I have some Digi-Key SE3320-ND 60 Khz xctls (C-2 60.000KC-P).

My best try at measuring the series resonant frequency shows
59998 Hz. Perhaps the 60.002 Khz ones are series resonant at
60000 Hz. I don't have any of those to measure.

To help us out, it would be best if you describe your
method of testing the resonance frequencies and the
accuracy of your frequency meter/counter.
....

Measuring the exact crystal resonance frequency is
NOT a simple exercise at 60 KHz. I would suggest
looking closer at the Digi-Key links for technical
data direct from the manufacturer. Those are found
on the Digi-Key final part-number page just below
the electronic catalog page PDF link. Manufacturer's
data yields the parallel capacitance, maximum series
resonance crystal equivalent resistance, and either
the equivalent series inductance or the equivalent
series capacitance. Digi-Key is excellent in their
links to manufacturer's data in my estimation.

A -2 Hz "error" in frequency is about 33 PPM (Parts
Per Million) or 0.0033 %. That seems to be within
manufacturer's stated tolerance.

I see what you're asking.

The Digi-Key pages says 100 ppm (+/- 6 Hz @ 60 Khz). The Epson
web page says they are photolithography-finished and at least
one Epson data sheet says the standard frequency tolerance is
20 ppm. Possibly they have no problem hitting 20 ppm and the
actual tolerance is much better than that (but not over
temperature).

20 ppm is still 1.2 Hz wide and the resonance probably is
sharper than that (Q 50K?).

This measuring project started when I tried to use the Epson
crystal data to calculate what the series resonant frequency
would be of the 60 Khz parallel specified crystals. After
much mucking around with various numbers I decided to measure
it.

And you're right, I've been ignoring calibration. Here's how
I'm measuring the resonance:

I have a homebrew LC VCO running at 6 Mhz. It's full frequency
range is about 5.99180 to 6.0053 Mhz. This is divided by 100
(two 74LS90's) and then low pass filtered resulting in a sine
wave around 60 Khz.

The signal level is attenuated via 10k/1k resistors and then fed
through the crystal with a 10k load on the other side. There's
some additional loading from the x100 gain amplifier and then
into a scope.

The circuit around the xctl looks like:
.1 10k
from-e-follower-lowpass--||----/\/\/\/\/----+---| xctl |----+--- to
x100 amp


1k 10k


gnd gnd

I can see a noise widened trace on the scope plus some
switching spikes/artifacts. As I tune the VCO, the noise
trace is flat except at one specific frequency, which is
about 1 Hz at most wide where the noise band becomes a sine
wave.

I'm measuring the frequency of the 6 Mhz VCO with a
Ramsey C-125 frequency counter. It's a standard ICM7216D
counter with a cheap 10 Mhz crystal as the time base. It's
uncalibrated (other than the factory, not sure of the date,
possibly in the 70s?).

Ok, how to calibrate the frequency counter?

And how stable is the frequency counter?

I'm living in a cloud of RF noise, plus computers. In addition
the frequency counter is a real RF noise generator too
(multiplexed LEDs in addition to the counting circuitry).

By moving the counter and short wave radio to a different room
I managed to hear the 2nd harmonic of a 5 Mhz crystal oscillator
on 10 Mhz with WWV. It sounded like the beat frequency was lower
in frequency than the 100 Hz WWV modulation pulses. So an upper
bound of +/- 100 Hz at 10 Mhz would put the upper bound on the
frequency counter of 10 ppm.

I'm not really happy with this calibration, I'll have to see what
I can do to improve it.