[SI-LIST] Re: TDR's and ESD Protection

John:
I've seen more TDR sampling heads blown by ESD damage, than any other =
kind of victim device.  Cables with charged center conductors can do it, =
but shorting the connector pins should take care of that, except if =
there's an open circuit between conductor and associated pin, or an =
operator error, even once.

You are hardly the first person to use TDR in a production environment, =
but I'm no expert there either.  Can't point out any particularly robust =
instruments or methods.  There might be external in-line ESD clamps on =
the market.  You could try a momentary-contact switch or RF relay =
between TDR head and test connector, grounding the center conductor and =
isolating the instrument until ready to measure.

Cables do generate voltage pulses (presumably piezoelectric) if, say, =
you drop them onto a bench or floor.

Good luck.
Rich Feldman

-----Original Message-----
From: JOHN SAWDY [mailto:JSAWDY@xxxxxxxxxxx]
Sent: Tuesday, March 26, 2002 1:32 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] TDR's and ESD Protection


We are experiencing a high failure rate on our Tektronix SD-24 TDR heads
when we test cable assemblies.  Our cables assemblies sometimes are 40 =
to 80
feet in length and consist of multiple coax and twinax cables within =
each
assembly.  We typically use the TDR to measure differential electrical
length and skew of the finished assembly.=20

We use wrist straps and conductive mats on the benches.  We short all =
pins
to ground before we connect to the TDR to try to remove any residual
charges.

I am wondering if anyone else has some experience with this and if they
might have a solution.=20

I have also heard that "mechanical noise" generated by movement of the =
cable
may be a culprit but I am not sure of the magnitude of such noise.  Can
anyone confirm that this is a possibility?

Does anyone have any experience to suggest that one brand may be more =
robust
then another or that one model of head may be more robust.

Looking forward to your responses,
John

John F. Sawdy, Senior Applications Engineer
Meritec, a division of Associated Enterprises
voice mail: 440-354-3148 x 267
fax: 440 -354-0687
mailto:jsawdy@xxxxxxxxxxx



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