[SI-LIST] Re: 100 ohm differential GHz trace width and gap
- From: pwelling@xxxxxxxxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 21 Jan 2002 15:13:10 -0700
Doug/James,
I do the same.
I still prefer 50 Ohm Line-to-Ground & 100 Ohm Line-to-Line traces. Another
caution is if you fill the planes around the traces (for field containment)
you may have to calculate for co-planar trace impedance.
I also prefer to manage the traces near the pair (same layer) and traces in
adjacent layers. Keeping in mind what the return reference planes/path are,
I try to run these against ground planes and in a quiet area of the PWB
(sometimes this is not possible). Routing against power planes or routed
voltages adds common mode noise that you don't need especially in a receive
Differential Pair route.
Philip Ross Wellington
Mgr. Signal Integrity & EMI
L-3 Communications CSW
-----Original Message-----
From: James_R_Jones@xxxxxxxx [mailto:James_R_Jones@xxxxxxxx]
Sent: Monday, January 21, 2002 2:59 PM
To: doug.hopperstad@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: 100 ohm differential GHz trace width and gap
Doug,
I prefer less coupling to the ground plane, ie. narrower traces. My
motivations for this would be to have more coupling between the + and -
portions of the differential transmission line. There are common mode noise
rejection advantages, and improved AC return current paths. You should see
less SI degradation when crossing moats with tightly coupled diff T-lines.
One gotcha that I might warn about, however, is losses. If your traces
become to narrow, you may see some attenuation due to skin effect. The
usable area of the trace at high frequency is at the "skin" of the traces
and reduces with narrow traces. It should be possible to view this effect
when measuring narrow traces on a TDR. You will see a slightly rising slope
on a trace with consistent impedance.
I recommend using a good 2D field solver to get answers to specifics: Ansoft
2D or Apsim RLGC. Ansoft takes a bit longer to run, but has cool field
plotting options that helped me to visualize field directions and
magnitudes.
Hope this helps,
James R. Jones
-----Original Message-----
From: Doug Hopperstad [mailto:doug.hopperstad@xxxxxxxxxx]
Sent: Monday, January 21, 2002 2:49 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: 100 ohm differential GHz trace width and gap
I am interested in some feedback regarding routing 100 ohm differential
GHz signals on thin vs wide trace widths. I am currently looking using 8
mil traces with a 12 mil gap between the +/- pair. The routing is fairly
open and does not require very tightly coupled traces. I understand that
the tighter the traces are as a pair the common noise is improved. The
board material is FR408. A proposed core would use a dual-stripline with
a 7 mil dielectric between the planes (gnds) and routing layers and a 7
mil (or higher) dielectric between the routing layers. The routing is
orthogonal and tandem crosstalk is controlled. A stripline approach is
also an option if needed.=20
Doug Hopperstad
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