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Which small ceramic capacitors have the worst microphonics?

Discussion in 'Electronic Design' started by Joerg, Apr 14, 2013.

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  1. Joerg

    Joerg Guest

    Does anyone know which tiny MLCC have the worst microphonics? Brand, series?

    I want to do some tests to see how well these things can sense low pitch
    sound, mainly via changes in capacitance but also piezo effects. Main
    reason is that I need a fairly low impedance, high capacitance and a
    very small size. Ideally less than 0.010" width and height but for right
    now I can test with larger ones. 0.050" or so would be fine for testing
    the concept, maybe even a little larger. A capacitance higher than
    0.01uF would be nice. The frequency response doesn't have to go beyond
    100Hz.
     
  2. John S

    John S Guest

    My suggestion, Joerg, is to get the greatest capacitance in the smallest
    size to start with.
     
  3. John S

    John S Guest

    I should have given some reasoning behind this. Sorry.

    The high capacitance in small size can only be achieved, AFIK, with high
    dielectric constants. That means a lot of variation with applied voltage
    and with mechanical variation as well. They are transducers that were
    not designed as such.

    Cheers
     
  4. whit3rd

    whit3rd Guest

    Well, the worst microphonics will be for quartz crystals and ceramic resonator
    equivalents (because they are poled). Microphonic response at frequency F
    depends on the crystal being asymmetric (so that it knows up from down).
    A non-polarized MLCC would see sound input at F and give feeble electrical
    output at 2F. Raise the bias voltage on the MLCC , maybe beyond the recommended
    applied voltage, to see electrical output at F.

    Quartz crystals and ceramic resonators are usually packaged so you cannot
    stress them with external soundwaves, but the internal elements are sensitive.
    Ceramic resonators are permanently 'poled' with no need of a bias voltage source,
    and alpha-quartz has asymmetric crystal structure which amorphous polycrystalline
    quartz lacks. Sadly, you can't make a halogen lamp envelope into a good
    microphone.
     
  5. Joerg

    Joerg Guest

    Yes, that's what I probably do. But it is often surprising, one brand
    has 2x the microphonics and all else is pretty much the same. So I was
    wondering if people knew something, such as "brand XYZ always has tons
    of microphonics" or something like that.
     
  6. Joerg

    Joerg Guest

    I can't have any bias voltage in this case. But to my surprise I just
    read a report where a C0G cap is as "bad" in microphonics as the
    ferroelectric X7R sibling. This turf seems to be full of surprises.
     
  7. Joerg

    Joerg Guest

    One would think so because it's ferroelectric but I found a Kemet study
    showing it may not be so in all cases.
     
  8. Perhaps the brands from the unknown (read Chinese) manufactors may be worse since they might not have inhouse control to minimize that parameter.

    Is this for a one-off or a production item. I wouldn't sleep nights if it were for a production item.

    Regards

    Klaus
     
  9. Joerg

    Joerg Guest

    Long term it is for production but right now only a feasibility study.
    If it works we can then determine a pathway to obtain caps of consistent
    behavior. If necessary a custom run. Some deviation is fine because
    every unit will be calibrated.

    I always sleep well :) ... Drives others crazy, I can say good night,
    turn around, and 15 seconds later ... zzzzzzz
     
  10. Guest

    I'm the same way, even during periods of high stress on the job.

    But, I have been told that could be a signal that one has sleep apnea, and it could be dangerous.

    Some specialist say that if you can fall asleep fast, that is because you get to little sleep and you should get more sleep. I get along fine with 6 hours per night, but I may just be kidding myself....

    Regards

    Klaus
     
  11. Joerg

    Joerg Guest

    I thought all Vikings could do that :)

    Yep, especially if you have cases in the immediate family. The only way
    to find out would probably be in one of those sleep diagnosis centers
    that some hospitals run. Essentially you have to stay overnight.

    6h is quite low. I try to get in 7h which isn't so easy anymore. Since
    California had a business exodus most of my clients are now 2-3
    timezones ahead. This means I now have to get up latest by 6:30am. And
    then the dogs want to be walked as well.
     
  12. mook johnson

    mook johnson Guest


    hasn't been my expereince. I once has some large like .5X x 1" stacked
    X7Rs from AVX in the lab as motor bus capacitors. When I ran the motor
    I could hear the RPM of the motor from the cap back across the lab. :)

    Replaced them with Kemet, Novacap and another custom cap supplier, and
    they were very similar.

    Can you gain some sensitivity by tuning the mounting structure (pcb I
    assume) to have a high mechanical Q at the frequency if interest?
     
  13. Joerg

    Joerg Guest

    I've had differences and those most likely had to do with the number of
    layers and things like that. Some caps were taller than others.


