EMRFD Message Archive 8874
Message Date From Subject 8874 2013-08-01 11:00:18 Bob L Is This A Method to Achieve Proper Crystal Filter Terminations? Proper input and output terminations are important to good crystal filter performance.
From a module point of view would it be better to include an input and output IF amp as part of the filter module? Would this ensure the filter part of the module has proper impedance matching?
Would we expand the building audience if we displayed some of our literature in this way?
8879 2013-08-01 16:05:24 Kerry Re: Is This A Method to Achieve Proper Crystal Filter Terminations? I'm not sure of your question/comment.
Certainly impedance-matching at the filter is important as it is elsewhere in the chain.
EMRFD espouses L-network matching and I find this very good.
Here is a filter module;
It's actually two filters; the other is behind the shield and the two are switched by the DIP switches at each end.
The matching networks are the fixed capacitor, the trimmer capacitor and the inductor at each end.
Adjustment can be made in different ways. The most elegant way, I think, is to disconnect the filter ends from the matching networks, terminate the matching networks with the filter design impedance and adjust the trimmers for maximum 50-ohm RL at the input BNCs.
Impedance of the modules before and after the filter is set by the methods described in EMRFD; 50-ohm amplifiers and/or attenuation as appropriate.