From: Daniel, Erik S. ([email protected])
Date: Wed May 10 2000 - 06:20:13 PDT
Martyn-
I assume that you are asking about mismatch in the widths of the striplines
as opposed to mismatch in the driving circuits. Realize that the two are
closely related. Please allow a brief digression on this issue as I think
it may help.
In principle, ANY structure consisting of two signal conductors and ground
plane(s) which is uniform along the direction of signal propagation will
support two distinct TEM "modes", each with a well-defined characteristic
impedance. Only in the specific case that the structure is symmetric with
respect to the two signal conductors can these modes be called "even" and
"odd". This is the desirable target for a number of reasons. A couple of
the most important are that 1.) an odd-mode signal radiates less, and 2.)
conversely, and odd-mode signal is less susceptible to noise, which tends to
couple to the even mode. Note, however, that even if one has a perfectly
symmectric transmission line capable of supporting perfect odd and even
modes, #1 requires good matching of the driving circuit (so that it drives
the odd mode exclusively with little drive of the even mode), and #2
requires good CMRR of the receiving circuit (or else noise coupled into the
even mode will be sensed). The unfortunate conclusion of all of this is
that differential t-line matching and differenctial i/o design are
intimitely related, and it is difficult to specify one without specifying
the other.
In practice, in my somewhat limited design experience, it seems that
mismatch tolerances of ~10% in the transmission line structures have been
acceptable. This includes a rather aggressive analog PCB design in which
crosstalk and noise immunity at the 100 dB level was desired in the 0-2 GHz
frequency range. I'll be interested to see what some of the more
experienced members of this list report as acceptable tolerances.
A side note: I notice that you are from Polar. If I understand correctly,
Polar's TDR instrument RELIES on the fact that differential lines under test
are symmetric, as it does not excite the traces in the odd-mode, but rather,
excites them single-ended and computes odd and even mode responses. (See an
earlier thread in the SI-LIST for more discussion). Can you or someone in
your company comment on this? Specifically, I think many of us subscribing
to this list would be interested to know just how your particular instrument
performs this calculation so that we might be more aware of its strengths
and weaknesses.
- Erik
==================================================================
Erik Daniel, Ph.D. Mayo Foundation
Voice: (507) 284-1634 Guggenheim 1011B
Fax: (507) 284-9171 200 First Street SW
E-mail: [email protected] Rochester, MN 55905
==================================================================
> -----Original Message-----
> From: Martyn Gaudion [mailto:[email protected]]
> Sent: Tuesday, May 09, 2000 5:47 PM
> To: [email protected]
> Subject: [SI-LIST] : Match of differential pairs
>
>
> Hi everyone,
>
> Can anyone help on the following subject? My customers
> are in the PCB manufacturing industry and are increasingly
> called to build differential striplines, however on fine line
> boards it is very hard to make an exact match of the signal
> pair.
>
> This obviously has effects on cmrr, emc etc, but I cannot find
> any documents that define how much mismatch is acceptable on
> a differential pair.
>
> National have a good app note on LVDS for example but when
> matching is discussed it simply says good match is necessary.
>
> This was easy on 7 or 8 mil line width, much less so at 3 to 4 mil.
>
> Look forward to your input
>
> Kind regards
> Martyn Gaudion
> [email protected]
> www.polarinst.com
>
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