| Author |
Message |
Tim Shoppa
Guest
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 Posted:
Tue Mar 01, 2005 1:23 am Post subject:
Using non-overtone crystal in overtone mode? |
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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.
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Tim Wescott
Guest
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| Posted:
Tue Mar 01, 2005 2:07 am Post subject:
Re: Using non-overtone crystal in overtone mode? |
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Tim Shoppa wrote:
| Quote: | 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.
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.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com |
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Tom Bruhns
Guest
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| Posted:
Tue Mar 01, 2005 2:33 am Post subject:
Re: Using non-overtone crystal in overtone mode? |
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Tim, would a 100MHz oscillator module do for you? See DigiKey
CTX318LVCT-ND, for example.
Cheers,
Tom
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John Fields
Guest
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| Posted:
Tue Mar 01, 2005 2:34 am Post subject:
Re: Using non-overtone crystal in overtone mode? |
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On 28 Feb 2005 11:23:50 -0800, "Tim Shoppa" <shoppa@trailing-edge.com>
wrote:
| Quote: | 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.
|
---
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
---
| Quote: | If anyone knows of a place that ships off-the-shelf 100 MHz fifth or
seventh overtone crystals, I can avoid this whole exercise.... :-)
|
---
Anybody who makes crystals ought to be able to help you out; here's a
start:
http://www.icmfg.com/
--
John Fields |
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Tim Wescott
Guest
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| Posted:
Tue Mar 01, 2005 3:27 am Post subject:
Re: Using non-overtone crystal in overtone mode? |
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John Fields wrote:
| Quote: | On 28 Feb 2005 11:23:50 -0800, "Tim Shoppa" <shoppa@trailing-edge.com
wrote:
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.
---
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
---
If anyone knows of a place that ships off-the-shelf 100 MHz fifth or
seventh overtone crystals, I can avoid this whole exercise.... :-)
---
Anybody who makes crystals ought to be able to help you out; here's a
start:
http://www.icmfg.com/
|
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:
http://literature.agilent.com/litweb/pdf/5965-7662E.pdf.
Perhaps you're thinking of SAW devices?
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com |
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RST Engineering (jw)
Guest
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| Posted:
Tue Mar 01, 2005 3:29 am Post subject:
Re: Using non-overtone crystal in overtone mode? |
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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
| Quote: |
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.
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John Fields
Guest
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| Posted:
Tue Mar 01, 2005 3:51 am Post subject:
Re: Using non-overtone crystal in overtone mode? |
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On Mon, 28 Feb 2005 13:27:04 -0800, Tim Wescott
<tim@wescottnospamdesign.com> wrote:
---
No, I was thinking they vibrated in thickness compression. Thanks for
the correction.
--
John Fields |
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Larry Brasfield
Guest
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| Posted:
Tue Mar 01, 2005 4:09 am Post subject:
Re: Using non-overtone crystal in overtone mode? |
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"Tim Wescott" <tim@wescottnospamdesign.com> wrote
in message news:112734jd5n2lf2f@corp.supernews.com...
....
| Quote: | 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: http://literature.agilent.com/litweb/pdf/5965-7662E.pdf.
|
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.
--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me. |
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Tim Wescott
Guest
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| Posted:
Tue Mar 01, 2005 4:26 am Post subject:
Re: Using non-overtone crystal in overtone mode? |
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Larry Brasfield wrote:
| Quote: | "Tim Wescott" <tim@wescottnospamdesign.com> wrote
in message news:112734jd5n2lf2f@corp.supernews.com...
...
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: http://literature.agilent.com/litweb/pdf/5965-7662E.pdf.
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.
I pointed that out in a previous post. But hey -- wouldn't it be fun to |
have an oscillator that yodels?
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com |
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J M Noeding
Guest
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| Posted:
Tue Mar 01, 2005 4:55 am Post subject:
Re: Using non-overtone crystal in overtone mode? |
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On Mon, 28 Feb 2005 13:29:15 -0800, "RST Engineering \(jw\)"
<jim@rstengineering.com> wrote:
| Quote: | 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
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.
a CB xtal will probably operate on 100MHz, althouth I've only seen |
applications for 45 and 81MHz
-jm
---
J. M. Noeding, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/c.htm |
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douglas dwyer
Guest
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| Posted:
Tue Mar 01, 2005 5:03 am Post subject:
Re: Using non-overtone crystal in overtone mode? |
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In message <1109618629.998142.124510@o13g2000cwo.googlegroups.com>, Tim
Shoppa <shoppa@trailing-edge.com> writes
| Quote: | 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.
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.
--
dd |
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Frank Moe
Guest
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| Posted:
Tue Mar 01, 2005 5:07 am Post subject:
Re: Using non-overtone crystal in overtone mode? |
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"Tim Shoppa" <shoppa@trailing-edge.com> wrote in message news:<1109618629.998142.124510@o13g2000cwo.googlegroups.com>...
| Quote: | 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.
|
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 |
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Tim Wescott
Guest
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| Posted:
Tue Mar 01, 2005 6:10 am Post subject:
Re: Using non-overtone crystal in overtone mode? |
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Active8 wrote:
| Quote: | On Mon, 28 Feb 2005 12:07:49 -0800, Tim Wescott wrote:
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.
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.
IDNKWTFIAS: I Don't Know What I Am Saying. |
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com |
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Active8
Guest
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| Posted:
Tue Mar 01, 2005 6:10 am Post subject:
Re: Using non-overtone crystal in overtone mode? |
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On Mon, 28 Feb 2005 13:29:15 -0800, RST Engineering (jw) wrote:
| Quote: | 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
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.
--
Best Regards,
Mike |
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Active8
Guest
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| Posted:
Tue Mar 01, 2005 6:10 am Post subject:
Re: Using non-overtone crystal in overtone mode? |
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On Mon, 28 Feb 2005 12:07:49 -0800, Tim Wescott wrote:
| Quote: |
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.
|
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.
--
Best Regards,
Mike |
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