On Sun, 14 Apr 2013, garyr wrote:

I'm trying to build a 80/40 meter receiver and I'd like to be able to
switch the RF input between the two bandpass filters. This sort of thing is
usually done with relays but I'd like to avoid that if possible. A
SN74CBT3306 would be ideal but I think the on-resistance would be too high,
thought it's hard to tell from the data sheet. Are there other devices that
would be suitable for this application? What do you think about using
2N7000's for this job?

The input and output capacitors of the double-tuned filters are 2200 & 2700
pF so the stray capacitance of the switching circuitry would not affect the
response very much.

Thanks in advance

I don't recall every seeing CMOS switches used for switching LC cicuits.
Maybe for switching stages, but I can't think of an example of that.

If it's not a switch, which has the disadvantage of not being close to
where the circuit needs switching, it's a relay. If it's not a relay,
then switching diodes are called in. Lots of people were using small
signal diodes for switching in the early days, then PIN diodes came along
they got the glory. The one issue seems that under some circumstances,
they can add distortion.

In the very old days, the tuned circuits were plugged in, very troublesome
but it avoided switches. Once semicondcutors came along, they were cheap
and small enough that duplication often made sense, switch whole stages,
including the tuned circuits, rather than switching the coils alone. That
gives you more leeway, with active stages in there, any loss from CMOS
switches goes way down.

Michael VE2BVW