[Premium-Rx] 6790 filters

[Tisha Hayes]

Charles wrote:
" In the 6790 itself, the IF filter switching circuitry leaks pretty
horribly -- a consequence of feeding all of the filter inputs in
parallel and switching only their outputs with diodes.  If you have
the 16 kHz filter installed (most of the ones I've measured are more
like 20 kHz at 6 dB), it spoils the ultimate out-of-band attenuation
of the narrower filters from their cutoffs up to 25 kHz or more.  The
simple solution is to remove the "16 kHz" filter.  Presumably the
narrower filters all leak too, when you are using an even narrower
filter, but I have never been bothered by it.  However, my widest
filter is 6 kHz.  If you have an 8 or 10 or 12 kHz filter installed,
that might also exhibit the problem.

It has long been known that if you are going to switch only one end
of your IF filters, it should be the input end, for this very
reason.  For a radio of this stature, it is (IMO) an unpardonable sin
not to switch both ends, and even worse to switch only the wrong
end.  All you would need to modify a 6790 to switch both ends is a
handful of cheap components.  If I were doing this, I would change
the existing diodes to something more suitable than the installed
1N916s, and use them for the input side, as well. "
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It is indeed a quandary with filter isolation. It is one little advantage I
had found to the mechanical filters in the R-390A is that you do not have a
way to get "blowby" on the filters as the input and output ends of the
filters are in completely isolated little sides of the chassis. I have a
few radios like the Harris RF-350K transceiver that uses relays to switch
both the inputs and outputs of the filter but as there really is no current
through the relays they tend to close but really remain open due to the
oxide layer. I had considered making a retrofit plug-in relay using metal
cased mercury relays to eliminate the connection problem, make band
switching must faster (same relays are used on the LPF board) or to go with
PIN's.

That would be a great aftermarket component, an electromechanical relay
replacement with a base adapter PCB to common relay pinouts to use a high
quality PIN diode. Then you could tear out those DIP relays or the freaks
like in the Harris gear and go with a solid state solution.

Another thing I had seen with some filter designs is the lack of a pair of
trimmer caps for the input and output impedance's of the filters.  Some
receivers just use a single trimmer cap for the entire bank of filters and
only on the output, that is not how the real world works.

I have added fingerstock to some receivers to improve isolation between
stages, conductive foam or rubber gaskets, even feed through caps between
stages on certain lines to improve performance.

-- 
Ms. Tisha Hayes/ AA4HA
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