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

[Larry H]

In my previous post I reported some AGC measurements as follows:


RF in Uv: 1    10  100    1,000    10,000    100,000 
    min:  0    2.3    5.4      9.2    12.4        16.1 
    ctr:  0    3.0    6.0      9.7    12.9        16.8 
    max:  0    5.0    7.9    11.2   14.2        17.5 

The agc voltages for the 1 uv level are wrong.  It should read as follows:

RF in Uv: 1     10    100    1,000    10,000    100,000 
    min: .20    2.3    5.4     9.2        12.4         16.1 
    ctr: .20    3.0    6.0     9.7        12.9         16.8 
    max: 2.7    5.0    7.9     11.2    14.2         17.5 


The .20 v in min and ctr do not affect the way the AGC operates, but the 2.7 v in max makes a big difference.  The AGC in this rx is designed to delay the application of agc voltage to the rf amp, mixers, and IF amps in order amplify weak signals as much as possible to overcome the internal noise in it.  Any voltage above .5 v will degrade its ability to keep the noise as low as possible on weak stations.  You can of course reduce the RF gain to compensate for this, but that too degrades its ability to bring in weak stations as best it can.  The RF gain can be turned down a little without much of an issue on weak stations, to around 9.

So, this is another reason to NOT turn the IF gain up too high.  Interesting enough, the IF gain setting on my 390A needs to be lower for proper AGC operation than for the S/N threshold.  I set my gain at about 5/8 from min and with 1 uv in = .3 v agc.  This is working very well for AGC and S/N.
Regards, Larry