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Null fundamental of tuning-fork crystal

Is it possible to add something to the Colpitts oscillator topology to null the fundamental of a standard 32.768 kHz "tuning fork" crystal, and run the oscillator at the crystal's next highest mode (191kHz?)

Reply to
bitrex
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Does a tuning fork crystal have a mode at 3x the fundamental? I've always wondered.

At any rate, circuits abound for 3rd-overtone oscillators for AT-cut crystals. I'd assume that something similar would work for a tuning-fork crystal, assuming that it really does have a resonance at the 3rd overtone.

In general, you make a tuned-base, tuned-collector amplifier with the crystal as the element on the collector. It depends on the B-C capacitance for feedback, so it may need some external capacitance to start singing.

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
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Reply to
Tim Wescott

Hmm. Not much out there. This is what my Google-Fu found. Q is lower (?!?):

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
http://www.wescottdesign.com
Reply to
Tim Wescott

and interestingly, in the case of crystals anyway, the THIRD OVERTONE is __not__ exactly at the third harmonic of the fundamental.

m
Reply to
makolber

Does it have such a mode? That's the 5.8th harmonic.

Overtone oscillators usually have a resonant LC circuit to center the loop gain on the desired harmonic.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

... and that's why we say overtone, and not harmonic.

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
http://www.wescottdesign.com
Reply to
Tim Wescott

Bottom right of second page

Reply to
bitrex

A scanning microscope might want very low motional amplitude. An oscillator won't control that well. It should maybe be driven by a separate source.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

What's the application?

I wanted to use a tuning-fork crystal as a cryogenic (liquid helium) temperature sensor, but I couldn't get the customer interested. So we stuck with silicon diodes.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

Just a real cheap but somewhat stable clock to drive a switched capacitor filter and a binary counter. Crystals in the 100-200kHz range are expensive but watch crystals are literally a dime a dozen. Problem is 32kHz is too low a "sampling rate."

I was thinking of just resonating a watch crystal at an overtone. You can crystal-lock a bipolar 555 and get a buffered output that can drive some current if the frequency is well under 1Mhz.

20 cents for the whole subcircuit would be a nice price
Reply to
bitrex

A 2MHz crystal I looked at had its 3rd overtone 2.5% below the

3rd harmonic. That's *far* away.

Jeroen Belleman

Reply to
Jeroen Belleman

Very unlikely. Tuning forks are mass-spring resonators, not transmission lines. It's like blowing across a beer bottle and expecting the next resonance to be at 3x. (The beer bottle (Helmholtz) resonance is also a mass-spring type--the mass of the air in the neck and the compressibility of the rest of the air.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

The benefit to using an oscillator is that you can scan much faster. Changing the force gradient between tip and sample changes the coefficients of the differential equation governing the resonance, so it responds instantly instead of taking Q cycles to do it.

The speed difference is startling.

Cheers

Phil Hobbs (designer of the world's first commercial atomic force microscope)

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Use it to injection-lock a higher frequency oscillator.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Hey, thats a good idea. Maybe not phase lock the free-running oscillator, but at least lock its frequency to some multiple of the lower frequency

Reply to
bitrex

Well, yes, exactly. That's what I meant, more or less.

Except that the next resonance up on a beer bottle isn't an octave -- it appears to be a function of the shape of the bottle. I just checked a standard long-neck beer bottle (F# above middle C fundamental, an the C# two octaves above middle C for the first overtone). A one-liter cheap wine bottle sounded much lower in the fundamental, and higher than the beer bottle on the first overtone. I'm pretty sure that for a sudden transition, the first overtone is close to what you'd get if you just cut the neck off of the bottle. For a one-octave overtone you probably need a cylindrical tube.

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
http://www.wescottdesign.com
Reply to
Tim Wescott

What I said before about prior art from regular old 3rd-overtone crystals. You'd have to take care to get oscillation in bending and not torsion -- one of the issues of using "any old" crystal in more-than-3rd overtone service in amateur radio is that you get undesired modes of vibration close to the desired one, with the accompanying mode jumping.

By the time you've gotten your analog electronics to the point where the oscillator will reliably start at the right frequency, you may be much better off with a fundamental oscillator and a PLL.

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
http://www.wescottdesign.com
Reply to
Tim Wescott

+1! A 555 should be stable enough to injection lock at 5x the crystal frequency.

Or use the crystal oscillator and a much-maligned 4046. Use the XOR phase comparator to lock to the 5th-harmonic of a 32.768kHz square wave

-- I think the 4046 oscillator is stable enough for this to be a valid approach.

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
http://www.wescottdesign.com
Reply to
Tim Wescott

Yeah, it's easier just to sync the higher frequency. I have a counter already that can tap off a division

Reply to
bitrex

Oh hell. In that case, a much-maligned 4046 should do. Except, if DigiKey is to be believed, it takes up 3/4 of your 20-cent parts budget.

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
http://www.wescottdesign.com
Reply to
Tim Wescott

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