As I did not want to buy many crystals and go through all the difficult sorting, matching and calculations I decided to go for some crystal filters rather cheaply available on ebay.
In contrast with a receiver a SA resolution filter should not be as steep as possible otherwise you may miss some signal easily when you are using a too large frequency range
The NDK 10F7.5A looked suitable so I bought some. Measuring them on my VNA they al seem to be on the same center frequency (10.7MHz) which is nice!
The input impedance is, according to the datasheet, 1.5kOhm/5pF so using the online matching calculator the matching circuit should be something like this.
The 5pF of the crystal should be subtracted from the calculated value of C1 to get the actual C1
In order to confirm the matching circuit I mounted one of the filters on my universal test jig, connected the VNA and connected a tunable inductor and capacitor of about the correct value.
This simple setup lets you tune all components till you get the right performance.
After some fiddling the polar input impedance chart looked like this
Tuning could still be a bit better but the filter loss is very acceptable
As I had more of these filters the obvious next step is to use more then one. Two connected directly in series with the impedance matching at the input and output of the whole filter I got a rather disappointing result.
Way to wide, not symmetrical and too much loss
But then I remembered about connecting a small capacitor to ground in the middle of the filter
And using this ancient variable capacitor I was able to tune the filter
Adding a third filter stage and tuning for minimum loss created a somewhat wider but certainly steeper filter. I can not yet get rid of the pass band ripple but have not yet tuned the impedance matching capacitors and the input/output impedance is still a bit too high.
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