[Premium-Rx] RA6790/GM IF filters

[Charles Steinmetz]

Craig wrote:

> I have a whole set of Blue Collins CF solder in mechanical filters  *  *  *
 >
> My RACAL RA6790/GM has only five plug-in crystal filters installed (16 KHz
> Sym, 6.8 KHz Sym, 3.24 KHz Sym, 1.2 KHz Sym and 400 KHz Sym)  *  *  *
>
> To make a long story short, can I mix crystal filters with mechanical
> filters in the RA6790/GM providing I install the appropriate  input/output
> caps for the Collins LSB and USB filters and run BITE to recognize the new
> filter bandwiths.

Sure.  Make sure you put the LSB filter in Position 1.  The USB filter 
can go anywhere else, but I find it most orderly to put it in Position 
2.  Remove the loading capacitors from any crystal Position where 
crystal filters are installed.

I recommend that you remove *both* the 16 kHz filter and the wire link 
in filter Position 7 to get the best out of all the other filters.  The 
designers cheaped out on the 6790, and instead of using diode switches 
at both ends of the filters, they chose to feed the IF signal to all of 
the filter inputs all the time and switch only the filter outputs.  The 
raw IF signal coming through the 16 kHz filter and the wire link leak 
through the output switches and spoil the response of all the narrower 
filters.  Under current band conditions there is really no opportunity 
to use such wide IF responses anyway, so you are not giving up anything 
in practice by removing them.

Best regards,

Charles