RE: [SI-LIST] : Match of differential pairs

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From: Martyn Gaudion (Martyn@gaudionm.freeserve.co.uk)
Date: Wed May 10 2000 - 16:26:21 PDT


Erik,

Many thanks for your answer, yes I am referring to mismatch in
stripline or differential microstrip width - not length... My interest was
that we had seen an increase in mismatched lines on fine line boards
and I am looking to see if anyone has any data on allowable mismatch.

It is helpful to see you have found as much as 10% imbalance in differential
striplines has still given acceptible results.

Second, yes thanks for giving me a chance to pick up on the subject of
differential
TDR. As you explain there are two primary methods one using two opposing pulses
and the other using a single pulse to drive each line in turn and
calculating the
resultant differential impedance.

There are advantages and disadvantages to both methods but we have
done extensive testing on thousands of coupons and correlated results from
both types of systems. We find the answers from both methods correlate well.

As you say a TDR driving with equal and opposing pulses will give a resultant
impedance when testing a differential pair - however IT WILL NOT FLAG
IMBALANCE.
- Unless you also compare the single ended results.

Its also worth noting that measuring differential impedance with two opposing
pulses requires the pulses to be accurately matched in time and amplitude.
When
using two pulsers its relatively straightforward to match the edges in time
but there
is no guarantee that the pulse aberrations will be identical. This can be
a source of
error in impedance measurements when using this technique.

The Polar CITS TDR uses a single pulse and calculates the single ended
impedance,
the coupling, and can also calculate imbalance. However at present we do
not display imbalance
as it is only recently with the increasing use of very fine line structures
that we have
noticed an increase in the number of boards with more than 1 or 2%
imbalance between
the traces in a given differential pair.

As a traceable reference for TDR comparisons we would recommend
that NIST traceable air lines either singly or in pairs are used
to verify the results between TDRs. This allows a comparison
to be made based on standards rather than simply between products..

Kind regards

Martyn Gaudion
Polar Instruments
Tel: + 44 1481 253081
Fax: + 44 1481 252476

www.polar.co.uk
email: <martyn.gaudion@polar.co.uk>

At 08:20 10/05/00 -0500, you wrote:
>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: daniel.erik@mayo.edu Rochester, MN 55905
>==================================================================
>
>
> > -----Original Message-----
> > From: Martyn Gaudion [mailto:Martyn@gaudionm.freeserve.co.uk]
> > Sent: Tuesday, May 09, 2000 5:47 PM
> > To: si-list@silab.eng.sun.com
> > 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
> > martyn.gaudion@polar.co.uk
> > www.polarinst.com
> >
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