**From:** Mark Gailus (*[email protected]*)

**Date:** Mon Jun 26 2000 - 10:02:37 PDT

**Next message:**Arani Sinha: "[SI-LIST] : SPICE format to MNA stamps"**Previous message:**Joan Vicent Castell Balaguer: "Re: [SI-LIST] : guard traces"

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

PM ---------------------------

Mark Gailus <[email protected]> on 06/23/2000 01:01:07 PM

Please respond to Mark Gailus <[email protected]>

To: [email protected]

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

[email protected]

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**Next message:**Arani Sinha: "[SI-LIST] : SPICE format to MNA stamps"**Previous message:**Joan Vicent Castell Balaguer: "Re: [SI-LIST] : guard traces"

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