[SI-LIST] Re: Planes under oscillator parts
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Bryan Ackerly <backerly@xxxxxxxxxxxxxx>
- Date: Wed, 01 Jun 2011 02:52:29 -0700
What you want to avoid is excess capacitance that will throw off the
oscillator or even prevent it from starting up properly. To that end
you can use your head to estimate plate capacitance between the traces
and ground placed on the adjacent layer, or further down in the
stack-up. If the parasitic capacitance is less than 5-10% of your load
capacitor's value, then you are pretty safe putting a ground polygon
there. Otherwise leave that area clear.
Yes, if you don't put any ground in it means that the field from that
little circuit will radiate more efficiently. The power level is
extremely low, so this is usually not an issue. It depends on what the
rest of your packaging looks like. If you have wires coming out of your
box, then likely you need to concern yourself much more with CM noise on
those wires than what can radiate from the 0.25" sq area or less that a
crystal and two capacitors take up.
Steve.
On 5/31/2011 10:05 PM, Bryan Ackerly wrote:
> I was looking at a data sheet for a Renesas R78 processor today and it
> indicated that the area under the oscillator crystal and load capacitors
> should be completely devoid of power or ground planes.
>
> I understand that there could be an issue with modeling load capacitance from
> a plane but otherwise I would have thought this was rather bad advice from an
> EMC perspective.
>
> Has anyone else heard of doing this? Is it based on any particular solid
> reasoning?
>
> --------------
> Bryan Ackerly.
> Schneider Electric
>
>>>
>>>
>>>
>>>
>>>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
