdf) if the target is to have 0.5 of the input amplitude for the output (improperly -6dB again).

>From the computed values required, it can be understood that the problem will now be to procure (or have those made on purpose) the needed resistor values.

73, Jacques, VE2JFE in Montreal

De : Ing. Giovanni Becattini <giovanni.becattini at icloud.com> 
Envoyé : 9 octobre 2024 04:14
À : Jacques Fortin <jacques.f at videotron.ca>
Cc : Bob Camp <kb8tq at n1k.org>; Jim Whartenby <old_radio at aol.com>; r-390 at mailman.qth.net
Objet : Re: [R-390] Official specs
Importance : Haute

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 <jacques.f at videotron.ca <mailto:jacques.f at 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 <kb8tq at n1k.org <mailto:kb8tq at 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 at mailman.qth.net <mailto:r-390 at 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 <jacques.f at videotron.ca <mailto:jacques.f at 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-bounces at mailman.qth.net <mailto:r-390-bounces at mailman.qth.net>  <r-390-bounces at mailman.qth.net <mailto:r-390-bounces at mailman.qth.net> > 
De la part de Jim Whartenby via R-390 Envoyé : 7 octobre 2024 12:42 
Cc : r-390 at mailman.qth.net <mailto:r-390 at 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 at mailman.qth.net <mailto: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 <mailto: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 <mailto: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 <mailto: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 <mailto: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 <mailto: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 <mailto: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 <mailto: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

______________________________________________________________
R-390 mailing list
Home: http://mailman.qth.net/mailman/listinfo/r-390
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:R-390 at 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 at 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 at 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 at mailman.qth.net

This list hosted by: http://www.qsl.net Please help support this 
email list: http://www.qsl.net/donate.html

-------------- next part --------------
A non-text attachment was scrubbed...
Name: 50to125MatchingPads.pdf
Type: application/pdf
Size: 2237783 bytes
Desc: not available
URL: <http://mailman.qth.net/pipermail/r-390/attachments/20241009/4df2746c/attachment-0001.pdf>