[R-390] Official specs

Jim Whartenby old_radio at aol.com
Fri Oct 4 23:13:38 EDT 2024 

BobI don't know why you are speculating "Since you can use any signal generator" when MIL-R-13947B(SigC) on page 20 specifies the Measurements Corp Model 82 signal Generator, or it's equal.  Your concern with matched impedances between the generator and the R-390 may appear to be valid but this seems to have been accounted for in the R-390 spec with higher input voltages to account for the mismatch losses.  Collins and the Signal Corps specified the 125 ohm input impedance of the R-390 and they were surely aware that the standard impedance of high end signal generators was normally 50 ohms.

Check out http://hparchive.com/Boonton/BRC-The-Notebook-03.pdf which is a 1954 Boonton Radio explanation for the use of Dummy Antennas.  Everything you need is in the first four pages.  It explains why the mismatch between 50 ohms and the receiver input impedance reduces the loading on the receiver input tuned circuits and recovers the Q of the receiver RF input tuned circuits.  This also affects favorably the S+N/N measurement.  There is a reason for the apparent madness.
Jim

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

    On Friday, October 4, 2024 at 07:23:24 PM CDT, Bob Camp <kb8tq at n1k.org> wrote:   

 Hi

Based on a *lot* of interaction with DOD source inspectors over the years … it’s very much a “that depends” sort of thing. The guy you get this month may be *very* different than than the guy who comes in next month. 

Since you can use any signal generator, there is room for “trouble” with minimal specs. Typically the way this works out is a request to clarify things. Unfortunately that stuff does not get into the official specs. Yes, some of us have pointed that out as a problem …. never got addressed AFIK. 

Bob


> On Oct 4, 2024, at 5:19 PM, Jim Whartenby via R-390 <r-390 at mailman.qth.net> wrote:
> 
> Bob
> You are over thinking the testing.  The generator is terminated but not in it's characteristic impedance.  Collins and the agency letting the contract agreed on a test method and wrote it down in the test procedure.  Anything that would affect the testing like VSWR has already been considered and accounted for.  That Hams don't use the proper termination for the signal generator, according to the spec, which affects the perceived sensitivity is another issue.  Spec is spec as they say.
> 
> Not all Government Source Inspectors are knowable about the equipment that they are reviewing and putting their inspection stamp on.  Not all Signal Corps equipment is inspected by Army GSIs.  Every once in a while, Army equipment will have an anchor stamp on it and vice-versa.  Sometimes certain specs are wavered if not considered critical, most often they are not.  Again, spec is spec.
> Regards,
> Jim
> 
> Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 
> 
>    On Friday, October 4, 2024 at 02:43:36 PM CDT, Bob Camp <kb8tq at n1k.org> wrote:  
> 
> Hi
> 
> Ok …. but …..
> 
> If it is simply a resistor, then the generator is unterminated. It’s designed and calibrated to run into a 50 ohm load. If all that’s there is a series resistor the generator is not properly set up (and thus not calibrated ….). Yes some generators deal with this better than others. 
> 
> My guess is that there’s more to the load circuit than just that resistor. Without a schematic …. who knows ….
> 
> Bob
> 
> 
>> On Oct 4, 2024, at 12:53 PM, Jim Whartenby via R-390 <r-390 at mailman.qth.net> wrote:
>> 
>> Bob
>> But we do know what the interface is between the signal generator and antenna input.  On document page 20, the balanced RF input is in series with a 125 ohm non-inductive resistor and the unbalanced RF input is in series with a 50 pF capacitor.  Both interfaces do not include the generator's output impedance.
>> 
>> As for the sensitivity not being what the R-390/URR or the R-390A/URR is capable of, well there are perhaps 100's of tests that the receiver must successfully pass before it is accepted.  Tightening any of the specs to exactly what the receiver may be capable of passing will guarantee that no one receiver will ever pass all of the acceptance tests.  Then there is the added problem of your test equipment's error tolerance and being traceable back to the NIST standards.  Is your 10 microvolts from the signal generator really 10 microvolts?
>> 
>> Unless otherwise stated, the specification calls out a value that the receiver must do better than.  A sensitivity of just under 6.5 microvolts for a 10 dB S+N/N with an audio power output of 10 milliwatts seems reasonable over the range of 2 to 32 MHz for either balanced or unbalanced RF inputs.  This is perhaps typical for just about all HF receivers built for the military, at least for what I am aware of, but I am sure that there will be the rare exception.
>> 
>> Regards,
>> Jim
>> Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 
>> 
>>    On Friday, October 4, 2024 at 09:39:26 AM CDT, Bob Camp <kb8tq at n1k.org> wrote:  
>> 
>> Hi
>> 
>> The gotcah here is that we really don’t know what the interface between the signal generator and radio looked like for these official tests. Despite the document going into a lot of detail, they did not include a schematic or a part number. (or at least not one that I could find). There are a *lot* of ways they might have been doing things …..
>> 
>> Bob
>> 
>>> On Oct 4, 2024, at 10:19 AM, Ing. Giovanni Becattini via R-390 <r-390 at mailman.qth.net> wrote:
>>> 
>>> But when we perform the classical sensitivity test with a modulated signal, do we get the AM or CW value?
>>> 
>>> Following the instructions I got a value of -104 dBm, i.e. 1.41 uV. Because I used the DA-121, the real value should be even better, far from 5 uV. And surely my R-390A is not as good as it could be .
>>> 
>>> Is my reasoning correct?
>>> 
>>>> Il giorno 4 ott 2024, alle ore 16:13, Barry <n4buq at knology.net> ha scritto:
>>>> 
>>>> I don’t think of 1uV as “bad” but most of these radios will beat that. 
>>>> 
>>>> Thanks,
>>>> Barry
>>>> 
>>>>> On Oct 4, 2024, at 8:55 AM, Ing. Giovanni Becattini via R-390 <r-390 at mailman.qth.net> wrote:
>>>>> 
>>>>> Wow, thanks!
>>>>> 
>>>>> By the way, do you know why the sensitivity values were so bad? Are we the ones who are taking measures that are too lenient or are they the ones who were too conservative?
>>>>> 
>>>>> Thanks again
>>>>> 
>>>>> Gianni
>>>>> 
>>>>>> Il giorno 4 ott 2024, alle ore 15:37, Larry Haney <larry41gm2 at gmail.com> ha scritto:
>>>>>> 
>>>>>> Hi Gianni and Barry,  Tom Marcotte and Al Tirevold (SK) obtained and cleaned up the specs for the 390A and put it on our website.  Here's the link: mil-r-13947b– (r-390a.net) <https://www.r-390a.net/mil-r-13947b.pdf>. Unfortunately, the entry in References page indicates it is for the R390(), but it is also the the 390A.  I will be changing that shortly.
>>>>>> 
>>>>>> Regards, Larry
>>>>>> 
>>>>>>> On Fri, Oct 4, 2024 at 4:49 AM Ing. Giovanni Becattini via R-390 <r-390 at mailman.qth.net <mailto:r-390 at mailman.qth.net>> wrote:
>>>>>>> Have ever been released official specs for the 390A?
>>>>>>> 
>>>>>>> The manuals report few data and neither too correct, for what I can understand (e.g., sensitivity)
>>>>>>> 
>>>>>>> Thanks
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