Ground plane under crystals

[Richard H.]

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

 

[legg]

On Sun, 05 Dec 2004 22:09:41 -0700, "Richard H." <rh86@no.spam> wrote:

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?

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

 

[Richard H.]

Larry Brasfield wrote:

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.

It commonly does not matter. Does it matter to you?

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!

 

[Dave VanHorn]

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.

 

[Richard H.]

"Nicholas O. Lindan" wrote:

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

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!

 

[Meindert Sprang]

"Richard H." <rh86@no.spam> wrote in message
news:41B52869.EA6BE8AC@no.spam...

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.

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

 

[Richard H.]

Dave VanHorn wrote:

The reason that I put it there, is to form a shield
for the crystal leads which run back to the
processor.

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

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.

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!

 

[Geo]

On Tue, 07 Dec 2004 05:23:15 GMT, "Nicholas O. Lindan" <see@sig.com> wrote:

I don't know of any reason for a ground plane under the
crystal. TTBOMK it is a flourish added by the layout guy.
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

 

[Richard H.]

Spehro Pefhany wrote:

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.

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!