[Premium-Rx] 6790 filters

[Charles P. Steinmetz]

Tisha wrote:

>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.

Well, the problem with the 6790 is not the filters, it is the 
switches (in the 6790's case, 1N916 diodes used as switches).  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 -- not by the filters themselves.

>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.

This is called "dry circuit" operation.  Proper self-wiping 
gold-plated relay (or switch) contacts designed for dry circuit duty 
work superbly and last a long time.  HP uses relay switching for the 
input and output pads of its test equipment (spectrum analyzers, 
signal generators, etc.) with excellent results down to -150 dBm and 
further.  (This is not to say, "never a failure" -- the 3225A sig 
gen, in particular, needs its relays cleaned occasionally, and the 
contacts [both mechanically, and the plating] are very easily 
damaged.  But I have never had a problem with other HP dry circuit 
relays or switches.)

>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.

I'll take relays any day over PIN switching.  I think of PIN 
switching as something used by manufacturers who are more interested 
in cost than quality, and the use of regular signal diodes for RF 
switching as something used by manufacturers who just don't care how 
their gear works.  Sometimes, other considerations dictate the need 
for solid state switching (vibration, magnetic shielding, power 
limits, contract spec, etc., etc.), but IMO it is a last resort.  I 
have almost always managed to find a way to avoid it.

Best regards,

Charles