On Tue, 20 Jun 2006 13:58:31 -0500, Ben Jackson wrote:

I'm still working on my HF APRS beacon idea (25MHz-14.85MHz = 10.151)
and I've done some experiments using a 74HC86 quad XOR with one gate for
each oscillator and one as a mixer. In spice simulations, the '86 as a
mixer produces lots of products resulting from the odd harmonics of the
inputs, some of which are close to or below the 10.151MHz I want. If I
run that output through a 2nd order Butterworth 30m bandpass filter I
get a clean result.

You ant to do that.


My question is about the tradeoffs between filtering the exciter output
to the PA vs filtering after the PA. Obviously a post-amp filter has
to handle more power, but has the opportunity to eliminate amplifier
distortion. But eliminating unwanted inputs to the amp also reduces
unwanted outputs.

if you do it agter the PA you get those products plus IMD to create
more products and it will be very hard to secure a clean signal
without high losses.

Are there rules of thumb for suppression of unwanted signals at each
stage of an amplification chain?

Start clean, then clean up anything that results from the stage(s).
Usually if you have a clean source and run that through multiple
linear stages you should get the same only bigger with only a
small number of artifacts that are easy to clean up.

Allison