| Author |
Message |
Richard H.
Guest
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 Posted:
Mon Dec 06, 2004 10:09 am Post subject:
Ground plane under crystals |
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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 |
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Larry Brasfield
Guest
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| Posted:
Mon Dec 06, 2004 10:25 am Post subject:
Re: Ground plane under crystals |
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"Richard H." <rh86@no.spam> wrote in message news:41B3E995.9E5EA9F7@no.spam...
| Quote: | Should the can on a crystal be tied to ground, or left floating?
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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.
| Quote: | 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?
|
The bare pad does not make much sense when you
think about it. So I hesitate to guess the "why".
| Quote: | At <50MHz, would mask really make a difference in the RF absorption?
|
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.
| Quote: | Of course, the can is quite noisy on a 'scope, so it'd seem sensible to
ground it. What's the best approach?
|
I would ground it unless it did not matter.
It commonly does not matter. Does it matter to you?
--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me. |
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legg
Guest
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| Posted:
Mon Dec 06, 2004 5:15 pm Post subject:
Re: Ground plane under crystals |
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On Sun, 05 Dec 2004 22:09:41 -0700, "Richard H." <rh86@no.spam> wrote:
| Quote: | 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 |
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Phil W
Guest
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| Posted:
Tue Dec 07, 2004 2:50 am Post subject:
Re: Ground plane under crystals |
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Soldering it makes for good mechanical stability anyway.
PW
"legg" <legg@nospam.magma.ca> wrote in message
news:31j8r0hqie0eq8n9tucm8vnc30s1kqv549@4ax.com...
| Quote: | 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 |
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Richard H.
Guest
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| Posted:
Tue Dec 07, 2004 8:50 am Post subject:
Re: Ground plane under crystals |
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Larry Brasfield wrote:
| Quote: | 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! |
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Dave VanHorn
Guest
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| Posted:
Tue Dec 07, 2004 8:56 am Post subject:
Re: Ground plane under crystals |
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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. |
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Nicholas O. Lindan
Guest
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| Posted:
Tue Dec 07, 2004 10:23 am Post subject:
Re: Ground plane under crystals |
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"Richard H." <rh86@no.spam> wrote
| Quote: | I see they offer a version with a ground pin.
|
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.
--
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer: Electronics; Informatics; Photonics.
Remove spaces etc. to reply: n o lindan at net com dot com
psst.. want to buy an f-stop timer? nolindan.com/da/fstop/ |
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Meindert Sprang
Guest
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| Posted:
Tue Dec 07, 2004 11:11 am Post subject:
Re: Ground plane under crystals |
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"Richard H." <rh86@no.spam> wrote in message
news:41B52869.EA6BE8AC@no.spam...
| Quote: | 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 |
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Richard H.
Guest
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| Posted:
Tue Dec 07, 2004 11:13 am Post subject:
Re: Ground plane under crystals |
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"Nicholas O. Lindan" wrote:
| Quote: | A bare-wire strap around the crystal is often used in
lieu of the third pin.
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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! |
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Richard H.
Guest
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| Posted:
Tue Dec 07, 2004 11:23 am Post subject:
Re: Ground plane under crystals |
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Dave VanHorn wrote:
| Quote: |
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!
| Quote: | 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! |
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Paul Taylor
Guest
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| Posted:
Tue Dec 07, 2004 1:30 pm Post subject:
Re: Ground plane under crystals |
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On Mon, 06 Dec 2004 23:13:49 -0700, Richard H. wrote:
| Quote: | "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.
|
I had heard that the metal link around the can was there to stop the pins
on the crystal from opening due to vibration + fatigue.
Can't remember were I got this information from now, but I did later come
across an example of this. A friend gave me a TV remote that suffered
from an intermittant fault and I was asked to see if I could fix it. I
opened the unit up and found a crystal with one of its leads open
circuited and the other almost in the same state. The crystal can lay
horizonal to the board, with its leads bent and then soldered - there
was no wire fitted for mechanical fixing. The fatigue occurred at the
point where the leads met the can.
After seeing the TV remote fault, I assumed that the mechanical fixing
issue was indeed the main reason for the strap, rather than for
grounding.
Of course, a lot of SM cystals available today don't suffer from
vibration in the same way and can't be easily grounded either!
Regards.
Paul.
--
Remove _rem_ before replying by email. |
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Dave VanHorn
Guest
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| Posted:
Tue Dec 07, 2004 7:18 pm Post subject:
Re: Ground plane under crystals |
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| Quote: | 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 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.
| Quote: | 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.)
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Noise?
> Thanks! |
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Nicholas O. Lindan
Guest
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| Posted:
Tue Dec 07, 2004 7:31 pm Post subject:
Re: Ground plane under crystals |
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"Paul Taylor" <pault_rem_.ngea@virgin.net> wrote
| Quote: | After seeing the TV remote fault, I assumed that the mechanical fixing
issue was indeed the main reason for the strap, rather than for
grounding.
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Ditto
--
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer: Electronics; Informatics; Photonics.
Remove spaces etc. to reply: n o lindan at net com dot com
psst.. want to buy an f-stop timer? nolindan.com/da/fstop/ |
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Richard H.
Guest
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| Posted:
Tue Dec 07, 2004 7:56 pm Post subject:
Re: Ground plane under crystals |
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Paul Taylor wrote:
| Quote: | I had heard that the metal link around the can was
there to stop the pins on the crystal from opening
due to vibration + fatigue.
|
That makes sense, of course. Curiously, the examples at-hand (20MHz
crystals) connect this bare strap to ground as well.
| Quote: | Of course, a lot of SM cystals available today
don't suffer from vibration in the same way and
can't be easily grounded either!
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Indeed. There seem to be plenty of examples doing it both ways, but
certainly the question is removed with the plastic SMD packages - either
they have a ground pad or they don't. :-) |
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Richard H.
Guest
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| Posted:
Tue Dec 07, 2004 8:35 pm Post subject:
Re: Ground plane under crystals |
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Meindert Sprang wrote:
| Quote: | 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.
|
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.
| Quote: | 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.
|
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! |
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