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

[Bernd Neubig]

Hi Gerhard,
I am rather sure that it is a 5 MHz 3rd overtone crystal.
the resistance should be in the 80 to 110 Ohm range and Q about 1.5 million. You can see the resonance without ringing in a span of 100 Hz or smaller with a sweep time of 10 sec minimum. 
See attached the response of a 5 MHz SC3 crystal in HC-40/U package.
Indeed the 5.45 MHz is the B-mode which has a temperature coefficient of -30 ppm/K
Because the crystal blank  has  a plano-convex shape. The overtones are quite far away from 3 times or 5 times the fundamental mode. 3rd overtone is about (rough guess) 2.9 time of fundamental mode.
To find them you must really carefully sweep around a few 10 to 100 kHz span with slow sweep time a narrow bandwidth

Regards
Bernd
DK1AG

-----Ursprüngliche Nachricht-----
Von: time-nuts [mailto:time-nuts-bounces at lists.febo.com] Im Auftrag von Gerhard Hoffmann via time-nuts
Gesendet: Mittwoch, 26. Februar 2020 01:42

To get a first impression, I soldered the crystal to an SMA plug and put it on an
R&S ZVB-8 network analyzer and measured S11. I could see the 5 MHz resonance
as a 15 dB dip.  There was also a resonance at 5.45 and a smaller one another 90 KHz
higher. the +10% suggest that it is an SC cut.
But I could not see anything at 1 or 1.6666 MHz, so it should be a fundamental crystal?
Is that common?
I made most measurements at room temperature. I can turn the hot air solder
station down to 91°C which is not far away from the crystal's 87.7°C
inflection point, and I could see some variation on the 5.45 MHz resonance vs. temp.
I must build a fixture for the hot air because the sweep time at 1 Hz bandwidth
is close to eternal.
Is the un-harmonicity (???) between fundamental and overtones stronger with SC-cuts
than normal AT? I also could not see anything at 15 MHz. Next I'll make a board
for the PI fixture as described by Bernd Neubig in his crystal cookbook.
BTW I could see some more dips with >= 10 Hz resolution. I hope that does not mean
that the ZVB needs service.




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