RE: [SI-LIST] : Differential TDR probes

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From: Ron Miller (rmiller@Brocade.COM)
Date: Fri Oct 20 2000 - 12:12:23 PDT


Hi John

Yes, soldering the shields together is good.

I thought you were saying that the shield must also be probed to
ground on the board(3 contacts) instead of just the differential signals.

The grounds though tied together do not have to be connected to the board.

Ron

> -----Original Message-----
> From: John Fisher
> Sent: Friday, October 20, 2000 11:31 AM
> To: Ron Miller; zanella, fabrizio
> Cc: si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : Differential TDR probes
>
> Ron,
>
> Certainly shorter is better! Depends on what you are looking for. Soldering the coax shields together helps as if it were a passive diff probe, which it also is in case of TDR, ensures that any noise picked up on one shield is not seen as differential mode noise back at the scope but as CM and thereby rejected due to CMRR. Keeping everything between TDR & DUT as short as possible ensures fastest edge and most resolution of closely spaced physical/electrical points of interest.
>
> John
>
> At 05:40 PM 10/19/00 -0700, Ron Miller wrote:
>
>
> John
>
> soldering the shields together every 3 inches implies some multiple of that.
>
> Much better to keep the length below 3 inches so as to not roll off the TDR
> edge. That is why the extension cables are needed to allow the head to get
> near the work.
>
> Ron Miller
>
> > -----Original Message-----
> > From: John Fisher
> > Sent: Thursday, October 19, 2000 3:39 PM
> > To: zanella, fabrizio
> > Cc: si-list@silab.eng.sun.com
> > Subject: Re: [SI-LIST] : Differential TDR probes
> >
> > Fabrizio,
> >
> > We have built these using equal length sections of semi rigid coax with sma's at one end. The semi rigid copper outer shield is soldered together approx. every 3 ". Prepare the ends by careful cutting of the shield with an exacto knife, than solder the center conductors to the launch point vias or pads. Tektronix shipped some of these to us as accessories when we bought their IPA-510 a few years ago. I might be able to find a tek part number but someone like Micro-Coax could make some for you or find an off the shelf semi rigid assembly with sma's on both ends and then just cut it in half. Be carful not to bend it too much and kink it. These can also make nice 50 Ohm 10x passive probes with the addition of a series 450 resistor at the end.
> >
> > Regards
> >
> > John
> >
> > At 03:44 PM 10/19/00 -0400, you wrote:
> >
> >
> > We're having problems getting a good high frequency launch when probing pc
> > boards and cable differential pairs when taking TDR measurements.Has anyone
> > found good differential TDR probes which allow you to measure very closely
> > spaced points on pc boards or cables?
> >
> > Thanks and regards,
> > Fabrizio Zanella
> > Hardware Engineering
> > EMC Corporation
> > fzanella@emc.com
> >
> >
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