| Author |
Message |
Richard H.
Guest
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 Posted:
Tue Dec 07, 2004 8:52 pm Post subject:
Re: Ground plane under crystals |
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Dave VanHorn wrote:
| Quote: | I don't want the osc pads shorting to the can. That
would make for a lot of antenna, I'd think.
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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?)
| Quote: | 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.
[...]
Noise?
|
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! |
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Spehro Pefhany
Guest
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| Posted:
Tue Dec 07, 2004 9:12 pm Post subject:
Re: Ground plane under crystals |
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On Tue, 07 Dec 2004 08:52:51 -0700, the renowned "Richard H."
<rh86@no.spam> wrote:
| Quote: | Dave VanHorn wrote:
I don't want the osc pads shorting to the can. That
would make for a lot of antenna, I'd think.
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?)
|
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
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com |
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Richard H.
Guest
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| Posted:
Tue Dec 07, 2004 9:31 pm Post subject:
Re: Ground plane under crystals |
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Spehro Pefhany wrote:
| Quote: | 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.
|
Hmmm. Good point, above & beyond all the discussion about grounding.
| Quote: | 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.
|
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! |
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Dave VanHorn
Guest
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| Posted:
Wed Dec 08, 2004 12:08 am Post subject:
Re: Ground plane under crystals |
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| Quote: | 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?)
|
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.
| Quote: | 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.
|
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.
| Quote: |
BTW, I didn't find any images on your site that showed your approach to
crystal ground pads. Can you provide a link?
|
I don't have a convenient way to get there from my cad software. |
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