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

[Larry H]

When I adjust the IF gain, I initially set it for the 150uv/7v level.  Then I verify that the AGC is working correctly by injecting a small signal (with no modulation) in the antenna and increasing it until the AGC voltage just starts to move the c/l meter.  At this point the RX noise should have mostly diminished.  If not, reduce gain and check.  If the gain needs to be reduced too much, something is wrong in the AGC.  I then check for signal overload by increasing the the sig gen output in steps up to .1v with modulation while scoping the audio output.  If the c/l meter is not close to max, something is wrong.


This is all pretty straight forward in AGC mode, but we make our sensitivity checks in MGC.  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.

Regards, Larry