[R-390] R-390(A) gain distribution

[James A. (Andy) Moorer]

You misunderstand what the IF stages in the R-390 series do. Don't forget 
that the original R-390 had 6 (!) stages of IF.

Several of the IF stages have very low gain - no more than 1 or 2. These 
stages provide additional selectivity - not additional gain. In the R-390 
and the R-392, there is even a stage with 3 (!) tank circuits, coupled by 1 
pf capacitors. You lose a lot of gain with low coupling like that, but you 
gain in selectivity.

The overall IF section needs to provide a gain of 1000 or so, but the IF 
selection is almost entirely responsible for rejection of adjacent signals. 
The Collins engineers needed to isolate the tank circuits from each other so 
they would resonate independently and each contribute another 6 dB/octave of 
rejection. If they are coupled too well, they stop acting as separate 
circuits and behave as a single tank circuit.

James A. (Andy) Moorer
www.jamminpower.com

----- Original Message ----- 

But 4 stages of 455kHz IF... why so much gain there?  Most receivers
of the era had 2 (cheapie) or 3 stages and do just fine.

How much gain is typically the 3-mixer chain capable of?  Each mixer
gets fed from the AGC control line, right?

It's been a while since I looked at the Collins cost reduction report...
is there any thought given in there as to eliminating an IF stage?