"AndyS" wrote in message
ups.com...

Joel Kolstad wrote:
For an IF filter, I'm planning on using a an off-the-DigiKey-shelf
crystal
filter. I need (well, want) to match the filter to 50 ohms.

Could someone verify I'm interpreting the data sheets & terminology
correctly?


The same can be done with the source impedance. However,
the source may not like the Xtal filter as a load, since, even tho you
may have the transformation exactly right, the load presented to
the source is very complex, and may be very high ( or low) as soon
as you leave the xtal filter bandpass frequency..... Consequently ,
I have always used a load resistor in the output of the source
amplifier, either as part of the coil Q, or as a fixed value resistor
...... just to
keep the load from going very very high outside of the bandpass
and making the source amplifier go unstable......Sure, I lose a bit
of gain, but a stable amp is more important than an extra few db
of gain....


A large increase in a receiver amplifier load impedance just outside the
passband of a crystal filter (or other types of filter) can cause adjacent
channel susceptibility. The network between the amplifier and the filter
should be designed to prevent this occurrence. There are networks that can
reverse this effect, so the impedance drops instead of increasing.
Simulations and impedance measurements can verify.

Bill W0IYH