[R-390] Official specs

Jim Whartenby old_radio at aol.com
Mon Oct 28 11:34:27 EDT 2024 

As hard as I have tried, I feel that I have not made the issue clear.  The dB is defined as a power measurement.  The formula 10 log (Pout / Pin) when solved for the DA-121 finds an insertion loss to be 9 dB.  When the voltage form of the same equation (20 log (Vout / Vin) is used, the DA-121 insertion loss is now 5.1 dB.  They both cannot be right so which insertion loss is correct?

Langford may not be using the DA-121 but he is attempting to properly terminating the SG with an additional resistance to match the R-390 measured real input impedance.  In doing this he found that the listed 125 ohm number is close to what he measured.  Knowing the actual voltage measured at the 125 ohm port and the total resistance needed to do the impedance match, he could then solve for the SG output voltage in the 50 ohm system needed to do the test.  When he did his impedance test, the high impedance was found to be 220 ohms and the low was 90 ohms.  This is not all that shabby for the time when the R-390 was designed.  

Even with his "improved" measurement method, Langford was still not convinced that this accuracy is sufficiently close to define the actual R-390 125 ohm input resistance.  Of course we now have, some 70 years later, the tiny spectrum analyzer that replaces perhaps a $50k+ piece of test equipment for the cost of a family's meal at McDonalds.  As mentioned by others, the use of a balun to convert 50 to 125 ohms would give higher accuracy results.

I'm off to do other things, too much time has been spent on this narrowly focused discussion,  Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Monday, October 28, 2024 at 05:17:51 AM CDT, Larry Haney <larry41gm2 at gmail.com> wrote:   

 Nice find, Jim.  Dr. Lankford's procedure does not use an impedance adapter to facilitate the measurements, but in his method that's OK because he measures the signal voltage level with an accurate scope at the input of the 390.  I assume that he is using the dmm optional package to measure RMS voltage or converting p-p to rms, because his measurements are in the ball park, however he should have clarified that.  The important aspect is that he's measuring the voltage at the input of the 390.
Regards, Larry
On Sun, Oct 27, 2024 at 5:42 PM Jim Whartenby via R-390 <r-390 at mailman.qth.net> wrote:

Bob
How does the data form a few weeks ago compare to the following from some 20 years ago?


Dallas Lankford on ReceiverSensitivity Measurement 




This note appearedin the R-390 email reflector on QTH. It is the most cogent discussionof receiver sensitivity I have ever seen. I reproduce it here (withpermission) with minor typographical corrections:  
James A. Moorer





"There has beena lot of confusion about how to measure the AM sensitivity of anR-390A. Unfortunately the manuals have contributed to this confusion.The 1970 Navships 0967-063-2010 manual has a sensitivity measuringprocedure on pages 4-2 and 4-3 which involves setting the signalgenerator (URM-25D) to minimum output. This is equivalent to themethod of turning the signal generator on and off which is used atseveral web sites to find the 10 dB S+N/N ratio. However, theNavships manual does not mention a 10 dB S+N/N ratio, but rather a 10dB rise, which it is. What the Navships and web sites measure is the10 dB S+N1/N2 where N1 is the noise due to the signal and receiver,and N2 is the no-signal receiver noise. Also, the 50 ohm impedance ofthe signal generator is not matched to the 125 ohm nominal (100 - 300ohms) antenna input impedance (through a UG-636A/U and UG-971/U) ofthe R-390A. Consequently, the signal generator reading is not thenumber of microvolts that appears across the R-390A antenna input.The Army manual TM 11-5820-358-35 gives a Sensitivity Test, not aprocedure for measuring the 10 dB S+N/N ratio. The earlier Armymanual TM 11-856A in paragraph 166 has what it calls an AMSensitivity measurement procedure. However, there are at least twothings wrong with it: (1) a DA-121/U attenuator (8.9 dB) two waymatch (52.2 ohms to 128.8 ohms) is used between the URM-25D andR-390A, and (2) the 0.8 volt noise indication in step (f.) is notmaximized with the antenna trimmer, nor is its value checked afterthe signal generator is adjusted for 2.5 volts, as it must be.




Hereis a correct method for measuring the AM sensitivity of an R-390A.




