[R-390] Distortion on MGC mode and another question on sensitivity

[Mark Huss]

You don't need a cage. Disconnect the signal generator, leaving the 
cable and attenuator connected to the receiver. If the Diode Load is 
more positive than -7 volts, you can live with the leakage, but will not 
be able to do a good S+N/N test. Connect generator, and adjust output to 
-7V at Diode Load. What you are injecting is (Signal + Leakage Noise + 
Receiver Noise) / Receiver Noise. Since the signal adds to the leakage 
noise, the result is -10.

One thing you might want to try is this. Disconnect generator, cable and 
attenuator from the receiver. Note Diode Load voltage as N1 This is 
Receiver Noise. Connect cable and attenuator, but not the generator. 
Note Diode Load Voltage as N2. Perform following formula; 
(N2-N1)+N2=NS3. Connect generator and increase output until Diode Load 
voltage equals NS3. Read generator output and note as S1. This is the 
amount of leakage you have from your attenuator/cable in microvolts. 
Increase generator output until you read -7V on the Diode load. Note 
reading of generator output as S2. S2 +S1 is the total rf input for -7V 
at Diode Load.

N1= Receiver Noise
N2= Receiver Noise plus Leakage Noise.
NS3 = Receiver Noise + Leakage Noise + Signal Noise where Signal Noise = 
Leakage Noise.
S1= Leakage Noise in Microvolts.

When you get the proper shielding on the attenuator (have you considered 
wrapping in Aluminum foil?) and coax cables (good ones), see how close 
it came out!

n4buq at knology.net wrote:
> Thanks for the replies about the MGC distortion.  I was thinking someone 
> had posted a while back that the R390* radios should not overload on strong 
> signals, period, but I suppose that's only in AGC mode.
>
> Another question:  When I'm testing for sensitivity, I'm looking for the 
> signal level that will result in -7V on the Diode Load; however, when I 
> connect the signal generator, there is atmospheric noise that is introduced 
> before I inject any signal which results in a few volts on the Diode Load.  
> Is it correct to increase the generator's output level to -7V total and 
> read the sensitivity from there, or should the signal generator result in a 
> 7V increase over the existing noise?  In other words, if I'm getting -3V in 
> atmospheric noise, then do I take my sensitivity reading at -7V or -10V?
>
> Part of this problem is the rigged-up mechanism I have for connecting the 
> generator to the receiver.  I'm in the process of making a housing for the 
> attenuator and feeding everything with coax thus eliminating the 
> little "antennae" I introduce with the current setup.
>
> I assume the -7V reading is accurate (or at least more accurate) if the 
> test is done in an RFI-proof environment with a perfect signal generator 
> that doesn't leak.  Lacking those two things, what is the proper way to 
> measure this?
>
> Thanks again,
>
> Barry
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