[SI-LIST] Re: TDR's and ESD Protection
- From: Noel Verbiest <nverbies@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx, rfeldman@xxxxxxxxxxx
- Date: Tue, 26 Mar 2002 17:25:49 -0800
That's indeed correct. The culprit phenomena is the piezoelectric
behavior of a coax. The friction between the different materials used
in the cable will give rise to triboelectric currents which - in turn
- will become (very high sometimes) voltages if the cable is left
open.
Several manufacturers therefore make "low noise" coax cable, in which
the insulator materials are treated with graphite in order to reduce
the friction.
If "low noise" coax is not an option, then keep the center and shield
of the coax cable shorted until the very last moment before doing the
measurement.
When the cable is connected to the TDR, and the short is removed, the
cable should be as motionless as practically possible: no vibration
(because of a fan motor for example), absolutely no flexing of the
cable, no tapping or accidental touching with a hand or tool.
Of course, the longer the cable, the worse it becomes.
My $.02
Noel Verbiest.
>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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