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

[Dana Whitlow]

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

On Thu, Feb 27, 2020 at 10:46 AM Richard (Rick) Karlquist <
richard at karlquist.com> wrote:

> OTOH, you could build a simple Colpitts
> oscillator and see where it oscillates.
> That's what they did back in the dark
> ages.
>
> Any time nut should be up for that.
>
> Rick N6RK
>
> On 2/27/2020 5:35 AM, Bob kb8tq wrote:
> > 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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