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

[Bob kb8tq]

Hi

Ok, so just to run the math:

5 MHz / 2.9 = 1.724 MHz

If the Q at the fundamental is 500K (a wild guess) then 1.724 MHz / 500,000 = 3.4 Hz

In a world where a synthesized sweeper *might* be stepping in 10Hz steps, that’s an 
easy one to miss.

Bob


> On Feb 26, 2020, at 11:40 PM, Bernd Neubig <BNeubig at t-online.de> wrote:
> 
> 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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