[SI-LIST] Differential microstrip with coplanar ground traces ... unexpected results
- From: "Bob Welte" <welte@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 2 Oct 2002 06:26:39 -0400
Hello to All
Would anybody like to share their knowlege or experience with using
coplanar ground traces in a microstrip configuration?
Are there any "rules" for the width or impedance, and number/spacing of
vias for the ground traces of such a configuration?
A short explaination of our design:
We have 100 ohm diff microstrip; signals are .013", with .015" spacing,
ground traces are spaced .010" from signals and are .010" wide, with vias
to the ground plane (.012"
below) at about every .5". Our digital signals are 2.5Gb/s with the
fastest rise time at 100ps.
We expected the coplanar grounds to better contain the fields, and allow
closer pair to pair spacing without the risk of large crosstalk. We
believed that we had taken the coplanar grounds into account when designing
the configuration, but now thnk that the software we used assumed that the
coplanar grounds were perfect. With the narrow ground traces, and the vias
to the ground plane, we now believe that the coplanar grounds are actually
quite inductive at our frequencies. We have both microstrip and stripline
coupons, and expected the microstrips to be slightly better since they have
no vias in the signal path. But in fact the striplines show less
discontinuity (TDR), and have less jitter when transporting a pseudorandom
bitstream.
Thanks
Bob Welte
IBM Microelectronics
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