Crystal Filter Test Jig

The following circuit shows a simple test jig that may be used to test most crystal filters.


If the filter requires purely resistive terminations then the optional inductors will be required to tune out the stray capacitances at the input and output points.

Connect up as shown, set the scope for 10mV sensitivity and the generator for 100mV output. Tune the generator around the filter frequency until an indication is seen on the scope. Select external FM and adjust the frequency deviation so that the filter passband is clearly visible. Now adjust the variable resistors and capacitors for the best filter passband response (least ripple) - there may be some interaction in the adjustments.

If zero pF is required then add the inductors and adjust them for resonance  - the inductors will need to resonate at the filter frequency with around 5 - 20pF of tuning capacitance.

Reverse the filter and repeat the measurements to check if it is symmetrical or not.

If the resistors end up at maximum then they may need to be increased in value.

The termination impedance may now be estimated from the trimmer settings and measuring the resistor values with a test meter.If you have a lot of local RF interference then the jig may need to be screened.

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