[R-390] sensitivity, product detector, etc

[Mark Huss]

Well, first, the RF Gain also controls the IF Gain, too. So what you are 
doing is shutting off the RF Amp, and the first, second, and third IF 
amps, then cranking up the gain on the third IF Amp. Then when you 
adjust the RF Gain up, you are increasing the gain on all those stages. 
Don't know how that would affect the noise floor of the receiver. One 
good thing is that the R-390 is acting as one honking big preselector.

Did not know about the command line switches on the G303I. That would be 
handy. And this is the first I heard about problems of overloading in 
the MW band. Are you close to broadcasters?

William A Kulze wrote:
> 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
>
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