Quartz Resonator Test Fixture

A recent QEX article (Jan/Feb 2017 2016; sorry ’bout that), Crystal Measurement Parameters Simplified, Chuck Adams K7QO) suggested a simplified version of the K8IQY crystal parameter test fixture would work just as well for low-frequency quartz resonators:

The resistive pads eliminate the fussy toroids and their frequency dependence.

Tossing a handful of parts on a small proto board:

I found two absurdly long hunks of RG-174 coax with BNC connectors, so that’s how it connects to the outside world; sacrificing a short SMA jumper would reduce the clutter, but that’s in the nature of fine tuning. At the frequencies this fixture will see, coax properties don’t matter.

I can’t think of a better way to mount those AT26 cans than by soldering the wire leads directly to a pin header; pushing them under spring clips seems fraught with peril, not to mention excessive stray capacitance.

Measure the actual in-circuit capacitance for the 33 pF cap (shown as 39 pF in the schematic, it’s not critical), which worked out to 34.6 pF.  That’s the external series capacitance Cx.

The overall procedure, slightly modified from the original:

• Measure C0 with resonator in capacitance fixture
• Solder resonator to pins
• Remove jumper to put capacitor Cx in series
• Find series-resonant peak = Fc
• Install jumper to short Cx
• Find series-resonant peak = Fs < Fc
• Remember the peak amplitude
• Unsolder crystal
• Install suitable trimpot = Rm in socket
• Adjust trimpot to produce same output amplitude

Crunch the numbers to get the crystal’s motional parameters:

Rm = trimpot resistance
Lm = 1 / [4 π2 (Fs + Fc) (Fs - Fc) (C0 + Cx)]
Cm = 1 / [(2 π Fs)2 Lm]
Q = [2 π Fs Lm] / Rm

Then you’re done!