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?

Say I'm getting an ECS-10.7-7.5A filter, data sheet he
http://www.ecsxtal.com/pdf/mono.pdf ... I'm reading the "termination
impedance" as 1800 ohms in parallel with 5pF or, at 10.7MHz, 1800 || -j2975 =
1318 - j797.2 ohms. From reading the application notes on ECS's site, the
idea here is that -- for the filter to work as designed -- it wants to "see"
1318 - j797.2 ohms on both the input and output, correct?

I'm also thinking that just because the filter wants to see a certain
impedance doesn't necessarily imply that the filter's own internal impedance
is the complex conjugate.

Thanks,
---Joel

Andy writes:
Joel, whenever I put in a Xtal filter, I make a parallel tank on the
input
and output so I can tune the reactance of the tank to minimize the
ripple in the filter. This is usually part of the source amp, anyway,
and having a high Q tank on the output is only a couple more parts.

You are correct in that the filter "wants" an 1800 ohm source and
load resistance. This can be achieved by impedance matching, or
by termination resistors ( whilch will lose a lot of your power ). It
is
good for the entire termination to be the input of the next device,
transformed up or down.

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....

Finally, the last adjustment is made by sweeping the bandpass
while diddling the input and output tuning until the flattest response
is noted, or at least the response the manufacturer specifies......
I peak the response at the center of the banpass first, then
sweep back and forth to get a flatter response. Usually the
peaking at the center is very close, tho..and if you have no
way to sweep, it will probly work well for voice type signals...
With data signals, well, it just depends.....

It may be useful to build a little jig to test the filter in, using
fixed resistors for source and load, and parallel tanks on both
sides, just to see if the filter does what you think it does. There is

a lot of loss here, but you would only be looking for the response.
If you don't have a sweeper, maybe you can just rock the
generator back and forth with one hand while diddling the
tanks with the other......

Good luck..

Andy W4OAH