[SI-LIST] Re: Stange behavior in skew measurement

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Wed, 11 Jan 2012 07:44:31 -0800

An 8200 with 80E04 heads is capable of consistent and accurate 
measurements.  The only thing that Ihave seen with the 8000's and 8200's 
is that you have to be careful that the internal temperature stays 
within the calibrated range.  Assuming that you have then I would look 
at the cables.  How long are your cables, and how good are your cables?  
Consistency is a good reason to spend the money on a good set of WL Gore 
cables.   And even with Gore cables you want to avoid tight radii, and 
variations in bend from the time you calibrate to the time you are done 
measuring your DUT.

You can try switching cables, and/or using semi-rigid, and/or keeping 
the cables in a very straight and uniform orientation to see if they are 
the main contributor.

Steve.


On 1/11/2012 7:12 AM, Havermann, Gert wrote:
> Hello SI-Fellows,
> can someone explain to me the following effect I found in a skew measurement:
>
> I use Tek TDS8200 with 80E04 TDR Sampling Heads. I usually make 5 separate 
> measurements: 2xdifferential, 2xcommon and 1xsingle ended. For diff and 
> common I make a separate deskew up front, due to the non synchronized 
> timebase when switching modes. After deskew I measure the skew at the open 
> end of the DUT (I know its not the most accureat way, but it is quick and 
> served my needs very well in the past).
>
> For the two common and differential measurements I always switch dut ports to 
> check my deskew. Usually both measurements show about the same value 
> regardless of the port assignment. This time I had 16ps difference, and in 
> addition, one measurement showed just a few fs skew, and the other 16ps. Just 
> for sanity check I deskewed a bit more, and still one measurement showed just 
> fs while the other changed.
>
> Any Ideas or hints?
>
> BR
> Gert
>
>
>
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