[R-390] Measuring Sensitivity

[Craig C. Heaton]

Barry,

Have you looked at the "Pearls of Wisdom". There is a section, pdf file with
sensitivity adjustments and I believe your mismatch situation is discussed.

http://209.35.120.129/Pearls/index.htm

I may have this all wrong but here comes my $0.02. Since I use the R-390/A's
in the shack for ham radio use, feed the antennas with open wire, use a
tunner/transmatchs to make the transmitters happy: (read 50 ohms impedance
here) this what I do.

Looking at Chuck Rippels website I feed the balanced input as pictured, 50
ohms coax unbalanced and one side grounded to the frame. Then the output of
my sig-gens URM-25D & HP 8640B are 50 ohms unbalanced, I believe; align the
R-390/A's with no adapters, just BNC connectors and coax to set up the IF
deck and peak the RF coils/caps thru the balanced antenna input(now
connected unbalanced). Treat the R-390/A's as though they have 50 ohm
unbalanced antenna inputs.

Could be all wet here, both sig-gens outputs end up less than 0.1uV on all
bands. Using flowertimes's method of calculating S+N vs N ratio for the IF
and RF sections, I've got 30dB thru the IF and 20dB thru the RF. Must be
doing something right?

Craig,

Nomex is now on!

-----Original Message-----
From: r-390-bounces at mailman.qth.net
[mailto:r-390-bounces at mailman.qth.net]On Behalf Of n4buq at knology.net
Sent: Friday, March 09, 2007 6:27 AM
To: r-390 at mailman.qth.net
Subject: [R-390] Measuring Sensitivity


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
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