[time-nuts] How to get unknown frequency quartz crystals oscillating

Bernd Neubig BNeubig at t-online.de
Sat Jun 4 12:49:02 EDT 2016

Tim Shoppa wrote:
>The Pierce logic-gate-biased-active oscillator is pretty reliable to start and will oscillate somewhere with most crystals from kHz to MHz.
>As you found out, it will often come up on one of many overtones.
>To reduce chance of coming at an overtone, a series resistor from logic gate output to the crystal is often enough. If not, a RC low-pass will cut down even further (although of course adding phase shift.)

This is certainly the easiest and fastest way for a go/no-go test and to find the approximate resonance frequency. 
In the attached circuit diagram make CX1 and CX2 about 10 pF and RGK several MegOhms.
The inverter gate should be preferably an unbuffered HCMOS or other fast inverter.
For crystals in the MHz range you can replace RV by a short, for kHz crystals make it a few kOhms. If testing small watch crystals @ 32768 kHz or around, RV should be 100 kOhm at least. RV reduces the crystal drive level (RF current) to an acceptable level to avoid overloading or even damaging of the crystal. For low frequency crystals the RV-CX2 lowpass also avoids start-up at the overtone.
It is recommended to add a second inverter gate at the output to isolate your oscilloscope or counter input from the oscillator stage. Add some >330 ohm in series to the output of the 2nd inverter, if you connect a coaxial cable. Then terminate the coax at the oscilloscope or frequency counter end with 50 Ohms, so the square wave form will be roughly maintained.

In this circuit the crystal will not operate at its series resonance, but at a load resonance with load capacitance of something between 8 pF and 10 pF (depending on the inverter input and output capacitance plus the stray capacitances of your test fixture).
If you want to operate the oscillator at a (low) overtone, such as 3rd (or maybe 5th), you must add a series combination of 10 nF plus an inductor in parallel to CX2. The 10 nF is to avoid DC short-circuiting of the output. The inductor together with CX2 must have a resonance frequency mid between fundamental mode and 3rd overtone (not at one of them). So the tuned circuit acts like a capacitor at the 3rd OT and is inductive at fundamental mode (thus the phase condition for oscillation is not fulfilled at the fundamental mode)
Have fun

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