QER Crystal Filter Designer

The QER filter is a special case of ladder filter that has a very smooth passband response and is very scalable just by adding additional center sections. All capacitor values are equal and it isn’t as termination sensitive as some other filter typologies. Note that the extra paralleled crystals on the end sections don’t count as filter poles, so a 4 pole filter has 6 crystals. I find that 6 poles are really necessary for good sideband rejection. This is a good rule of thumb and isn’t peculiar to this filter topology.

A simple way to build up a filter is to design a 2-pole unit (no shunt crystals, 1 shunt cap) first and get it working. Since all caps are equal, you can start off with arbitrary cap values, say 50-80 pf fo an SSB filter, proportionally higher for CW. For development purposes, put a 500 ohm pot in series with each of your 50 ohm test equipment outputs/inputs and initially set them to mid-way. Adjust the pots for minimum ripple, or better yet, best match in the passband, then measure the pot values. This is your desired termination impedance. It will be higher for an SSB filter and lower for a CW filter.  Lower or raise the cap values for wider or narrower bandwidth respectively, re-adjusting the termination resistor values as you go. Be aware that the filter bandwidth will shrink as you add xtal/cap sections. so start off with a bandwidth about 1.5 x what you want to end up with. This matching process using variable resistor terminations will have large filter loss during development (25-30 dB), but you’ll get it all back when you match the filter impedance to the circuit load using either a transformer or LC networks. Add xtal/cap sections one at a time until you get the filter complexity and performance you’re looking for.

Here’s a simple 4-pole, 2 kHz BW design. Note: If you build this design, use your own measured crystal parameters. When I analyzed this 9 Mhz design, I used the motional parameters from a 6 Mhz crystal I had data for.

For more theory about crystal filter design, see this page on this site: Crystal Filter Design Simplified

 

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The LTSpice design file can be found here. After downloading, start LTSpice, then navigate to the file. There are notes in it that make it pretty self-explanatory and easy to use.

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