In article ,
says...

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.
--

"I believe history will be kind to me, since I intend
to write it." - Winston Churchill


i just read this thread and had a few thoughts. i don't know what kind
of interference you're dealing with, nor do i know what restrictions are
placed on your competitions.

i was thinking about front end overload, also. the types of interference
vary in different countries, but those paging towers and, taxis, etc.
all play hell on a front end. you really need a spectrum analyser to
figure out what you're deaaling with.

as for those ceramic resonators, they're ok, but in the better
receivers, they're followed by if xfmrs to get rid of the spurious
response of the ceramic filter.

since you are thinking of just starting from scratch, here's what i'd do
if you're not bound by rules.

http://www.aerocomm.com
http://www.radiometrix.com

cost? i dunno. i figure if you can afford one, good. then you can spend
more time on the robotics and weaponry.

if you can use these, the question becomes a matter of whether you can
retrofit one of these in time. i'm not ready to look into using them yet
and therefore haven't gotten into the details. so i don't know what
you'd have to do to get a PWM signal in and out, but since they handle a
higher data rate (1 Mbps) than std R/C PWM, you could bust the PWM
signal up into little "chips" and reconstruct it on the receiving end.
maybe an integrator/LPF would be all you need. on the other hand, maybe
you can just send" the PWM directly. I'm sure the applications engineers
could help.

hope this helps.

mike