Hi As long as we are beating up on the poor old SC for mode issues:
We have an C mode ( = the one that normally gets used in a precision oscillator) We have a B mode ( = the “thermometer” mode that gets used in some MCXO’s) Hmmm …. why did they start with B … hmmm …. Well yes indeed there *is* an A mode. For a given overtone it’s *above* the B mode. Exactly why they are lettered high to low … at least they are in order. Page 3-15 in: Quartz Resonator & Oscillator Tutorial - Time and Frequency ...tf.nist.gov › sim › 2010_Seminar › vig3 <https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=2ahUKEwiYxa7Z-vbnAhWJlnIEHaTSCAkQFjABegQIARAB&url=https%3A%2F%2Ftf.nist.gov%2Fsim%2F2010_Seminar%2Fvig3.ppt&usg=AOvVaw0LXXz8jKe0sNHtk3Cj18ok> Gives a nice plot of how it fits in. Why mention it? Well it’s the answer to the next up question “why is the oscillator passband so tight?” ( = you have to kill a very close mode just above the 3rd OT C mode and a closer than you would think mode below it). Bob > On Feb 29, 2020, at 6:30 AM, Bernd Neubig <[email protected]> wrote: > > 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 > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
