Paul Burridge wrote in message . ..
Hi chaps,

I've decided to bite the bullet and try to build an RF filter for
40Mhz. This filter will ideally have a very, very sharp characteristic
at one single spot frequency +-20Khz and attenuate the crap out of
anything either side of this. It'll need to be tunable over a range of
say 200Khz. Can anyone give me a steer on what type of arrangement
would be best suited to fit this purpose?
Thanks,

p.

Filter at 40 MHz with 40 KHz BW means a very high Q, up to 1000.
Cavity filter at 40 MHz is big! You can try active RF tuned stage (at
40 MHz) with a positive feedback, also known as Q-multiplier. In
addition to high selectivity this will also result in a high gain,
which may overload your actual receiver. In this case you can
attenuate the signal at the output of the RF stage. You have to make
sure Q-multiplier would start to oscillate. It is an issue by itself.
73,
Vlad
kb9olm