"Arid ace" wrote in message
news But because at that time circuits weren't
simulated, it was overlooked that due to the diode capacitance, filters
supposed
to be switched out,
Hi
Such effects do exist with any switches.
Behaviour of filters with a very mismathed input and output impedances is
very
unpredictable.
For switching IF filters logic would be to cascade them and shunt the too
narrow ones.
I have never seen this implemented.

On Wed, 17 Apr 2013 02:06:04 +0200, "bilou" wrote:


"Arid ace" wrote in message
news But because at that time circuits weren't
simulated, it was overlooked that due to the diode capacitance, filters
supposed
to be switched out,
Hi
Such effects do exist with any switches.

14p DIP reed relays (2 relays / package) already suggested by their
construction, to short circuit filters not in use, in order to prevent spurious
responses. That was in the 80s. Too expensive for mass production though.

Behaviour of filters with a very mismathed input and output impedances is
very
unpredictable.

What matters is the spurious response via parasitic and other undesirable
capacitances or common inductances.

For switching IF filters logic would be to cascade them and shunt the too
narrow ones.
I have never seen this implemented.

With each IF filter adding ~6 dB loss, either something has to be done about
gain correction or S-meter display. I built the 455 KHz IF with 2 filters for
each bandwidth and a low gain (~12 dB max) dual gate mosfet amplifier between
the filters. Switching bandwidth included removing the supply of the mosfets not
in use. The result was excellent - I verified that with a spectrum analyzer.