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# crystal filters input output match circuits

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#### 7rots51

crystal filter matching

In crystal filter datasheet we have termination impedance
(for example : 1200 OHM || 3pf)

for 2 pole or 4 pole filters how we can calculate matching network using L,C?

Is there anu good appnote or tutorial on this topic?Can I use Zmatch software for this purpose?

crystal filter circuit

This is a simple parallel impedance match problem and can be easily solved by hand or software as long as you know the I/P and O/P impedances you want to match to. The only thing to watch for is the required BW

V
Points: 2
impedance matching of crystal filter

7rots51 said:
In crystal filter datasheet we have termination impedance
(for example : 1200 OHM || 3pf)

for 2 pole or 4 pole filters how we can calculate matching network using L,C?

Is there anu good appnote or tutorial on this topic?Can I use Zmatch software for this purpose?

Hi,

Here is a useful page on your topic:

In case of crystal filters you can use simple L or Pi or T filter networks for mathing if there is a big difference in the needed and the actual filter impedances and you can use wideband transformers for matching if there is no big difference.
In your case if your goal is to get 50-100 Ohm (i.e. low) impedance from the 1200 Ohm filter impedance, this means a big difference in impedances, so the simplest solution would be to use a low- to high L match at the input of the crystal filter and a high-to low L match at the output. This L match is nothing else but an opened parallel resonant LC circuit. In direction low-to high matching the low impedance is connected in series with the L and the high impedance is connected in parallel with the C. See this link for instance with (Java applet) online calculation:
http://bwrc.eecs.berkeley.edu/Research/RF/projects/60GHz/matching/ImpMatch.html

I do not know the Zmatch software (perhaps now that you are aware of the L or Pi matching network properties you can answer your question if you know Zmatch) but here is a very simple to use and free, still a very good filter program with built-in L or Pi filter impedance matching features, among many other things:

Finally here is another online L match calculator to get prompt L and C values:

unkarc

PS The component values in a matching network like an L or Pi is not changing with the number of poles a crystal filter is designed: once a crystal filter has the optimal terminating impedances by design like your 1200 Ohm || 3pF, the matching network values remain the same, independently from the number of poles within the crystal filter.

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