RE: [SI-LIST] : D/W vs. S/H

From: Larry Miller ([email protected])
Date: Thu Nov 30 2000 - 07:24:12 PST

S/H is used to calculate the differential (odd-mode) impedance between
traces in differential pairs.

W/H determines the even-mode (Zo) impedance of the trace. Whether the trace
is a stripline (the meat in a ground plane sandwich) or a microstrip (butter
on the top of a ground plane surface), the ratio of W/H is very important.

Get the UltraCad Impedance calculator (or one of your choice-- there are
many around the PCB materials [like search on FR-4] Web sites, and they are
all free downloads or active website applications) and play around for
awhile with the numbers. You'll soon see what the various dimensions do to
trace impedance, inductance and propagation velocity and get a feel for how
this all works out.

Larry Miller

> -----Original Message-----
> From: Loyer, Jeff W [SMTP:[email protected]]
> Sent: Wednesday, November 29, 2000 12:55 PM
> To: [email protected]
> Subject: [SI-LIST] : D/W vs. S/H
>
> Doug's query brought up a related question to my feeble mind...
>
> Is there any reason to specify distance between traces relative to their
> width? As far as I know, the most critical dimensions to consider are: 1)
> distance between the edges of two traces, relative to 2) distance between
> the trace and its ground plane(s). The width of the conductor is not a
> significant factor, unless you're using center-to-center separation, where
> you'll have to take into account the width. I don't understand why we
> wouldn't specify S/H instead of D/W (see below).
>
> ______________________________________________________ GND
> ^
> |
> (H)
> |
> v
> ___________ <--- (S) ---> ___________ Signals traces
> <-- (W) -->
> <---------- (D) ---------->
>
> Jeff Loyer
> (253) 371-8093
>
>
> -----Original Message-----
> From: Doug Hopperstad [mailto:[email protected]]
> Sent: Wednesday, November 29, 2000 12:27 PM
> To: [email protected]
> Subject: [SI-LIST] : RE: Crosstalk Bus spacing
>
>
> When determining the minimum spacing between traces on a digital bus, is
> it
> best to setup the three traces as follows:(The design is using a
> stripline)
>
> "A": Aggressor trace
> "V": Victim trace
> "A": Aggressor trace
>
> ------------------------------------------------------- Ground Plane
> layer
> ------(A)------ ------(V)------ ------(A)------ Trace layer, 0.5
> ounce.
> ------------------------------------------------------- Ground Plane
> layer
>
> Should both Aggressors be in-phase with each other or should one of them
> be
> inverted to get the worst case crosstalk. I am simulating with both
> applications and getting much more crosstalk on the victim trace when both
> aggressors are in-phase.
>
> The clock edge rate is 950pS and the trace width is set at w = 5 mils. The
> Plane to trace layer spacing is 6.5 mils. This provides a nice 50 ohm
> trace
> impedance.
> The distance between traces is set at 5 mils (1w). I have been playing
> with
> 2w in the simulations as well.
>
> Is it traditional to set the trace-to-trace spacing on the bus traces,
> i.e.
> bits(0:x) for example, at 1w the trace width. The bus-to-adjacent traces
> have been set for 2w spacing. The clock spacing is set for a 3w minimum.
>
>
>
>
>
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