[R-390] sensitivity, product detector, etc

[William A Kulze]

Hello group,

I was just going through the postings and the discussion on sensitivity
measurement, Softrock and IF gain adjustment caught my interest.

Here's the latest setup I've been playing with. Now, mind you, I'm no
engineer, not even sure I'd qualify as a tech anymore, but something about
this seems like it makes sense to me. I'd be happy to hear anyone's
thoughts on the subject. About a year ago I purchased a WiNRADIO G303i. By
using command line switches I can disable the demodulator and use my
choice of 3rd party demod software. Been playing around with FlexRadio
Power SDR. I've connected the R390A IF out to the WiNRADIO (tuned to
455kHz) through a couple of surplus 20db attenuators. Turning the agc off
on the 390a and winradio, with the antenna removed from the 390a and RF
gain to minimum, I adjust the 390a IF gain until I just start to see an
increase in the noise floor on the panadapter view. I set the 390a RF gain
for the same. I adjust the manual IF gain on the winradio to place the
noise floor at -140dbm on the panadapter. Any signal that comes in seems
cleaner and easier to copy. I only did this a couple of days ago and only
had that night to play with it, so I've no long term results and
definitely no measured tests. But it seems (I hope) that I'm getting as
much gain as I can without injecting any significant noise level. I can
increase the rf gain on weak signals, but the noise just comes up with it.

I don't know what litle microvlts and such is going on, but DRM signals
seem to have a better SNR than before.

FWIW re: MGC distortion, it's already been mentioned that this is normal
when feeding too much signal into a circuit. AGC just -A-utomatically does
what you would have to do -M-anually with the rf gain control. What I
think is meant by not overloading these radios is that in AGC mode it
would take a VERY strong signal to start producing harmonic images where
they don't belong. With my outside wire antenna on the winradio I get MW
signals in many different places that I shouldn't. The 390a does just fine
with the same input.

I know some of this is not exactly 390 topic, but I'm having fun making
the most of both old and new.

Regards,

Bill