[SI-LIST] : guard traces

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From: Mark Gailus (Mark_Gailus@notes.teradyne.com)
Date: Mon Jun 26 2000 - 10:02:37 PDT


---------------------- Forwarded by Mark Gailus/NNH/Teradyne on 06/26/2000 12:43
PM ---------------------------

Mark Gailus <Mark_Gailus@notes.teradyne.com> on 06/23/2000 01:01:07 PM

Please respond to Mark Gailus <Mark_Gailus@notes.teradyne.com>

To: si-list@silab.eng.sun.com
cc: (bcc: Mark Gailus/NNH/Teradyne)

Subject: [SI-LIST] : guard traces (corrected message)

I found two errors (so far) in my message of 6/26/00.

One is in the formula for the half wave resonance example which should be:
7.14 GHz ( = (0.5) /((1cm) (70ps/cm)) ), and NOT
7.14 GHz ( = (0.5) (1cm) / (70ps/cm) ).

The second is that the portion of a guard trace between two grounded vias will
resonate at ALL harmonics (1f, 2f, 3f, 4f, 5f,...) of the lowest resonant
frequency which occurs when the portion is a HALF wavelength.

Sorry about that.

A corrected version of the entire earlier message follows:

*******************************************************************************************************

Just to further emphasize some useful things that have already been pointed out
by other si-list subscribers:

A two dimensional field solver does not by itself include the effects of the
vias that connect a guard trace to ground. The 2D solver can typically treat
the guard trace as one of the following:

 (1) as an "ideal" ground which is more or less the same as connecting the guard
trace "continuously" to the ground plane(s) along its entire length,

 (2) as a "floating" conductor which is more or less the same as not connecting
it to ground anywhere and in addition cutting it into many short disconnected
portions so that it does not carry any significant total current in the
end-to-end direction,

  or (3) as another "signal" trace.

Case (1) is a reasonable approximation if the portions of the guard trace
between vias are well below any resonant frequencies (see below), case (3)
allows a more detailed transmission line circuit model of the actual situation
to be created (also see below) if this seems necessary.

A "guard trace" between two signal traces can definitely INCREASE coupling
between the signals at frequencies for which the guard line resonates.The
resonant frequencies for a portion of the guard line are those for which the
distance between successive ground vias is a multiple of a half wavelength in
the board. For example, a guard line on an inner (stripline) layer of an FR-4
board with a delay of ~70 picoseconds per centimeter, would have its lowest
resonant frequency at 7.14 GHz ( = (0.5) /((1cm) (70ps/cm)) ) for a 1cm ( 0.4
inch) spacing between ground vias, and 714 MHz for a 10 cm ( 4 inch) spacing
between vias. ( Of course each portion also resonates at approximate multiples
of the lowest frequency, depending upon other details like via inductance.) So
the spacing and number of ground vias desired will depend upon the frequency
content of your signal, your tolerance for crosstalk, etc.

 As has been pointed out, these resonant coupling effects can be simulated in
any SPICE or other circuit simulators by using a two-dimensional solver to
derive a coupled line model for the portion of interconnect between a pair of
ground vias, WHILE TREATING THE GUARD TRACES AS YOU WOULD ANY OTHER SIGNAL LINE,
and then hooking up these models with appropriate via-to-ground models in the
right places (as a first cut just a low resistance or possibly a small
inductance to ground is good).

I hope this is helpful.

Best regards,

Mark Gailus
mark_gailus@notes.teradyne.com

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