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Mechanical sensitivity of crystals...

Discussion in 'Electronic Design' started by Mike, Aug 25, 2004.

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  1. Mike

    Mike Guest

    I have a 40MHz PLL with a bandwidth less than 1Hz. The reference frequency
    is 20MHz, which is generated with a crystal oscillator module.

    When I tap the PC board with my finger - a light tap, not hard - the PLL
    temporarily loses lock. I puzzled over this for a while, then bypassed the
    crystal oscillator module with an external clock. When the external clock
    is used, the board is insensitive to my finger tap. This is repeatable, and
    a second board behaves identically. The oscillator solder joints appear to
    be good, and the signal output is okay.

    My theory is that there is a small change in the crystal oscillator
    frequency when I tap the board. The PLL has a very low bandwidth, and can't
    track the frequency change. It loses lock for a short time, until the
    crystal oscillator frequency recovers (or at least stops moving) and the
    PLL reacquires lock.

    The problem is, I've never heard of crystals having mechanical
    sensitivities like this.

    Has anyone else seen anything like this? Is this effect documented
    anywhere?

    -- Mike --
     
  2. Leon Heller

    Leon Heller Guest

    Crystals are piezo devices and will be affected by mechanical shock.

    Leon
     
  3. What do you think crystals are?.
    They mechanically distort using the piezo effect, and the vibration, is
    the basis of their behaviour. The crystal is effectively 'ringing', with
    the AC waveform matching their natural mechanical frequency. As such
    anything disturbing their movement, _will_ produce effects. This is the
    basis of many of the accelerometer devices on the market.
    In your case, mechanically 'shock mount' the crystal. Use flexible wires
    instead of the existing legs, and cushion the crystal. There are
    commerically available spring mounts, which hang the crystal on a rubber
    membrane to prevent exactly this type of problem. If you look at crystals
    supplied for high shock enviroments, they typically have larger cases,
    with the crystal element itself shock mounted internally. This mechanical
    sensitivity, is the very basis of how they work.

    Best Wishes
     
  4. Iwo Mergler

    Iwo Mergler Guest

    Yes, the crystals get noisy, when you tap them. It's
    known as microphony. You can buy crystal microphones. ;^)

    In most applications that isn't noticeable, but when
    trying to do GPS receivers with cheap crystals, it was
    a common occurence for the tracking loops to lose lock
    on such occasions.

    1Hz bandwidth at 20MHz sounds very narrow to me. Most
    crystals are specified to be 10-50ppm accurate over
    temperature and age. The cheap computer-grade oscillator
    modules are known to go beyond 100ppm.

    You seem to rely on a 0.05ppm reference from a crystal?

    Kind regards,

    Iwo
     
  5. James Meyer

    James Meyer Guest

    Although the crystal its self is a piezo device, the frequency of a
    crystal oscillator is much more likely to be affected by the mechanical
    sensitivity of the rest of the other components in the oscillator circuit.

    Connect just the crystal to the board using an inch or two of very thin
    wire bent into a zig-zag pattern and support the crystal separately from the
    board so that you can isolate the mechanical shock to the board only. I suspect
    the sensitivity to shock will appear only for the board.

    Jim
     
  6. Leon Heller

    Leon Heller Guest

    He's using an oscillator module, not a crystal.

    Leon
     
  7. Mike

    Mike Guest

    The actual system is somewhat more complex - I omitted details that weren't
    relevant. It's a data recovery system. The crystal oscillator drives a
    synthesizer that provides a frequency within a few ppm of the data
    frequency. A data recovery PLL uses a phase interpolator to generate the
    data frequency from the synthesizer output. A third PLL with a very low
    bandwidth follows the data PLL, and performs jitter attenuation. The
    absolute accuracy of the crystal doesn't have to be perfect, but sudden
    frequency changes, even by only a few Hz, can be enough to throw the final
    PLL out of lock.

    -- Mike --
     
  8. Tim Shoppa

    Tim Shoppa Guest

    They certainly do, they are mechanical devices after all.

