On Wed, 22 Mar 2006 10:29:56 +0000, John Wilkinson
wrote:

Hi,
I am using a couple of 45MHz crystal filters in my first IF amp.
Has anyone any experience of using these, and give me any advice on
problems they encountered.
I think I have a problem with screening, as they require 700 Ohms
termination resisistance.

When the manufacturers spec a termination resistance, do the filters
themselves look like say 700 Ohms, or do I need to set this externally with
a resistance?

The filters -do not- look like 700 ohm, at least not in the stopband.
Unless a filter has a dissipative loss, the only way it can attenuate
is by reflecting out-of-band signal(s). To do this, it must be highly
mismatched.

The termination impedance is what the filter needs to see to perform
as specified. You must set this either with a resistance and/or an
amplifier designed to present this impedance to the filter.