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Ground plane under crystals

Discussion in 'Electronic Design' started by Richard H., Dec 6, 2004.

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  1. Richard H.

    Richard H. Guest

    Should the can on a crystal be tied to ground, or left floating?

    I usually see ground planes under crystals, sometimes with solder mask,
    sometimes intentionally bare/tinned. If the can should be grounded, it
    seems sloppy to rely on contact instead of a joint, so why the bare pad
    instead of solder mask? At <50MHz, would mask really make a difference
    in the RF absorption?

    Of course, the can is quite noisy on a 'scope, so it'd seem sensible to
    ground it. What's the best approach?

    As a point of reference: http://www.ecsxtal.com/pdf2/hc-49us.pdf
    I see they offer a version with a ground pin. Maybe that's the solution
    to my quandry, but I don't see it done commonly.

    Thanks,
    Richard
     
  2. If you care about close-in phase noise, ground it.
    Otherwise, if you will be happy with any frequency
    that falls within the xtal tolerance, don't bother.
    The bare pad does not make much sense when you
    think about it. So I hesitate to guess the "why".
    It will certainly force non-contact, at least under any
    normal vibration. It would slightly reduce capacitance
    between the can and ground (assuming the pad is
    grounded). This, in turn, will make the capacitance
    between the terminals and ground slightly lower and
    potentially much more stable. I doubt that loss in the
    dielectric will have any significance, at any frequency
    that crystals are ordinarily operated.
    I would ground it unless it did not matter.
    It commonly does not matter. Does it matter to you?
     
  3. legg

    legg Guest

    I've seen crystal oscillators stopped (temporarily) or staggered by
    noisy switch closure or other randomly pulsed EMI, when the case was
    not grounded. I ground them, even if just with a solder blob - this is
    easier if the ground plane has no mask.

    RL
     
  4. Phil W

    Phil W Guest

    Soldering it makes for good mechanical stability anyway.

    PW
     
  5. Richard H.

    Richard H. Guest

    I don't know yet. I strive to ask "why", rather than ignoring a common
    practice, or copying it blindly.

    It would seem to have benefits either way, but casual contact seems very
    haphazard and soldering the can doesn't seem right. I suppose I'll get
    the flavor with the ground pin and remove all doubt about whether it is
    / should be grounded.

    Thanks to all for the comments!
     
  6. Dave VanHorn

    Dave VanHorn Guest

    The reason that I put it there, is to form a shield for the crystal leads
    which run back to the processor. On my boards, you'll see that this ground
    under the crystal is an isolated finger, and touches nothing except the
    crystal caps, and the nearest uP ground pin. From there, it joins system
    ground. I normally don't connect the cans to the plane, in fact I use
    insulators under the cans to make sure nothing makes any unapproved
    connections.
     
  7. Grounding the can is a generally good idea -- as it is
    a bad idea to have a large metallic object that is not
    tied to a known voltage.

    The extra pin also serves to hold the crystal in place.
    An unsupported through-hole crystal will otherwise put a
    mechanical cantilevered load on it's leads when the board
    is subject to vibration/getting dropped.

    A bare-wire strap around the crystal is often used in
    lieu of the third pin.

    I don't know of any reason for a ground plane under the
    crystal. TTBOMK it is a flourish added by the layout guy.
    It also saves on etchant.
     
  8. I have never done it and recently, one of my boards was tested for IEC60945
    (very stringent EMC test). It came out right.
    Millions of receivers/transmitters have been produced in the past were
    crystals were plugged in sockets. No ground connection. Many of the boards I
    see have no ground connection. The only ones that have, are the ones where
    the crystals are mounted horizontally. Here it is just done to prevent them
    from moving under high G loads.
    Oh, and SMD crystals do not even have the possibility for a ground
    connection.

    And based on the nature of a crystal (low frequency compared to it's
    simensions and high Q) I'd like to thing they don't radiate at all.

    Meindert
     
  9. Richard H.

    Richard H. Guest

    Indeed - I'd forgotten about that, and a couple boards I've just grabbed
    from the junkpile also tie this strap to ground.