    Crystal mounting can make the problem worse or better. Usually
    just supporting the crystal by its two leads and letting it flop around
    is the worst. Soldering the crystal can to the board is better.
    Best is to use a three-lead (TO-39-type) can, although these
    are usually only used in high-shock mil-spec situations. Even in the
    "best" case there are still microphonics.

    Tim.
     
  9. John Larkin

    John Larkin Guest


    I had a similar problem on a 155.52 MHz (OC-3 rate) PLL, used in a
    laser timing module. I used a nice low-phase-noise Vectron VCXO in an
    adaptive loop, around 3KHz in acquire mode, 300 Hz for track. Whenever
    I pulled a test SMB connector out of the board (snap!) it would lose
    lock. After much tapping, it was isolated to the VCXO and nothing
    else. If you just touch the VCXO with a small screwdriver, rather
    gently, it can lose lock. We wound up having a wire-form company make
    us a bunch of tiny custom springs to shock mount the osc from the PCB,
    one spring on each pin. Good thing we didn't use a surface-mount part!

    I've seen G-sensitivity specs on very good (SC-cut) oscillators, but
    such a spec is rare.

    John
     
  10. Mike

    Mike Guest

    I haven't opened the oscillator module, but I suspect it's a crystal, a
    couple capacitors, and an oscillator/driver IC mounted on a substrate. What
    else besides the crystal would be shock sensitive?

    -- Mike --
     
  11. Ken Smith

    Ken Smith Guest

    No the best is to take the package without the 3rd wire and shock mount
    it.

    Good crystals have a shift of about 5PPB/g at low frequencies. At higher
    frequencies, the shift can become larger as you get near the mechanical
    resonance of the packaging. Damping out the higher frequencies is really
    worth it if you are trying to get a stable frequency.
     
  12. Ken Smith

    Ken Smith Guest

    The first bet is the capacitors. If they are surface mount devices (very
    likely) flexing the substrate can change their value. A very stiff
    substrate and housing helds to prevent this.
     
  13. Mike

    Mike Guest

    Yes, I know that, but we're talking about ~0.1ppm frequency change caused
    by a relatively small mechanical movement. I'm not banging on the crystal
    itself, I'm moving the entire container up and down when I tap the PC
    board. I'm just using my finger, which is soft, so the impulse is mostly
    low frequency.

    I found a review paper on acceleration sensitivity of crystal oscillators
    on the IEEE UFFC web site. It looks like that has the information I'm
    looking for.

    -- Mike --
     
  14. colin

    colin Guest

    have u tried a diferent source of XCO ? ive taken one or two apart before
    now and they differ quite a bit some have quite elaberate springs contacting
    the crystal some seem less isolated. however if the crystal would move about
    to much inside the case the capacitance wld be altered slightly wich wld
    alter the freq slightly. i would gues the surface mount modules have no room
    for much isolation.

    ive noticed ordinary crystals if you pres on the case it changes the
    frequency significantly.

    i asume the PLL is doing more than doubling the 20mhz frequency or you could
    just use a higher freq module or a doubler ?

    Colin =^.^=
     
  15. Jamie

    Jamie Guest

    yes i have.
    the crystal is simply mounted on a couple of thin wire legs.
    vibration of the stone can cause pressure on the stone from the
    mounted legs. this can cause a resonant shift.
    being close to the case when shook can also cause problems.
    they make different crystal style cases you may want to look into that/
    in any case i really don't think you are going to leek it with in
    1 Mhz
     
  16. Norm Dresner

    Norm Dresner Guest

    Any bad (solder) connection in your circuitry or in a component is a
    possible suspect.

    Also, at your frequencies a change of even 1 pF of stray capacitance could
    affect a tuned circuit. What mechanical oscillations are you creating by
    tapping on the board?

    Norm
     
  17. Norm Dresner

    Norm Dresner Guest

    In some crystals the actual piece of quartz is more-or-less suspended by the
    leads within a frame. Even simple mechanical motion of the container can
    couple into the crystal suspension. If you were really curious, you could
    set up a second unit and beat the to frequencies together to see how much
    you're changing the actual frequency by. It'd be instructive to see what
    demodulated frequencies are there.

