Re: [SI-LIST] : "Picket Fence" (Via Fence) for increasing

Ron Miller (rmiller@Brocade.COM)
Thu, 02 Sep 1999 14:45:59 -0700

> Elya

I am attaching an internal mail I sent out recently on stitch spacing.

The actual amount of coupling through the aperatures would be difficult
to assess, but would be similar to aperature coupling from a rigid
waveguide,
for which one could check the book by Mathias and 5 other guys, titled
something like "waveguide structures"... I cant remember the rest but
it
is available through Aertech House publishers, and is very old.

> Subject:
> Calculation of ground Stitch spacing for a PCB
>
> In order to minimize the emissions from a pcb from between sandwiched
> ground layers, stitching must be a small part ~ 1/4 or 1/8 of a quarter wavelength
> at the highest frequency of intrest.
>
>
> WAVELENGTH CALCULATION
>
> For 1 GBs data the fundamental is ~500 Mhz.
> Wavlength + 1/500Mhz ~ 2ns
>
> Wavelength Calculation
> EM waves travel at ~ (170 ps/in)in FR4
> Velocity = 1/(170 ps/in) ~6 mils per picosecond
>
> So, fundamental wavelength in inches =2ns X 6mils/ps ~12 inches
> **********
> SPACING REQUIREMENT CALCULATION
>
> For a quarter wave length, which is like an open we reduce this by 1/4 ~ 3inches
>
> to reduce this significantly we should be 1/8(1/4 min) of the quarter wavelength ~ 3/8 inches
> **********
> Looking at the eye pattern and at EMI measurements, the 5th harmonic
> must be considered, so the this is further reduced by 1/5 ~3/40 in ~ 75 mils.
>
> For a round number 100 mils looks like a good compromise or even 125 mils but not less.
>
> Ron
>
> ________________________________________
>
>
>

> Dear Members,
>
> I am involved in a project where we must apply the technique of
> "Picket Fence" (Via Fence) for increasing isolation between
> conductors on a PCB.
>
> I cannot find any refercnes (papers, books, etc.) on that issue. Are
> you familiar with anything on that issue? The problem, basically, is
> - how much isolation can we get between conductors embedded between
> ground planes, or between ground and power planes, when they are
> "isolated" by a "picket/via fence".
>
> If you have come across such material, or references, I would be
> grateful if you could refer me to them. I have gone through the
> "classics" and could find nothing there, and I need some references
> for the work we are involved in...
>
> Elya B. Joffe - EMC Engineer
>
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--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
     ==========0000-(_)0000===========
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA 
95131
rmiller@brocade.com,  rbmiller@sjm.infi.net

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