Null fundamental of tuning-fork crystal

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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?)

Re: Null fundamental of tuning-fork crystal
On Thu, 20 Apr 2017 15:37:40 -0400, bitrex wrote:

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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
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Re: Null fundamental of tuning-fork crystal
On Thu, 20 Apr 2017 14:56:45 -0500, Tim Wescott wrote:

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

<http://www.u.arizona.edu/~liusheng/research/reference/03%20CPL%202.pdf

--  
Tim Wescott
Control systems, embedded software and circuit design
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Re: Null fundamental of tuning-fork crystal

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

m


Re: Null fundamental of tuning-fork crystal
On Thu, 20 Apr 2017 13:20:31 -0700, makolber wrote:


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

--  
Tim Wescott
Control systems, embedded software and circuit design
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Re: Null fundamental of tuning-fork crystal
On 20/04/17 22:20, snipped-for-privacy@yahoo.com wrote:
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A 2MHz crystal I looked at had its 3rd overtone 2.5% below the
3rd harmonic. That's *far* away.

Jeroen Belleman

Re: Null fundamental of tuning-fork crystal
wrote:

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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  

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Re: Null fundamental of tuning-fork crystal
On 04/20/2017 04:23 PM, John Larkin wrote:
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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
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Re: Null fundamental of tuning-fork crystal
On 04/20/2017 03:56 PM, Tim Wescott wrote:
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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
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Re: Null fundamental of tuning-fork crystal
On Thu, 20 Apr 2017 19:15:43 -0400, Phil Hobbs wrote:

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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
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Re: Null fundamental of tuning-fork crystal
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How do you figure? :-)

A tuning fork is made of beams, which have distributed mass.

The overtones are likely to be very different from harmonics, partly due to  
structure (beam width becomes a considerable fraction of length) and partly  
because acoustics just tend to be dispersive.

We're rather spoiled, as EEs, that E&M is as linear and nondispersive as it  
is (and only supports transverse waves).  Though, I suppose an optical guru  
such as yourself might have a less innocent perpective. :^)

The beer bottle will also exhibit higher modes, it's just that they're less  
well coupled by the neck structure.  You'd need a bandpass on top of the  
neck to be able to "blow" that note -- just as a crystal oscillator needs a  
network to select the desired overtone.

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
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Re: Null fundamental of tuning-fork crystal
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s  

the  
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Of course. The acoustic modes extend all the way up to optical phonons.  

Mass-spring resonances don't exhibit overtones near harmonics of the lowest
-order mode. Simple ones don't exhibit higher resonances at all, unless the
 spring has mass or the mass has springiness. ;)

Cheers

Phil Hobbs  

Re: Null fundamental of tuning-fork crystal
On 4/20/2017 9:41 PM, snipped-for-privacy@gmail.com wrote:
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Which is always the case, no?

--  

Rick C

Re: Null fundamental of tuning-fork crystal
On 21/04/17 11:41, snipped-for-privacy@gmail.com wrote:
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... or the Young's modulus of the spring varies with stress.


Re: Null fundamental of tuning-fork crystal

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That's a nonlinearity, though, not a higher-order mode.  

Cheers

Phil Hobbs  

Re: Null fundamental of tuning-fork crystal
On Thu, 20 Apr 2017 19:40:25 -0500, Tim Williams wrote:

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It's easy to blow overtones on a beer bottle, jug, length of pipe, or  
whatever.  For low notes you blow lots of air at low pressure mostly  
perpendicular to the direction of the hole.  For higher notes you blow  
higher pressure air through a smaller opening in your lips, more downward  
toward the far edge of the lip of the hole.  Experiment around -- you  
should be able to easily find at least one overtone, and with experience  
you can get four or five different note out of the bottle before you  
either get tired or your housemates rip you to shreds.

--  
Tim Wescott
Control systems, embedded software and circuit design
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Re: Null fundamental of tuning-fork crystal
On Thu, 20 Apr 2017 15:37:40 -0400, bitrex

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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  

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Re: Null fundamental of tuning-fork crystal
On 04/20/2017 04:20 PM, John Larkin wrote:
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<http://laplace.ucv.cl/Cursos/FisicaExperimental_2/Old/AJP000415.pdf

Bottom right of second page


Re: Null fundamental of tuning-fork crystal
On Thu, 20 Apr 2017 16:22:32 -0400, bitrex

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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  

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Re: Null fundamental of tuning-fork crystal
On 04/20/2017 04:27 PM, John Larkin wrote:

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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


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