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

[Charles Steinmetz]

Larry wrote:

> When reducing the IF gain in AGC mode to produce good quieting on weak signals, I noticed that the S/N measurement in MGC mode improved.  I still do not know why this is happening because the IF gain adjust is way past any point where it should affect the real S/N in MGC mode.

That is exactly Roger's point about the "book" setting of the IF gain 
control.  It is high enough that the RF stages do not clearly dominate 
the noise figure when the receiver gain is wide open.  Thus, reducing 
the IF gain actually does improve the overall S/N.

HOWEVER, see below and my previous message re: atmospheric noise.  For 
the reasons given, you DO NOT want to reduce the IF gain too much or you 
will lose DR on strong signals without any gain, in practice, with 
respect to weak signals.

This gets complex, because the 390/390A has more gain-controlled stages 
than many boatanchors and also has staged AGC ("delayed AGC"), where 
some stages get a fair amount of gain reduction before other stages get 
any at all.  So, the stage-by-stage gain distribution changes in a 
complex way with both manual and automatic gain reduction, by design. 
Presumably, the stage that dominates the noise figure also changes in a 
complex way with both manual and automatic gain reduction.  As ever, the 
goal is to keep both ends of the DR positioned for the best performance 
compromise, at all input signal levels.

Bottom line -- beware of setting the IF gain for "best S/N" or "maximum 
sensitivity," because doing so tends to sacrifice strong signal 
performance -- which is already a weak point  with the 390/390A.  As I 
noted before, what you gain in S/N for the weakest signals on the bench 
is meaningless, because in actual use the receiver noise is swamped by 
atmospheric noise.  (If you operate exclusively above 20MHz, there may 
be a case for re-thinking the optimum IF gain because the band noise is 
much lower than at 20 meters and below.)

Best regards,

Charles