From: owner-[email protected]
[mailto:owner-[email protected]]
On Behalf Of Andy Talbot
Sent: 27
April 2007 12:36
Subject: LF: Filter expert
needed - Mystery of the lost real zero.
Is there anyone monitoring these groups/reflector who knows the deep
insides of filter design theory?
I've just synthesized a straightforward 50- 75MHz 5th order
Top-C-coupled bandpass filter to form part of a frequency
converter that will feed an SDR-IQ digital receiver. While
playing with the filter design using a circuit analysis package, and
recalling satellite microwave cavity filter designs from the
dim-and-distant past, I remembered that by arranging cross-coupling between non
adjacent resonators, it was possible to form pairs of real zeros in the
upper and lower stop bands respectively. These convert the filter to
an elliptic type response, and considerably improve the cut-off. I also
recall it was a very critical thing to set up.
So, I tried adding an additional cap between resonators 2 and 4, got a
wonderful 10 - 20dB improvement in the low end cut-off response around 40MHz
exactly where it was needed, BUT ONLY THE ONE REAL-ZERO COULD BE FOUND
!!! There is absolutely no sign af any second null above the passband.
Where's it gone? Shouldn't they always come in
pairs? This is really bugging me, I can't sleep at night, can
anyone explain?
Could it be because a top-coupled - C BPF is non-symmetrical?
