[R-390] R-390A IF gain setting and S/N measurement

[Tom M.]

The way I do it is to find a weak signal and optimize the setting for copy on the signal.  This may seem crude but it works well for me and matches the intention of the optimization, to be able to copy weak signals.
For ex, tune in to R. Australia on 17480, and adjust the IF gain for best copy.
You mileage may vary.
Tom N5OFF


      From: Larry H <dinlarh at att.net>
 To: R-390 Forum <r-390 at mailman.qth.net> 
 Sent: Thursday, July 14, 2016 10:40 PM
 Subject: Re: [R-390] R-390A IF gain setting and S/N measurement
   
Thanks for all your great input.  It has been very helpful.  This has prompted me to read through the Pearls and Y2KR3 again this past few weeks looking for an alternate and/or a more precise way to set the IF gain.  I always thought that the 150uv/-7v setting was a little high, but not sure how much.  And, I knew that setting it too low was not good either.  And, thanks to Charles Steinmetz, I now know that too low negatively affects its Dynamic Range. 

So, here's what I found:

1. A procedure by Chuck Rippel says to set it between -4 to -7 db on the line meter on RF noise (no antenna).


2. A lot folks agree that the 150uv/-7v setting was a little high.

3. A lot folks agree that keeping the gain as low as possible (bot not too low) improves the S/N.

4. A few folks agree that setting it too low negatively affects its Dynamic Range.

I found a lot of good info written by Roger Ruskowski, but it was mostly on how to measure RF and IF/audio for 30:1 and how to get there, but nothing on how to set the IF gain more accurately.

I found some very helpful info by Dallas Lankford, but there again nothing on how to set the IF gain.

Did I miss finding on how to set it? 


My tech manual says that after setting it for 150uv/-7v, and if 1 to 4 uv input to the balanced input yields -7v on the diode load, that is good, leave it.

I like Chuck's setting in 1 above, as that seems to be fairly close.  IE, a little less gain than the above 150uv/-7v book setting.  But that's a wide range and this is affected by the quality of the RF deck. 

Because the above are too granular, what I'm looking for is something like this: 

'If the S/N is OK, then set the IF gain for -7v with @ 2uv average input to RF balanced connection (with the sig gen impedance matched, of course)'.

Does this sound fairly close?

What I'm 'feeling' is that 2uv might be the sweat spot - this would yield a good balance between DR and S/N.

I do not know how to tell when the DR is being negatively affected.  How can I tell?  I assume this is about keeping distortion to a minimum on strong signals.  Can this be easily measured?

Comments please.
Regards, Larry
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