Re: decoupling/ bypass capacitors at connectors

Doug Brooks (doug@eskimo.com)
Thu, 06 Mar 1997 11:03:39 -0500

This is a very interesting and complex subject.

I call your attention to a study that specifically looked for the
effects of transfering a trace to reference a different plane,
without vias or close decoupling:

The Effects of Vias on PCB Traces
Printed Circuits Design Magazine (Miller Freeman)
August, 1996 starting page 22

It is also available in .pdf (Adobe Acrobat) form at:
www.eskimo.com/~ultra/homepage.htm --- follow the links
to technical notes.

Test boards currently exist at the University of Washington and
at the University of Missouri and might be made available to
qualified parties for additional study if anyone thinks that
the evaluation was incomplete or in error.

Doug Brooks

> For 0.5ns requirements, the stitching has no effect except immediately
> adjacent to the signal trace or connectors. The displacement current will
> find the most proximal structure to propagate the image current.
>
> Hans Mellberg
> Consultant
>

The stitching of ground planes every square inch of the board will have
significant effect. Imagine a signal on the top layer of the card
that is referenced to a ground plane immediately below. The signal
goes down a via, through the board and takes off on a trace referenced
to another ground plane on the bottom of the board.

The question is, what happens to the return current. If the rise time
is .5 nSec, the 'length' of the rise time will be 3 inches, assuming 6
inches/nSec. (If these are exterior traces, the velocity may be closer
to 9 inches/nSec, making the distance traveled in the rise time 4.5
inches.)

Suppose there is a ground plane stitch via within 1 inch of the signal
via (1/3 of a rise time distance). True, there will be an impedance
discontinuity as signal current must depart from the return current as
the currents go through the vias. But, the time of flight (1/6 nSec)
will allow for 3 reflections across the impedance discontinuity during
the rise time. If we can get 3 reflections during the rise time, the
impedance discontinuity has minimal effect on the waveform.

If one of the reference planes is power, then decoupling will be
involved in the current path. The fidelity of the edge will be
degraded if a 'short' path is not provided for the return current.
Ground and power plane bounce will occur at via locations if this
path is not provided.

Larry Smith
Sun Microsystems