EMRFD Message Archive 1489

Message Date From Subject
1489 2008-03-01 22:25:03 kerrypwr Matching Crystal Filter Input & Output
Yesterday I finished a 250 Hz Gaussian-to-6 dB filter similar to the
Measurement Receiver one.

The measured response is excellent and the plot is very smooth; IL is
5 dB.

But impedance matching is poor.

The design was done with the AADE program which provides for
input/output impedance matching; I used this facility to transform the
700-ohm filter impedance to 50 at each end.

Initial testing gave poor results; 1 dB return loss!

I played-about changing values for Ca on the input but that was
cumbersome so I substituted a variable capacitor.

I can now "tune" the return loss but the maximum I can achieve is 2.4
dB; still pretty bad. This value is obtained with the variable
capacitor set to within 10% of the Ca value given by the program.

Has anyone had experience of this and can offer some advice?
1493 2008-03-02 19:06:41 kerrypwr Re: Matching Crystal Filter Input & Output
After some fiddling with Ca values I now see 12 dB RL/1.7:1 SWR on the
input port.

Not too bad; is this as good as can be expected?