Are you the William Sabin of Single-Sideband Systems & Circuits?

tim ab0wr

William E. Sabin wrote:



Xtal filters are designed to operate correctly between specific values of
generator and load resistances. Trying to "match" the input and output
impedances almost always degrades the filter frequency response. Xtal
filters are rather strange in that respect because of the complicated
things that go on inside.

Transforming the generator and load resistances to 800 ohms using L
networks is an excellent idea. Add the 3 pF that the vendor suggests.

Bill W0IYH

"David" wrote in message
...
I am wanting to place a 45MHz, 2-pole crystal filter between the first
mixer output and 2nd IF input. (Mixer output is a SA602 and IF input is
SA605).

The mixer output is around 1500 Ohms and the IF input is around 4500 Ohms
with 2.5pF capacitance parallel.

The required source and termination for the filter is 800 Ohms par with
3pF. The insertion loss of the filter is 2dB max.

Would the impedances be close enough to use resitive pad between mixer
and filter (680R in series with 1k2) and a small cap to ground across the
filter input.
Then a 980R resistor to ground at filter output and no capacitance (use
SA605 2.5pF and stray to make up the 3pF).

This would give total loss around 4.7dB.

Or would I be better to use say a tapped capacitor or L match from Mixer
to filter with
added C on filter input.

Apparently the SA605 has a "swet spot" for noise at around 800R so maybe
the Res to ground on the input would be ideal ???

Thanks for any help, I have not used crystal filters before and unsure of
implications of the approach.

Regards

David Huisman