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Using non-overtone crystal in overtone mode?

Are "overtone" crystals cut differently than "fundamental" crystals? Or are they just specified differently?

In particular, say I took a garden-variety 20MHz fundamental microprocessor crystal and instead used it at its fifth overtone, trying to hit 100 MHz. The LC network is there to make sure that it's on its fifth overtone. Will this "misuse" mean that the oscillator will be harder to start up, less stable, more noisy, ???, than a crystal oscillator made out of a real overtone crystal? I don't mind if I "miss" 100 MHz by a several tens or hundreds of ppm, as long as it's stable there.

If anyone knows of a place that ships off-the-shelf 100 MHz fifth or seventh overtone crystals, I can avoid this whole exercise.... :-)

Tim.

Reply to
Tim Shoppa
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Overtone crystal cuts are not fundamentally different from fundamental crystal cuts, so to a 1st-order approximation they'll work. Crystals do have spurious responses that can cause mode jumping, and these responses don't necessarily map the same way the overtones do, so using a 20MHz crystal at 100MHz may or may not work, depending on the luck of the draw. Other than that I don't know of any differences.

IIRC Digi-Key has 100MHz crystals, but I may be remembering 100MHz oscillators. YMMV. IDNKWTFIAS. Caviat Emptor (so _that's_ what CE means! Here I thought it was a quality mark). Etc.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Reply to
Tim Wescott

Tim, would a 100MHz oscillator module do for you? See DigiKey CTX318LVCT-ND, for example.

Cheers, Tom

Reply to
Tom Bruhns

In an AT cut crystal the overtone modes are close, but not exactly on, the odd harmonics of the fundamental. Furthermore, all of the literature that I've read on AT cut crystals reports that they vibrate in the bulk of the crystal, in shear mode -- see figure 7 here:

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Perhaps you're thinking of SAW devices?

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Reply to
Tim Wescott

I don't know why off-the-shelf crystals are needed when Jan Crystal (Ft. Myers FL) will make the crystal to your specifications in a few days for the same amount of money. They can do fifth ot at 100 MHz. quite easily.

Jim

Reply to
RST Engineering (jw)

"Tim Wescott" wrote in message news: snipped-for-privacy@corp.supernews.com... ....

harmonics of the fundamental. Furthermore, all of

the bulk of the crystal, in shear mode -- see figure

One effect to watch out for with use of unspecified overtone modes is that the behavior of the resonator is not ideal; the presence or size of nearby spurs and the Q depend on how uniform the thickness is that determines frequency and the placement and size of contact metal. The wavelength is typically much less than the dimension along the non-shearing axis, so having a single mode of resonance near the nominal frequency or its overtones is not guaranteed, except by careful construction and verification. So, clearly, a guarantee about the behavior near the fundamental resonance cannot be extended to the overtone modes.

If I was trying to build a stable and pure oscillator operating at a crystal overtone, I would buy the crystal specified for the overtone I would be using.

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--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
Reply to
Larry Brasfield

harmonics of the fundamental. Furthermore, all of

the bulk of the crystal, in shear mode -- see figure

I pointed that out in a previous post. But hey -- wouldn't it be fun to have an oscillator that yodels?

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Reply to
Tim Wescott 

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You can use a fundamental mode crystal as an overtone oscillator, but
even if you can get it to oscillate, it won't be generating an
overtone at 100MHz, since overtone modes of oscillation aren't
harmonically related to the fundamental.  It's more like the slab of
crystal is vibrating like the drumhead of a steel drum with small
areas of the slab vibrating at higher frequencies, instead of the
entire slab virbarting at just one frequency.

Check out "Chladni patterns" if you're interested.

Here's some pattrens for violin tops and circular plates:

http://www.phys.unsw.edu.au/~jw/chladni.html
Reply to
John Fields

Tim,

to get optimum performance one would grind the 100MHz 5.OT finer or even polish it, and the thickness of the electrodes might be different to get optimum Q. But you should be ok by using a 20MHz fundamental in its 5th.

There are also manufacturers that make 100 in fundamental (up to about

200MHz), and many should have 100 in 5th as standard part...

Frank

Reply to
Frank Moe 

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No, I was thinking they vibrated in thickness compression.  Thanks for
the correction.
Reply to
John Fields

IDNKWTFIAS: I Don't Know What I Am Saying.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Reply to
Tim Wescott

That is total and absolute bullpuckey.

Jim

Reply to
RST Engineering

Dialog news reader's tip popped up to say that CE is "creative editing". It doesn't KWTF IDNKWTFIAS is, but I got everything but the IAS part.

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Best Regards,
Mike
Reply to
Active8

Jan's what I was about to suggest. I thought the rock I needed would have been off the shelf, but they made it and sent the test results.

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Best Regards,
Mike
Reply to
Active8

In message , Tim Shoppa writes

Overtone crystals are mechanical resonators and the overtone shear mode which has additional shear planes within the volume wont occupy exactly the same volume as the fundamental so the frequency will not be exactly

3X or 5X the fundamental but approx 2000ppm high or low? The fundamental crystal will not be so accurately polished or dimensioned as the overtone so it will not go well if at all also it may have higher levels of spurious. 
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dd
Reply to
douglas dwyer

a CB xtal will probably operate on 100MHz, althouth I've only seen applications for 45 and 81MHz

-jm

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J. M. Noeding, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/c.htm
Reply to
J M Noeding

Hello Tim,

As Douglas said the frequency may be a bit off unless you get a crystal made for 5th. An alternative for the 20MHz garden variety would be to make a 20MHz oscillator, run it into a fast gate and fish out the 5th the old fashioned way, with an LC circuit. Then run that through a gate again if needed.

Regards, Joerg

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Reply to
Joerg

Hello Jim,

Huh? See the 2nd paragraph (mode):

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Regards, Joerg

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Reply to
Joerg

Your Mileage May Vary

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Beware of those who post from srvinet.com!

Michael A. Terrell
Central Florida
Reply to
Michael A. Terrell 

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RST what???
Reply to
John Fields

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