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:[email protected]] 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
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