[R-390] Notch filter

[Bob Camp]

Hi

As far as I know there is no way to do an internal notch filter on the 
IF of the 390 without significant modifications to the radio. The IF 
output is probably your best bet combined with an external IF. In order 
to really do a good job with a notch filter it's better if the filter 
is at the IF.

Of course this is just the start of the process. Since you now have the 
entire tail end of the IF outside the radio some things have changed. 
The AGC now really should come from the external unit and feed back 
into the radio. The 390 is at least set up for this part of the 
process. I don't know of many boxes that are set up that way though 
....

The next thing is that the demodulator is now in the external box as 
well. That makes the mode select switch on the radio nonfunctional.

Finally feeding the audio back into the radio is a little problematic. 
It can be done but it is not as easy as it should be. A lot of the 
boxes just went ahead and did their audio. That takes out the volume 
control knobs and audio filtering as well.

That's a lot of stuff just for an IF notch filter. But if you are still 
game, keep on reading .....

Most of the good old IF notch filter designs worked with IF's around 50 
KC or so. They would mix down from 455 to a final low IF, filter, and 
then demodulate. I do not remember any good notch designs that worked 
directly at 455. They may be out there, but not on any radios I ever 
owned. If you go that way it's not just a notch, it's an entire extra 
IF strip.

One thing that has come along since the R-390 was born is digital 
signal processing. Most people look at this as a neat way to do 
filtering, but it's more than that. The one thing that most DSP units 
do very well is adaptive notch filtering. Properly done they also made 
good demodulators. You can do the process at either audio or at the IF. 
The IF processing is better,  processing at audio is cheaper.

Of course if you are going to do it all at audio then there is another 
solution. A lot of people use outboard audio filter boxes. The AGC does 
not work the way it should and you still need some strange stuff with 
speakers. The one I happen to like is the JPS NIR-10. They show up on 
various sites fairly cheap.

If you want to go the IF route about the only source I know of for 
outboard boxes is Sherwood Engineering. Their box works well with the 
R-390 and they have good support for their products. It's a small 
market and I'm a bit surprised  anybody can afford to make new gear 
like this.

Now if you still are not satisfied there is the roll your own approach 
.... 24 bit A/D's and 32 bit floating point DSP on an R-390. A little 
work with the good old C compiler and you'd be in business. Think of 
the possibilities ..... we could name it the R-390B

	Take Care!

		Bob Camp
		KB8TQ



On Aug 29, 2004, at 4:07 AM, Charles B wrote:

> Hi all,
>
> Is there any one add on that I can get to produce a notch filter 
> function for my R-390A.  I don't want to modify the receiver.  I have 
> the QF-1A unit, but it's kinda of old and I don't trust it.
>
> Thanks
>
> Chuck
>
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