[R-390] Measuring Sensitivity

[Barry]

I finally got a chance to build the 50:1 divider Roy suggested.

http://www.knology.net/~thelanding/R390A/50to1Divider.jpg

It consists of three 150-ohm, 1% resistors in parallel (because I didn't
have a 50-ohm resistor) and a 1-ohm, 1% in a divider configuration hooked to
female BNC connectors for input and output.  The cast aluminum housing is
quite nicely sized for the job and was easily machined.

It really made a difference in the noise I was getting using my old setup.
With coax all the way and the divider in a sealed box, there is very little
in the way of outside noise now when the radio is connected to the
generator.

Of course, none of my equipment is closely calibrated, but it isn't all that
far off either.  I have both an HP410B and a WV-98C (Senior VoltOhmyst)
connected to the Diode Load and both read within a few tenths of a volt of
each other at -7VDC so I think I'm getting a fairly accurate reading.

With the divider setup, I'm seeing about 1.5uV on the BC band for -7V on the
Diode Load point and around 0.7uV (yes, that's 7/10uV) on the 8mc band.
Again, a lot of this may be due to calibration errors, and possibly some
leakage, but I don't think it's all that far from a true reading.  By the
way, just to refresh, since the generator outputs 1/2 it's stated voltage
into a 50-ohm load, the 50:1 divider effectively makes the actual output
1/100 of the reading on the generator.

As far as leakage, I hooked up the radio's antenna input to a small loop of
wire fed by coax from the radio and "sniffed" all around the generator.  It
heard absolutely nothing.  I checked closely around the knobs, edges, top,
and back and could not detect anything coming from the generator.  Just to
check, the generator has a 2V output that can be used to drive a counter or
whatever.  I pulled the grounding cap off of that and the signal was
immediately heard with the loop several inches from this port.  Plugging the
port back up and running the loop right up against it still produced nothing
on the radio.  Yes, there is probably very slight leakages but the radio
can't hear them so not sure if they are affecting my measurements or not.

At any rate, this little project paid off very nicely in the convenience
department.  Thanks Roy!

Barry - N4BUQ