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Quartz Resonator Test Fixture: 3.58 MHz Crystal Test

Just to see if the resonator test fixture produced meaningful results, I plugged a 3.57954 MHz color burst crystal into the socket:

Quartz test fixture - 3.57954 MHz crystal

Quartz test fixture – 3.57954 MHz crystal

This is a staged recreation based on actual events; pay no attention to the Colpitts oscillators growing in the background.

Attaching goesinta and goesouta cables to the HP 8591 spectrum analyzer & tracking generator showed it worked just fine:

Quartz 3.57954 MHz - no cap

Quartz 3.57954 MHz – no cap

The reference level is -40 dBm, not the usual 0 dBm, due to the loss in those resistive pads. Unsurprisingly, the parallel resonance valley looks pretty ragged at -120 dBm = 1 nW = 7 µV.

Remove the jumper to put the capacitor in series:

Quartz 3.57954 MHz - 36.4pF

Quartz 3.57954 MHz – 36.4pF

The marker delta resolution surely isn’t 1 Hz, but 750 Hz should get us in the right ballpark.

Substituting a 72 Ω resistor, found by binary search rather than twiddling a pot:

Quartz 3.57954 MHz - 72ohm

Quartz 3.57954 MHz – 72ohm

Which gives us all the measurements:

  • Fs = 3.57824 MHz
  • Fc = Fs + 750 Hz = 3.57899 MHz
  • Rm = 72 Ω
  • C0 = 3.83 pF
  • Cpar = 3.70 pF

Turn the crank and the crystal motional parameters pop out:

  • Lm = 117 mH
  • Cm = 17 fF
  • Rm = 72 Ω
  • Q = 36 k

Looks like a pretty good crystal to me!

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  1. #1 by madbodger on 2017-03-29 - 09:28

    I assume you measured the capacitances with the other fixture? Interestingly, I’m building a Colpitts oscillator too: I had seen an example circuit (for a theremin) that used a Hartley oscillator, but I didn’t have any tapped inductors of the right values, so I rearranged the circuit into a Colpitts configuration, as it was easy to find two capacitors of appropriate values in my junk pile. Musing on it, I realized that it makes AC ground (as seen by the active element) the junction of the capacitors, magically providing an inverted signal from the bottom capacitor.

    • #2 by Ed on 2017-03-29 - 09:49

      Yup!

      Even a Darlington NPN with megohm biasing loads a 32 kHz tuning fork too much: it won’t start reliably. Works better at 60 kHz, so maybe I just ignore the problem & move on.