I measured the realcomponent of the R-390A antenna input impedance by connecting a 250ohm 2 watt Clarostat composition pot in the signal path, and used aUG-971/U (twinax to C) and UG-636AU (C to BNC). The 10X scope probewas connected across the UG-636. The URM-25D was set to someconvenient value that could be seen on the scope. The signal waspeaked (as seen on the scope) using the 390A antenna trimmer. The potwas adjusted so that the scope read half the open circuit voltage(the voltage from the antenna input side of the pot when disconnectedfrom the antenna input). The value of the pot was read using anaccurate voltmeter, call this value R1. The R-390A antenna inputresistance is R = R1 + 50 at that frequency.



I may have gottenthe high end numbers a little too high previously. My scope method isprobably not all that accurate because there is quite a bit ofuncertainty as to the half the open circuit voltage. A true RMSvoltmeter might be better. Now I am getting 180 - 220 ohms for thehigh values. Previously I got up to 300 ohms. The low values stillcome in around 90 - 100 ohms. Low values were found at 1.001, 1.999,and 3.999 MHz. High values were found at 1.5, 4.5, and 5.5 MHz.

Dallas Lankford, 2002

There is more to the above paper which can be found on James A. Moores  site:  https://jamminpower.org/noise.html


Regards,  Jim

Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Sunday, October 27, 2024 at 04:53:11 PM CDT, Bob kb8tq <kb8tq at n1k.org> wrote:   

 Hi
If you take a look at the info that was posted several weeks back:
The input impedance goes into the “many hundreds” of ohms and well below 100 ohms. That’s after using the trim cap. 
We’re not talking about +/- 10% here. We’re talking about “near open circuit” down to “pretty good match for 50 ohms”.
Bob


On Oct 27, 2024, at 12:59 PM, Jim Whartenby <old_radio at aol.com> wrote:
Bob
I am sure that you are correct, the input impedance to the R-390 has a tolerance so the balanced input varies around 125 ohms.  There should be enough of an adjustment in the "Antenna Trim" capacitor to get comfortably close to 125 ohms or even close to 50 ohms, at any frequency.  

But that was not the original issue.  The test spec is not an assumption, it is a mandate.  It says to use the DA-121 in order to comply with the test spec.  So if you wanted to sell R-390s to the Signal Corps, you had to pass their test spec as written and as witnessed by the government source inspector.

This discussion started with a challenge to this test spec.  The RF input to the R-390, according to the spec,  was higher then what the current owners experience with their receivers.  All I did was to point out was that the DA-121 has an insertion loss which was not overtly accounted for in the spec.  Ever since then, the insertion loss calculation has taken over the discussion.  Most have stated that the DA-121 insertion loss is 5 dB which appears to be true if the impedance change from 50 to 125 ohms is not taken into account.

Look at the R-390 "Pearls of Wisdom": https://www.r-390a.net/Pearls/sensitivity_alignment.pdf , check out pdf page 684.
"Because the DA-121 impedance adapter is a resistance 'L' pad and as such the output voltage is a percentage of the input voltage (45% of input, 9.1 db loss, or 4/9 of the input) for this adapter (each adapter is different). EG: if the sig gen reads 1 uV out, the voltage seen at the rx will be 0.45 uV. Of course, this is only true when the actual input impedance of the R-390A balanced antenna connection is 125 ohms. Well, as you probably know, the impedance varies from about 50 ohms to about 200 ohms, depending on the received frequency. Although I have not calculated it, I believe the variance is small enough to not make much difference. Regards, Larry"    It should be pointed out that the 45% mentioned above should actually be 55% or 5/9 of the input voltage.  Using the voltage divider rule, 125 ohms divided by (125 + 100 ohms) = 0.556.  So the 0.45 uV  mentioned above should actually be 0.55 uV.  

To account for the DA-121 insertion loss, one can multiply the signal generator attenuator dB reading by a little over 0.1 to get the actual power applied to the R-390 input.  If the DA-121 insertion loss was 10 dB then the conversion factor would be exactly 0.1 but the DA-121 is a bit less.  To convert the SG voltage reading to what the R-390 "sees" the conversion factor is 0.55 times the SG voltage.

As I have tried to point out, dB is always a power measurement.  Power equals the square of the voltage ratio IF and only IF the input and output resistances are the same.  When they are not the same then one has to use 10 log ((Vout / Rout) / (Vin / Rin)) to find the insertion loss (or gain) of the network.

I have read the above over perhaps a dozen times so hopefully any mistakes I made today have been caught.
Regards,  Jim

Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Sunday, October 27, 2024 at 09:09:46 AM CDT, Bob kb8tq <kb8tq at n1k.org> wrote:   

 Hi

We know that the R390() does not supply a 125 ohm load to the antenna 
or the test setup. Based on doc’s shown earlier, it typically is way off from
125 ohms. 

