Crystal filter thought

[brandon7861]

Has anyone ever made a crystal ladder filter with two additional shunt filters before and after it tuned directly above and below the pass band of the middle filter? I imagine it might round the edges of the center filter's pass band, but I think it would also steepen the roll off. I know finding enough crystals that close would be tricky. I do have 300 13.5MHz crystals, but I don't think there is enough frequency variation to make all three filters. Total waste of time?

 

[nomadradio]

I remember a magazine article from years ago that showed how to start out with a pile of crystals with the same marked frequency and sort them by individual resonant frequencies. The intent was to choose a set of four or six to make a lattice filter for CW. Your 13.5 Mhz crystals might have enough difference in resonant frequence to make a filter wide enough for sideband. Just wish I could remember where I saw that. Maybe 73 magazine? Maybe "Ham Radio". That one ceased publication decades ago, but had a lot of cool tech ideas.

73

 

[brandon7861]

I watched a couple videos on Cohn and QER filters. I am particularly interested in the QER filter as it has very little pass band ripple. Both seem simple enough to make with all caps being equal. The higher the capacitance, the narrower the bandwidth.

I am hoping I have enough matching crystals to make multiple filters of different bandwidths for the same radio without having to shift an IF to use them. The dozen I tried in an oscillator were within ±300Hz. I know I should have them within 20Hz for sideband, but I have heard within 100Hz will work.