On Sat, 26 Jul 2003 17:42:19 +0100, Paul Burridge
wrote:

On Sat, 26 Jul 2003 12:35:11 -0400, Joseph Legris
wrote:


What is the intended application?

I want to increase the sensitivity of a radio control rx. To this end,
I'm going to build an RF pre-amp, which needs to be highly selective
to avoid the probablility of increased ajacent channel interference.
Hence the need for a sharp filter in conjunction with the pre-amp.

This is a strange way of achieving your aim. You should be going for
adjacent channel selectivity in the IF, not the front end. That way
you can use a fixed frequency filter, and it can be as lossy as you
like - and it will be lossy to achieve these kinds of selectivity with
achievable unloaded Q.

If you put this filter at the RF stage where it will help with
adjacent channel selectivity,m it must be before the first amplifier,
and that will have killed your hoped-for sensitivity increase. If you
put it after that amplifier, then you may as well leave it where it
belongs - in the IF.

Radio design is always a compromise between sensitivity, selectivity
and large-signal performance. The standard topologies haven't emerged
by accident, I'm afraid.

d

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