    Unfortunately not in this case because I have to measure from DC for
    30-40Hz.
     
  14. miso

    miso Guest

    Much like chip design, you will probably find different parts of the PCB
    have more flex, which in turn effect the cap. Usually the corners are
    considered to be the quietest.

    If you can't put the critical cap in a corner and if the PCB is
    rectangular. there is probably more flex in the long direction of the
    board, so place the caps perpendicular to the long edge.

    There may be a way to parallel caps so the microphonics cancel.
    Obviously that would take more caps of a smaller value with different
    orientations.

    If you dig into chip design books and papers, you run into this kind of
    black magic.

    Perhaps flexible (elastomer) mounting of the PCB?
     
  15. Joerg

    Joerg Guest

    I actually want microphonics in this case and the cap won't be on a PCB.
    Some tests yesterday with what I have here showed that while slight
    pushing onto the surface of the cap does generate a detectable signal I
    could hardly measure any capacitance change. Capacitance change would be
    the only way to get DC out of it.
     
  16. Klaus Bahner

    Klaus Bahner Guest

    On 14-04-2013 21:54, Joerg wrote:
    [snip]
    All of these ceramic materials *are* ferroelectric, otherwise you
    wouldn't get the high epsilon_r. Slight changes in the composition of
    the compound affects temp coefficients and other properties, hence the
    classification into X7R, C0G etc., but they are all ferroelectrics.

    I think it is impossible to deduce microphonics from these general
    material properties at all. For example I would assume that manufactures
    use tricks like varying the orientation of the layers in a multilayer
    cap to minimize the overall microphonics effect.

    So most likely you have to contact the manufacturers and try to get into
    contact with one of the component design engineers knowing the gory
    details - which most likely is easier said than done :-(

    Another loose ideas could be the IEEE Trans. Ultrasonics, Ferroelectrics
    and Frequency Control. I seem to remember an article on capacitor
    microphonics there, but a quick search didn't find anything. Most likely
    my memory doesn't serve me well here.

    Or look for PVDF (Polyvinylidene flueride) sensors, usually used as
    piezo, but the should make a good microphone, too. Perhaps there is
    something on the market which can be mounted on a PCB. The price tag
    might be much higher than a capacitor, though.

    Regards,
    Klaus
     
  17. Joerg

    Joerg Guest

    My experience with that is that they won't release much, this is usually
    closely guarded internal reseach.

    Good idea. I am a member of that society but since the Transactions no
    longer come on paper it's easier to miss stuff when you don't make
    regular checking a habit. A paper kind of pushes itself under the nose
    for the evenings but the computers are off in our house after 6:30pm or so.

    In this case it can be several Dollars per capacitor. But PVDF would not
    offer very large capacitances when small (a few hundred um by 1mm or so
    in real estate). I remember some PVDF experiments we did in medical
    ultrasound. We never warmed up to that material. It had great bandwidth
    which is important in that market but shows quite poor acoustic
    efficiency. So we kept PZT.
     
  18. mike

    mike Guest

    You don't disclose the application, and the devil is always in the details.

    I have a couple of concerns.

    A device that small and stiff is gonna have about zero sensitivity
    below 100Hz. To make it work, you'll have to "bend it" by attaching
    it to something bigger and flexible. Operational bending is bad enough.
    Shock/vibration/resonance is gonna be a reliability nightmare.

    I'm skeptical about component usage far outside the design parameters.
    I've had enough production problems when the vendor actually improved
    a published spec.
    Exploiting an unspecified characteristic that's undesirable for
    the typical application is asking for trouble.
     
  19. Joerg

    Joerg Guest

    Sensing a local pressure in liquid. It has a DC value and then I'll have
    to measure up to 20-30Hz worth of changes. Very cramped space, hence the
    0.010" width.

    I can't work on bending (which I am aware is the normal modus operandi
    of a "singing cap"). I can only work with thickness changes which will
    eb quite miniscule. Ideally with capacitance change because the signal
    coming out of it via piezo generation is very tiny, at least here on the
    bench.

    Oh, I've done that a lot :)

    One could buy half a truckload of reels and store them in a gigantic
    nitrogen cabinet. That would last nearly forever. But the better method
    is to first establish principle of operation. Then it's time to sit down
    with a manufacturer. It would not be the first time where the response
    is "You want to do WHAT?!". After they heard the Dollar numbers involved
    that usually changed to "Oh, wow, let's see how we can do this".
     
  20. Joerg

    Joerg Guest

    I don't know what "cont ip" is. But I have a working measurement setup
    here, that's not the problem.

    I know, that's the reason for my posting. Which ones are the worst?
    Because worst is best in my case.


    EMI is not a problem, we'll run this in a modulated fashion.
     
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