RE: [SI-LIST] : Differential TDR "Measurements"

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From: Allen Fernandez (Allen_Fernandez@Jabil.com)
Date: Wed Apr 26 2000 - 05:50:32 PDT


Farrokh,

Even when differential traces are far apart, if the common-mode noise is
conducted (e.g. from the driver), you will still maintain common-mode
rejection at the receiver. If the common mode noise is radiated, however,
one trace could pick up more of this common-mode noise than the other,
resulting in less common-mode rejection.

The benefit of putting differential traces very close together is EMI
reduction. This results from one trace's magnetic fields canceling the
other's because the signals are 180 degrees out of phase with each other.
This benefit is offset by a reduced characteristic impedance adding
complexity to termination design by requiring accurate simulation and/or
characterization of the differential traces.

Regards,

Allen

-----Original Message-----
From: Farrokh Mottahedin [mailto:Farrokh.Mottahedin@quantum.com]
Sent: Tuesday, April 25, 2000 5:49 PM
To: 'si-list@silab.eng.sun.com'
Subject: RE: [SI-LIST] : Differential TDR "Measurements"

Approach 2 only works if there is no coupling between the differential
signals. This would be the case if the pair members are sufficiently far
apart that the impedance to ground dominates each signal. For example,
design a board with trace geometries such that each trace to board impedance
is 50 ohms, and the traces are separated far apart. Then a differential
signal travelling down these traces sees 100 ohms signal to signal. Now,
because the traces are distant from each other, there is no common mode
noise rejection. Therefore, this is not an electrically desirable solution.
If the traces are close enough to cancel common mode noise, then they
interact, and the coupling must be measured. Perhaps the fab vendor is
using known geometries of trace spacing and size to estimate the
interaction, but then this is not a measurement.

Approach 1 works best on a differential test trace pair that are uniform and
sufficiently long for reflections to settle. Usually 10 cm is sufficiently
long enough to allow a 100 ps risetime TDR signal to settle. ( If tr = 100
ps, bw = .35 / 100 ps = 3.5 GHz, lambda = c / f = 8.5 cm). When
differential impedance is measured on actual routed traces, the stubs and
vias and even sharp bends affect the measurements. The pcb vendor will
fabricate the board so that the test trace impedance is correct, but actual
signal traces may be different. This happened to me with a fab vendor a few
years ago. They measured the test coupon, and I measured the signals.

SCSI SPI-3 standard uses Approach 1, measured either with a TDR or with a
network analyzer.

Regards,

Farrokh Mottahedin

Quantum Corp.
500 McCarthy Blvd.
Milpitas, CA 95035
(408)324-7934
farrokh.mottahedin@quantum.com

-----Original Message-----
From: Zabinski, Patrick J. [mailto:zabinski.patrick@mayo.edu]
Sent: Monday, April 24, 2000 5:45 PM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : Differential TDR "Measurements"

We're working more and more with differential signals,
and subsequently dealing with more differential printed
circuit boards (PCBs). Over the past few years, we've
had difficulty with several PCB vendors
trying to obtain a controlled impedance 100 ohm
differential pair.

The problem generally boils down to "who's measurement
do we believe"? We measure one impedance, while the
PCB vendor measures another.

We've done some digging, and there appears to be two
approaches to measuring differential impedance, and I'd
like to hear what folks have to say about them.

Approach 1: inject two signals of opposite polarity,
one into the true and one into the complement. The
complement signal is substracted from the true, and
you read the impedance just like a single-ended
measurement.

Approach 2: Inject one signal into the true trace and
record its signal. Then, inject a signal into the complement
trace and record its signal. Then, with the magic of
mathematics, compile these two different captured signals
into an effective differential measurement.

The equipment we have in-house uses Approach 1, while
nearly every board vendor we work with uses Approach 2.
Can anyone shed some light into the accuracies, sensitivities,
etc. of these two approaches? Are there cases where one
approach is better/worse than the other?

Thanks,
Pat

-----
  Pat Zabinski ph: 507-284-5936
  Mayo Foundation fx: 507-284-9171
  200 First Street SW zabinski.patrick@mayo.edu
  Rochester, MN 55905 www.mayo.edu/sppdg/sppdg_home_page.html

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