LF Crystal Tester: Variable CX

Replacing the 22 pF series capacitor with a variable cap went smoothly after I got over having to rip-and-replace the adjacent socket and header, too:

LF Crystal Tester - variable CX
LF Crystal Tester – variable CX

The circuit remains the same, plus a test point to simplify measuring the actual capacitance:

Test Fixture - variable CX
Test Fixture – variable CX

I didn’t add a jumper to disconnect the crystal fixture, because (I think) it would add too much uncontrolled stray capacitance: removing the header would disconnect the socket / header wires.

The little red cap adjusts from (nominally) 3 pF to 28 pF over half a turn, without a stop. The rotor does have a marked side, but basically you’re supposed to tune for best picture and leave it at that.

The AADE L/C meter works fine, but in the low pF range everything affects the reading. The only way to measure the actual capacitance seems to be:

  • Clip one lead to the top of the 24 Ω terminating resistor
  • Hold the other within a millimeter of the test point pin
  • Zero the meter, note any residual offset
  • Touch clip lead to test pin
  • Note reading, mentally subtract residual offset

The as-installed range spans 6.5 pF to 28 pF. I think I can measure it to within ±0.05 pF, with a considerable dependence on maintaining the same pressure on the clip lead.

I suppose if you were doing this for real, you’d throw another Teledyne relay at the problem.


3 thoughts on “LF Crystal Tester: Variable CX

    1. Easier than high-impedance circuitry, though, where I also gotta stop breathing for a while!

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