[R-390] Re: [Milsurplus] Transplanting R-390 IF modules to R-390A

[Mark Huss]

Yes, the beads would not do anything about PTO phase Noise, it was just 
to keep garbage from getting into or out of the PTO. If memory serves, 
there was also a wiring change to the tube, and a few cap changes to try 
to reduce the phase noise. The thing I remember the first time I looked 
under the hood of an R725 was what seemed like hundreds of those beads 
everywhere. There were also contact fingers on the bottoms of the RF and 
Christal Osc chassis. I do know that the mechanical filters do have a 
very adverse effect on what is called Doppler effect DF'ing systems, 
such as the TRR-20. And though comparatively, the R-390 and R-390A PTO's 
do not have that much phase noise, it was really getting bothersome by 
the early eighties for the techniques being developed then. I also know 
that they went through a whole s**tpot of receivers, including Collins, 
trying to find one that had synthesis but less phase noise than the 
R-390A. Racal finally came through, but just barely, which is why they 
won the contract to replace the R-390A.
As to the rest of it, like I said, it could have been our imagination. 
It is a fact that getting an R-390A under 1uVS+N/N at 8kc BW would take 
a while, and getting the R725 to the same S+N/N was normal after the 
first alignment run-through. Mind you, we swept the R-725 while the 
R-390A just got three-point alignment, but what difference that would 
make, I don't know. Like I said, the general consensus was that the 
R-390A IF Deck had a higher noise floor because from the second IF tube 
on, the thing was as wide as a barn door. And yes, we did align them for 
the full 16 kc bandwidth.

Tim Shoppa wrote:
> Mikea wrote:
>   
>> On Fri, Feb 02, 2007 at 12:18:11PM -0500, Mark Huss wrote:
>>     
>>> That they 
>>> also decoupled just about every line out of the PTO and
>>> into the IF Deck  with ferrite beads probably did not hurt either.
>>>       
>
>   
>> Phase noise problems with the PTO? As in just junk
>> on the PTO output line, fixable by ferriting it, or other
>> problems that required changes to the PTO other than
>> just ferrites?
>>     
>
> Noise on an output that is fixable by "ferriting it" is not
> usually called "phase noise". Usually phase noise in a MW
> radio context refers to "close-in phase noise".
>
> Ferrites are usually used to reduce EMI susceptability
> and/or EMI emissions at a frequencies far away from
> the desired frequency.
>
> Several of a 390A's oscillators exhibit FM under B+
> and AGC variations. But again that's usually called
> FM and not "phase noise".
>
> I hesitate to send this e-mail to the list at all because
> I just make matters worse by mentioning actual problems
> in an attempt to get someone to define their mythical
> problem.
>
> Tim.
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>   


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