[R-390] Measuring Sensitivity

[n4buq at knology.net]

I'm not the smartest guy on this list and I need to understand some 
things.  I'm interested in measuring the sensitivity of my R390A and I have 
a signal generator (GR1001A) that has a 10-ohm output.  I have an adapter 
that houses a 40-ohm resistor that matches the output to a 50-ohm cable.  
When I feed the balanced input with this, there is an obvious mismatch 
between the 50-ohm cable and the input.

If I had 10-ohm cable (probably not a real factor with such short 
connections but...) and the input to the radio was 10-ohms, then I could 
read the level on my generator and know that 1uV is 1uV on the input, etc.  
Since this is not the case, short of an RF voltmeter capable of reading 
microvolts or other measuring instrument capable of the same, how do I 
determine how much signal is present at the radio for a given setting on 
the generator?  I realize I need to make up a matching network at the 
termination of the 50-ohm cable where it connects to the receiver, but not 
sure how all these matching networks play into the signal levels between 
the generator and the radio.

I assume that once this calculation is made, then when I feed the radio 
with a modulated AM signal with the BFO off and get -7V on the diode load, 
then I should be able to know the sensitivity level, correct?

The reason I ask is that I'm having to crank the generator up to around 
10uV to do the "switch the modulation on and off to get 10dB S+N/N readings 
and that seems high.  The radio "hears" quite well, but I'm wondering just 
how sensitive it really is.  The problem I'm having with the "modulation 
on, modulation off" method, is that the cable and generator make a very, 
very good antenna, especially on the BC band, making these measurements 
quite difficult.

Any advice here would be most appreciated.

Barry - N4BUQ