[SI-LIST] Re: bypass caps on unused pcb area

Joel, yes but not quite for the reason you state.  As long as the 
capacitor distribution is not egregiously non uniform, AND ignoring 
capacitance of attached ICs, then the bypass to plane cross-over 
frequency depends on:

1. The mounted inductance of each capacitor
2. The capacitance per square inch of plane cavity, which is to say 
225pF*eR/H ( H in mils )
3. The capacitor density per square inch

The two effects of leaving large unbypassed plane areas are:

1. The cross-over frequency comes down.
2. If the area has a long extent in one direction or another then modal 
resonance can be worse which is unpleasant if it aligns to something 
coherent like a clock or clock / 2.  The ways that cavities get excited:
a. An IC draws current through the cavity.
b. Signals inject into the cavity such as when a signal references the 
top plane goes through a via and then references the bottom plane, or 
signals cross moats.

Steve.
Joel Brown wrote:
> Is it a good idea to sprinkle bypass on areas of a pcb that are otherwised
> not used except for power / ground planes?
> The idea being to keep the power / ground planes from creating a resonance
> and radiating.
> Could such a resonace be exicted if there are no traces or components
> nearby?
> If so, how do we determine the interval of placement. We have no simulation
> tools to help us.
>  
>
> ------------------------------------------------------------------
> 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:
> http://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:     
>               http://www.freelists.org/archives/si-list
> or at our remote archives:
>               http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
>               http://www.qsl.net/wb6tpu
>   
>
>
>   


-- 
Steve Weir
Teraspeed Consulting Group LLC 
121 North River Drive 
Narragansett, RI 02882 

California office
(866) 675-4630 Business
(707) 780-1951 Fax

Main office
(401) 284-1827 Business 
(401) 284-1840 Fax 

Oregon office
(503) 430-1065 Business
(503) 430-1285 Fax

http://www.teraspeed.com
This e-mail contains proprietary and confidential intellectual property of 
Teraspeed Consulting Group LLC
------------------------------------------------------------------------------------------------------
Teraspeed(R) is the registered service mark of Teraspeed Consulting Group LLC

------------------------------------------------------------------
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:
http://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:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: