Paul Burridge wrote in message . ..
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?

It'll need to be a crystal filter, and your requirement that it must
be tunable means that you will convert down/up to an IF frequency and back
up/down again. (Well, you may not have to convert back up again but you
don't tell us your application).

Problem with the IF and conversion is the production of images. Images
won't be a killer problem because your tuning range is really quite narrow.

If you wanted to really cheapskate out some ceramic IF filters also seem
to meet your stated needs.

Tim.