**From:** Farrokh Mottahedin (*[email protected]*)

**Date:** Tue Apr 25 2000 - 14:48:42 PDT

**Next message:**Fred Balistreri: "Re: [SI-LIST] : Differential TDR "Measurements""**Previous message:**Vinu Arumugham: "Re: [SI-LIST] : Differential TDR "Measurements""**Maybe in reply to:**Zabinski, Patrick J.: "[SI-LIST] : Differential TDR "Measurements""**Next in thread:**Allen Fernandez: "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

[email protected]

-----Original Message-----

From: Zabinski, Patrick J. [mailto:[email protected]]

Sent: Monday, April 24, 2000 5:45 PM

To: [email protected]

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 [email protected]

Rochester, MN 55905 www.mayo.edu/sppdg/sppdg_home_page.html

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**Next message:**Fred Balistreri: "Re: [SI-LIST] : Differential TDR "Measurements""**Previous message:**Vinu Arumugham: "Re: [SI-LIST] : Differential TDR "Measurements""**Maybe in reply to:**Zabinski, Patrick J.: "[SI-LIST] : Differential TDR "Measurements""**Next in thread:**Allen Fernandez: "RE: [SI-LIST] : Differential TDR "Measurements""

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