[time-nuts] Are there SC-crystals out there in the wild that are not Overtone?

[Bernd Neubig]

You are right. As I am usually stating in my crystal seminars: "You are ordering a crystal with one particular frequency, but the manufacturer supplies with the crystals free of charge a bunch of additional frequencies, which are not mentioned on the marking."

There is a multitude of spurious in real crystals:
1. So-called an-harmonic spurious resonances, which are all above the desired frequency. For plano-convex crystals, as the usual high-precision  overtone crystals at 5 MHz or 10 MHz, there are two or three (at least) very strong spuriii about 100 kHz ~ 200 kHz above. In poorly designed crystals these modes could be as strong or even stronger than the main mode.
2. The other overtones including the fundamental mode with their an-harmonics are also always present. Higher overtones usually have higher resistance than the main mode, but the fundamental mode of a 3rd overtone could have a lower resistance than the desired 3rd OT. The B-Mode is a temperature sensor mode with -30 ppm/K f(T) slope
3. SC-cut crystals have a strong "B-mode only 9% above the main mode, which has comparable or even lower resistance than the desired "C-mode".
4. And finally there are the higher overtones of the low frequency vibration modes such as face-shear mode etc. Those can interfere with the main mode within a small temperature interval and will cause frequency dips and activity dips ("band breaks")
 
If you try to build an oscillator with overtone crystals you must always include a kind of trap or other selective circuits to allow only the desired overtone to work. For an oscillator using a SC-cut crystal you need to add additional selectivity to avoid operation at (or jumping to) the B-mode. This could be very tricky.

Best regards
Bernd


-----Ursprüngliche Nachricht-----
Von: time-nuts [mailto:time-nuts-bounces at lists.febo.com] Im Auftrag von Dana Whitlow
Gesendet: Samstag, 29. Februar 2020 00:47

Many crystals possess spurious modes not terribly far from the desired Hi-Q mode.
Since the spurious mode(s) are lower Q, oscillation on one of these can build up faster than oscillation in the desired mode, driving the sustaining amplifier into compression before the desired oscillating mode really gets going.  This will leave only the fastest-growing mode as the winner.  This is not speculation- I've seen it happen.

My point is that just building an oscillator with an unknown crystal has no assurance of running where  you really want it to, thus leading you astray.
Discovering all these
modes is a big part of the benefit of studying the crystal with a VNA or similar instrument before building anything.  Forewarned is forearmed- you then have a better chance of building an oscillator that does what you want it to do.

Dana