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

[Charles Steinmetz]

Larry wrote:

> The question had to do with measuring the S/N.  I tried to say that the 'measured' S/N changes depending on the setting of the IF gain, but that the IF gain control is way past the point where it could affect the 'real' S/N (as that is determined in the first few stages).  So what causes the 'measured' S/N to appear to change?

The S/N is only "determined" by the first stage IF THE GAIN DISTRIBUTION 
OF THE RECEIVER IS OPTIMIZED (see my previous post re: "determined" vs. 
"dominated").

The 390/390A IF gain adjustment has sufficient range to seriously 
de-optimize the gain distribution of the receiver, in which case the 
first stage (or even the whole RF section) will NOT necessarily dominate 
the noise performance.  You *are* measuring the real S/N of the radio in 
that configuration, but if the IF gain is set incorrectly the 
configuration isn't a useful one for other reasons (typically, because 
you have made the receiver's overload performance worse by several tens 
of dB).

Fully 50% of the 390As I've seen have had the IF gain set WAY too high 
by owners or techs seeking outrageous (and unnecessary) sensitivity 
numbers and thinking they were smarter than the Collins engineers.  It 
should be a rule that after adjusting the IF gain, 390/390A owners and 
techs must take the radios to Europe and try listening to the 40/41 
meter bands on a good antenna.

Best regards,

Charles