    The particular crystals I'm using are low-profile cans, so they install
    vertically instead of bending the leads. But the strap-around-the-can
    seems to endorse the practice of grounding the can.

    Thanks!
     
  10. Richard H.

    Richard H. Guest

    Interesting. I presume you've got some pics on your site, so I'll go
    check this out. Thanks!

    OK, I'll bite - why do you want the can floating instead of grounded?
    (What insulators do you use? And in addition to the solder mask, or
    only on bare boards?)

    I'd think grounding might help EMI emissions, but it seems you're
    concerned about causing circuit problems? Perhaps when the "noise"
    being grounded is 0 or 180 degrees to the crystal signal? (Speculating
    here... my knowledge of crystals and harmonics is limited.)

    Thanks!
     
  11. Dave VanHorn

    Dave VanHorn Guest

    I don't know where we get them, but they are little sheet insulators with
    holes for the leads.
    I don't want the osc pads shorting to the can. That would make for a lot of
    antenna, I'd think.
    I'm not worried about the can itself. The signals on the two leads are
    nearly the same amplitude, and 180 degrees apart, so in such a small area,
    they should cancel nicely.
     
  12. Richard H.

    Richard H. Guest

    All excellent points. Evidence suggests that bare pads under crystals
    and/or grounding of restraining straps may be overkill, precautionary,
    or maybe just superstition. And some folks go out of their way to avoid
    it. Very curious.

    In the end, this is probably the key - whether these emissions are even
    significant, to the circuit itself or for EMI. And with SMD crystals,
    there's really no question about how it should be done.

    Thanks!
     
  13. Geo

    Geo Guest

    If the crystal is mounted horizontally then I place copper on the top layer to
    stop me (or the autorouter) using the space for anything else.

    Geo
     
  14. Richard H.

    Richard H. Guest

    That's actually what led me to start wondering here, after a recent
    batch of boards that didn't have solder mask under the can.

    With solder mask, are the extra insulators really needed, or do you work
    much with bare boards? (i.e., concerned that vibration would damage the
    mask underneath?)

    Again, maybe a moot issue, or a bad example I've run across... on a
    recent board, I happened to probe the can with my scope and noticed a
    lot of signal on it. Apparently it's not strong enough to be an EMI
    issue, but it got me to thinking about grounding the can.


    BTW, I didn't find any images on your site that showed your approach to
    crystal ground pads. Can you provide a link?

    Thanks!
     
  15. For typical low-profile through-hole crystals, I use a keepout larger
    than the crystal can in the footprint so that no copper ends up under
    the crystal itself (on the top layer) other than the pads. I like to
    see a ground plane under the crystal on some other layer. The mask
    would "probably" be okay if it visually looks okay with no copper
    edges showing through on the sides of traces, and if vibration is that
    bad, the crystal leads will probably fail first.

    If there are no stand-offs under the crystal, it's possible for solder
    to come up through the holes and form a short to the crystal can. It
    probably would be a good idea to keep the pad size minimum on the top
    layer consistent with the design rules, and larger on the bottom if
    desired.


    Best regards,
    Spehro Pefhany
     
  16. Richard H.

    Richard H. Guest

    Hmmm. Good point, above & beyond all the discussion about grounding.

    I've wanted this for other reasons, and it seems to be the one thing I'd
    like that Eagle doesn't do yet... for some reason, it makes THT pads the
    same size on top and bottom.

    Thanks!
     
  17. Dave VanHorn

    Dave VanHorn Guest

    Depends on the pad geometry, and the way the bottom of the crystal is
    shaped.
    The insulators make sure it will never be a problem.
    I've used that point to check oscillator function.
    In essence, you're capacitively coupled to the oscillator, on both sides, so
    whichever side is stronger (output pin) dominates. It's probably best to
    ground the can, but then you need a manual solder blob, or a three-pin
    crystal with can ground pin. I haven't seen it be a problem.
    I don't have a convenient way to get there from my cad software.
     
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