[Premium-Rx] Racal RA6790GM questions

[Charles P. Steinmetz]

Heinz wrote:

>I want to upgrade my receiver with more filters. Also I am looking 
>for the A5 ISB board and the A6A1 remote control board.

One comment re: filters in the 6790 -- all of the filter inputs are 
fed, in parallel, all the time, so the filter outputs are all hot 
with signal all the time.  Leakage across the unselected filters is 
combined with the desired output of the selected filter by the switch 
matrix.  In particular, if you have a 16kHz filter installed or still 
have the shorting link in the final filter position, the stopband 
attenuation of the narrower filters is degraded out to the cutoff 
frequency of the 16 kHz filter (or to the cutoff of the roofing 
filter, in the case of the shorting link).

Accordingly, I recommend removing all filters wider than you will 
regularly use, and removing the shorting link in the final filter 
position (if present).

It would not be too hard to add switching diodes to the filter 
outputs, but it is much easier just to remove the wide filters (and 
shorting link).  It was widely known in the '60s that if you were 
going to cheap out and switch only one end of the filters, it should 
be the input end (for this very reason).  Why Racal did it the way 
they did is a complete mystery.  But this is just one of a fair 
number of ... ummm ... "less than optimal" design features in the 6790.

Best regards,

Charles