This is not at all uncommon in the world of receivers. 

Thus the *assumption* that the radio supplies a 125 ohm load is suspect. 

Welcome to why “1 uV” out of any signal generator probably is not what the 
input to the radio actually has applied to it. 

Do people head off and work out what’s “really there”? You could work it out
various ways.  That’s not how the spec on the radio is written. If the signal 
generator says 1 uV that’s the correct number to use. 

How is this relevant?

If I hook up a 50 ohm generator directly to the input of the R390(), it is running
from a 50 ohm source. Based on the doc’s shown a wile back, the input to
the radio is *always* higher than 50 ohms (and often by quite a bit). Loading
will have a very different impact on that 50 ohm source than on a 125 ohm 
source. 

If you *do* want to work this out in the “real case” ( = radio hooked up) 
*and* you want to do it only based only on power : You have a whole lot of work 
to do. One (as yet unmentioned) part of that is the input to the radio has a reactive 
component. That messes a bit with power math. 

Bob

> On Oct 27, 2024, at 9:40 AM, Larry Haney <larry41gm2 at gmail.com> wrote:
> 
> Jim,  I agree with this posting of yours *except* for the 1st and last
> statements.
> 
> 1.  First you said: 'What has been overlooked is that there is an impedance
> transformation from 50 to 125 ohms.'  We are all very aware of this fact.
> 
> 2.  Lastly you said: 'To convert the* SG voltage output* into the voltage
> actually seen by the R-390, multiply the SG reading by *0.1235* or divide
> the SG reading by 8.097, either way works.'  That is *not right* at all.
> You just went through a nice step by step explanation about how to
> determine the power loss, then you use that power loss ratio (0.1235) to
> determine the voltage seen by the 390.  *Wrong, wrong, wrong.*  The last 3
> steps in your procedure are: 1. dB = 10 Log ^ (.00247watts / 0.02 watts),
> 2. dB = 10 Log ^ 0.1235, 3. dB = -9.083.  *No real disagreement there*.
> The input watts to the da-121 = 0.02 watts, the output watts from the
> da-121 = .00247 watts, that's a 12.35% loss of *power* in watts, not
> voltage.  You *can not* use the 0.1235 *power loss* relationship to
> directly calculate the *voltage loss* relationship of the da-121 as you are
> doing in your last statement.
> 
> One way to correctly calculate the voltage coming out of the da-121 (Vout),
> would be to use the formula:
> 
> Vout = Sqr rt (Pout (watts) x impedance (ohms))
> 
> Where Pout is the power coming out of the da-121 (in this case, 0.00247
> watts) and impedance is the da-121 load impedance provided by the 390, 125
> ohms.
> 
> Vout = Sqr rt (.00247 x 125) = 0.5556 Volts
> 
>    .00247 x 125 = 0.30875
>    Sqr rt  0.30875 = 0.5556
>    Vout = 0.5556 volts
> 
> Vout is what's going into the 390 (in this scenario).
> 
> Regards, Larry
> 
> 
> --------------------------------------------------------------------------------------------------------------------------------------------------
> On Wed, Oct 23, 2024 at 9:35 AM Jim Whartenby <old_radio at aol.com> wrote:
> 
>> What has been overlooked is that there is an impedance transformation from
>> 50 to 125 ohms.  Any calculation that ignores this transformation is in
>> error.  The only solution that accounts for different impedances is by
>> looking at the respective powers at both input and output.
>> 
>> 
>> 1 volt into the DA-121 gives 0.556 volts out.  Looking at the power-in
>> verses power-out using the respective impedances:
>> 
>> 
>> Power = voltage squared / resistance
>> 
>> Pin = 1 volt ^2 / 50 ohms = 0.02 watts
>> 
>> Pout = 0.556 volt ^2 / 125 ohms = .00247 watts
>> 
>> dB = 10 Log ^ (Pout / Pin)
>> 
>> dB = 10 Log ^ (.00247watts / 0.02 watts)
>> 
>> dB = 10 Log ^ 0.1235
>> 
>> dB = -9.083
>> 
>> 
>> To convert the SG voltage output into the voltage actually seen by the
>> R-390, multiply the SG reading by 0.1235 or divide the SG reading by 8.097,
>> either way works.
>> 
>> 
>> Regards, Jim
>> 
>> Logic: Method used to arrive at the wrong conclusion, with confidence.
>> Murphy
>> 
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