FreeLists / si-list / [SI-LIST] Re: Power and Ground Plane area

["Dr. Howard Johnson" <howie03@xxxxxxxxxx>]

The crossover occurs at that frequency where the inductance
of the transmission structure becomes significant compared
to its resitance.

For a 4-mil-wide, 50-ohm transmission line in 1/2-OZ Cu on
FR-4 material (Er=4),

The inductance L = (170 ps/in. delay)*(50 ohms impedance) =
8500 ps-ohm/in.

The resistance R = 10 ohms/meter = 0.25 ohms/inch

We seek that frequency F where 2*pi*F*L = R

   F = R / (2*pi*L) = 4.6 MHz

This frequency will go down for fatter (lower R) traces, and
up for smaller (on-chip scale) traces.

NOTE: The formula is approximate -- I didn't take into
account the AC resistance of the return pathway on the
plane, or the proximity effect on the trace itself, each of
which increase the effective resistance at 4.6 MHz by about
30-percent.

Best regards,
Dr. Howard Johnson, Signal Consulting Inc.,
tel +1 509-997-0505,  howie03@xxxxxxxxxx
http:\\sigcon.com  -- High-Speed Digital Design seminars,
books, and articles


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Chris
McGrath
Sent: Wednesday, December 01, 2004 7:17 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Power and Ground Plane area



In a recent discussion with our layout house, they mentioned
that they
had recently had some training that stated that the return
currents can
begin to return beneath the trace (path of least inductance)
at
frequencies in the kHz range.  I am in the process of trying
to get
access to some of that literature to better understand the
parameters of
the discussion including the edge rate.

Has anybody else heard this and have any literature to share
(or links
to it)?  Usually, the return current is stated to follow the
path of
least resistance at "low frequencies" (including in the
Dr.Johnson
seminar notes) but I don't recall really reading anything
about where
the crossover occurs.

-Chris


> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx=20
> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Darshan
Mehta
> Sent: Wednesday, December 01, 2004 10:01 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Power and Ground Plane area
>=20
>=20
> Hello Everybody,
>=20
> I was reading one of the IEEE paper on topic "EMC
Analysis"=20
> by Dr. Tom Van. He says that Return current takes the
least=20
> impedance path. If the signal speed is 1 KHz, there is
no=20
> need of Power or Ground Plane because even if you have
Power=20
> Plane, the return current will not effectively utilize
the=20
> plane. So my question is, how we should decide at what
signal=20
> speed we should use Plane area for Power and Ground. Is
there=20
> is any specific formula for that. Thanks.
>=20
> Best Regards
>=20
>=20
> Darshan Mehta
>=20
> =20
>=20
>=20
>       =09
> ---------------------------------
> Do you Yahoo!?
>  Meet the all-new My Yahoo! - Try it today!=20
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