    Norm
     
  18. John Larkin

    John Larkin Guest


    Or listen to a harmonic on an FM radio. I bet it sounds like classic
    microphonics.

    John
     
  19. Mike

    Mike Guest

    Thanks for the replies, everyone.

    Yesterday's searching turned up several papers on vibration-induced phase
    noise. The Army has done quite a bit of work to understand these effects,
    and some crystal manufacturers (Vectron in particular) have oscillators
    specifically designed to suppress vibration-induced noise.

    In general, SC cut crystals with no shock-mounting are reported to have
    sensitivities in the range of 1ppb/g. Other cuts are typically somewhat
    worse. Vectron reports that with proper mounting, they can achieve 0.1 -
    0.01 ppb/g sensitivity.

    We did a simple 2g tip over test, measuring the frequency of the oscillator
    with the board right-side up and upside down. The result was a change in
    frequency of 2Hz. That's 50ppb/g (the change in force on the crystal is 2g)
    - roughly an order of magnitude larger than I was expecting based on the
    papers.

    By far the best reference I found was an article in the Miami Sun Post
    about "self-proclaimed de facto scientist" Carlos Dolz. Mr. Dolz placed
    three quartz watches in a centrifuge and spun them for six hours. The
    purpose was "to prove that a time shift into the future, or one into past,
    was possible ¡V in essence, 'a time machine.'" Apparently, this
    demonstration made it into a museum exhibit entitled "Playing with Time,"
    but the day it opened, after a single demonstration, Mr. Dolz was asked to
    pack it up and remove it. (This was apparently prompted by a Miami Herald
    article describing Dolz and the apparatus a few days before). During the
    demonstration, after six hours in the centrifuge, one watch gained 2.5
    seconds, one watch lost 2.5 seconds, and one watch stopped. Apparently,
    Dolz had been developing his 'time shift' theory in the museum lab since
    September 2003 and has conducted it six times successfully. Success is
    apparently when the watches in the centrifuge run _faster_ than a control
    watch. Dolz expected the watches to advance 4 seconds into the future over
    the course of a six hour test.

    This is it! We're seeing time-travel effects! Our oscillator is momentarily
    moving into the the future when I tap the board. We aren't yet sure whether
    the PLL's loss of lock occurs during the exit from the current space-time
    continuum or during reentry, but experiments are continuing.

    -- Mike --

    Some references:

    Wenzel html note on vibration induced phase noise:
    http://www.wenzel.com/documents/vibration.html

    The IEEE UFFC web site has lots of interesting info, including this paper
    on acceleration effects in quartz frequency standards:
    http://www.ieee-uffc.org/freqcontrol/quartz/vig/vigaccel.htm

    Vectron appnotes page:
    http://www.vectron.com/products/appnotes/
    This paper describes a Vectron shock-mounting system
    http://www.vectron.com/products/appnotes/Lowg_QRM.pdf

    More on self-proclaimed de facto scientist Carlos Dolz:
    http://www.miamisunpost.com/firststoryfrontpage.htm
    http://www.miami.com/mld/miamiheral...dade/cities_neighborhoods/west/8234518.htm?1c

    -- Mike --
     
  20. Greg Neff

    Greg Neff Guest

    This reminds me of a conversation I had with an engineer at Linear
    Tech (IIRC). He said that if you are using a high accuracy voltage
    reference IC, and you calibrate the board based on this reference (or
    calibrate the reference), then the calibration should be performed
    after the board is mounted in the enclosure. He said that any
    mechanical flex of the PCB during mounting could cause a corresponding
    flex of the voltage reference IC package, resulting in stress on the
    die and a slight change of the output voltage. So, if you have to
    calibrate the board prior to final assembly, then the PCB should have
    a 'U' routed out around the voltage reference reduce flex of the PCB
    under the reference. I don't know if the same principal will apply in
    your case, but it may be worth flexing the PCB to see if there is any
    effect (in addition to checking shock and vibration).


    ================================

    Greg Neff
    VP Engineering
    *Microsym* Computers Inc.
     
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