Crystal Capacitance Measurement Fixture

Crystal Capacitance Fixture
Crystal Capacitance Fixture

The trick to measuring small capacitance values is to get your body out of the loop: this fixture holds a crystal rigidly and makes solid contacts to both the case and leads while measuring the internal crystal-to-case capacitance. It’s sized for wire-terminated HC-49/U and HC-49/US cases, but you could obviously adapt it to other cases.

The meter is an AADE L/C Meter IIB, with binding posts on 3/4″ centers. The post caps are plastic, so the only reliable connection is on the bottom surface. I used double-sided 62-mil PCB material for the fixture base plate, with copper-foil tape wrapped around the sides and soldered along the top edge; the adhesive is allegedly conductive, but I suspect that’s for static dissipation and shielding, rather than for actually conducting signal current. Kapton tape over the copper foil prevents gashes on the sharp edges.

Notes:

  • Run a 1/8″ end mill across (both sides of!) the PCB and drill 1/4″ holes with a step drill at the appropriate spots.
  • A 25-mil brass shim stock rectangle soldered to the right half supports the crystal case to get the flange off the board.
  • Slice up an alligator clip with a Dremel cutoff wheel, drill a hole in the board to mount it with the screw that’s supposed to secure its wire, and it’ll hold crystal cans down with grim determination.
  • A snippet of phosphor bronze spring stock, bent in a slight arc with a tab soldered to the board at the far end, holds the crystal leads against the PCB. You could probably use brass shim stock.
  • The black strip on the far side of the binding posts is half of a wire-wrap IC socket, leads bent & clipped appropriately, then soldered to the underside foil. That’s where you stick crystals to measure their lead-to-lead capacitance (aka Co or C0). Works fine for through-hole caps, too.

A detail view…

Crystal Capacitance Fixture - Detail
Crystal Capacitance Fixture - Detail

Typical crystal case-to-lead capacitance is on the order of a few pF, so zeroing the fixture capacitance is important: (meter + fixture) weighs in at about 6 pF.

There’s another half pF of crystal-lead-to-fixture capacitance that’s nullable by positioning the cap leads just over the spring contact when zeroing the meter.

There’s essentially no stray capacitance due to a crystal in the socket strip along the back; zeroing the meter without the crystal seems to be adequate.

I find that pressing the Zero button with a screwdriver while bracing the other side of the case with another screwdriver gives the best results; doing it with fingers produces about +0.5 pF offset.

Done right, the meter reads within ±0.03 pF of zero and remains stable as long as you stay away while reading the digits!