[R-390] Why use a Roofing Filter?

[Bill Kulze]

Hi, All. Mating up the R-390a with an sdr is something I've done. Nothing real elaborate, IF out to SDR RF in, in this case a WinRadio G303i tuned to 455kHz. The biggest issue I ran into was attenuating the IF out signal to something more agreeable with an antenna input. One of the obvious advantages of the SDR is the panadapter. A visual reference is handy. With it I can find the optimal IF gain adjustment before you introduce noise. Same with the RF gain control. On My unit that is about 3 o'clock. You can see it all go to crap when either of those is too high. I usually ran it in MGC. Since the 'roofing filter' of the SDR is only 20kHz in this case, I just leave the radio at 16 kHz bandwidth, unless there is something very strong near something weaker I'm trying to copy, then the radio's filters can help.

The sdr comes in handy during alignment, also. It's sort of a poor man's spectrum analyzer. Leave the agc off in the sdr so you can see changes in amplitude better. You can sweep back and forth and see the shape of the IF bandpass.

In operation you have any mode of demodulation that the software is capable of available for use. I've also used the SDR by itself as a kind of station monitor, especially when using an analog rig, I can tell when I'm on frequency, and if I'm driving it too hard and putting out spurious signals.

Am I saying this is superior to the standalone R-390a? No, but it's something else to play around with.

Bill Kulze
W2NVD

-----Original Message-----
From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Bob Camp
Sent: Friday, August 01, 2014 8:37 AM
To: Dennis Wade
Cc: R-390-List list
Subject: Re: [R-390] Why use a Roofing Filter?

Hi

Ignoring the "why" of R-390 + SDR there are some right and wrong ways to do it.

----

1) You need a way to drive the AGC on the radio, or you need to pick off ahead of the IF.
2) You need a bandpass shaped dither source if you are going to pick off after filtering. 
3) You need pretty good shielding on your SDR to keep it's clocks out of the R-390

There are other nice to do things:

4) Compensating for the filter passband is nice if you pick off after a filter
5) Driving the S meter makes tuning a bit easier.
6) Using the audio volume control on the radio makes operation a bit more natural. 
7) Using the bandwidth control switch on the radio ....
8) Using the AM switch on the radio ...
9) Using the AGC switch ....

If you go the whole way down the list, that's a lot of playing with the radio. If you go part way, you have something that's a bit more of a hassle to operate than it could be. The net result of the list would be mating up a SDR IF up with the RF deck of an R-390.  The resulting radio would look cool. It would have the overload and drift issues of the original RF deck combined with the normal issues of an SDR. It *might* get some benefit from the RF filtering. 

Yes I've given this a lot of thought on and off over the years. 

Bob