[SI-LIST] Re: strange behavior... resent after server error
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 11 Jan 2012 09:32:12 -0800
Gert, so just to be clear:
Set-up as TDR only:
Port1 --> Cable1 --> Open
Port2 --> Cable1 --> Open
Port1_RoundTrip - Port2_RoundTrip = 0ps Calibration
Port1 --> DUT+
Port2 --> DUT-
Port1_RoundTrip - Port2_RoundTrip = 16ps
Port1 --> DUT-
Port2 --> DUT+
Port1_RoundTrip - Port2_RoundTrip = 0ps
What do you get for absolute values driving single-ended:
Port1 ---> DUT+
Port1 ---> DUT-
Port2 ---> DUT+
Port2 ---> DUT-
Steve
On 1/11/2012 9:16 AM, Havermann, Gert wrote:
> Scott,
> the differential delay of the + and – branch of my differential pair is 16ps
> when I feed + with TDR Port1 and –with TDR Port 2
> When I switch ports (- with TDR Port 1 and + with TDR Port 2) then I
> measure<1ps delay difference.
> In addition, the latter result doesn’t change much with varying skew at the
> TDR setting, the first result does change (as expected).
>
> I did the same test on many Testfixtures before, and I’ve never seen this
> kind of effect. My guess is that the mix of differential and single ended
> routing is causing this behavior, but I don’t have a real explanation for it.
>
> BR
> Gert
>
> Von: Scott McMorrow
> [mailto:scott@xxxxxxxxxxxxx]<mailto:[mailto:scott@xxxxxxxxxxxxx]>
> Gesendet: Mittwoch, 11. Januar 2012 17:52
> An: Havermann, Gert
> Cc: si-list@xxxxxxxxxxxxx<mailto:si-list@xxxxxxxxxxxxx>
> Betreff: Re: [SI-LIST] AW: Re: Stange behavior in skew measurement
>
> Gert
>
> Let me ask clarifying questions. Are you saying that
> 1. differential skew is 16 ps?
> 2. that the difference between differential and common mode delay is
> 16ps?
> 3. that you get different skew or delay measurements when you measure
> from different directions?
>
>
> Scott
>
> On Wed, Jan 11, 2012 at 11:24 AM, Havermann,
> Gert<Gert.Havermann@xxxxxxxxxxx<mailto:Gert.Havermann@xxxxxxxxxxx>> wrote:
> Steve, Jeff,
>
> thanks for the answer, but the cable is not the issue. I did use "cost
> effective" cable, but they are good enough for this test (and have been in
> the past). The cable was also not really moved during port switching as I
> fixed them with tape to avoid this kind of variances and the cables are only
> 5" long. We also have the phaseflex gore cables on our VNAs, and using them
> shows no difference.
>
> I also don't think the TDR Heads are the problem either. As I said, I
> deskewed after switching modes because I know of the timebase limitation of
> these heads.
>
> It must be something with the DUT that's causing this behavior. To give you
> more information, the DUT is a Test Fixture for a 25G Backplane and is done
> with FR408 flat weave material. Tracelength is 320mm, most of it routed
> single ended. Differential routed corners have deskew features added. The
> SMAs and transition to the trace shows +/-2 Ohm differential Impedance
> discontinuity and the Traces are all within 3% impedance tolerance.
> Geometrical Length matching was designed to be<1µm.
>
> One more information on the results: The single ended skew test (only one
> port attached the other terminated, and then difference of the signal
> runtime) showed skew<2ps, thus having a very low differential skew seems
> reasonable.
>
> BR
> Gert
>
>
> ----------------------------------------
> Absender ist HARTING Electronics GmbH& Co. KG; Sitz der Gesellschaft:
> Espelkamp; Registergericht: Bad Oeynhausen; Register-Nr.: HRA 5596;
> persönlich haftende Gesellschafterin: HARTING Electronics Management GmbH;
> Sitz der Komplementär-GmbH: Espelkamp; Registergericht der Komplementär-GmbH:
> Bad Oeynhausen; Register-Nr. der Komplementär-GmbH: HRB 8808;
> Geschäftsführer: Edgar-Peter Duening, Torsten Ratzmann, Dr. Alexander Rost
>
>
> ---------------------------------------------------------
>
> Absender ist HARTING Electronics GmbH& Co. KG; Sitz der Gesellschaft:
> Espelkamp; Registergericht: Bad Oeynhausen; Register-Nr.: HRA 5596;
> persönlich haftende Gesellschafterin: HARTING Electronics Management GmbH;
> Sitz der Komplementär-GmbH: Espelkamp; Registergericht der Komplementär-GmbH:
> Bad Oeynhausen; Register-Nr. der Komplementär-GmbH: HRB 8808;
> Geschäftsführer: Edgar-Peter Duening, Torsten Ratzmann, Dr. Alexander Rost
>
> -----Ursprüngliche Nachricht-----
> Von: si-list-bounce@xxxxxxxxxxxxx<mailto:si-list-bounce@xxxxxxxxxxxxx>
> [mailto:si-list-bounce@xxxxxxxxxxxxx<mailto:si-list-bounce@xxxxxxxxxxxxx>] Im
> Auftrag von Loyer, Jeff
> Gesendet: Mittwoch, 11. Januar 2012 17:00
> An: si-list@xxxxxxxxxxxxx<mailto:si-list@xxxxxxxxxxxxx>
> Betreff: [SI-LIST] Re: Stange behavior in skew measurement
>
> Agreed, and just to expand a little bit on this...
> I've found it useful to monitor the open TDR waveform while putting my cables
> through some contortions. Cables which are not "phase-stable" stick out like
> a sore thumb - you'll see timing shifts and variations in the rising edge of
> the pulse as you wiggle them around.
>
> Well behaved cables will appear rock-solid, and you can rest assured they
> will remain so as you move between reference and various DUT measurements.
> Cables which are not phase-stable will cause you all kinds of grief.
>
> This should be done periodically to all your cables, whether you're making
> TDR or VNA measurements, IMHO. Label the "bad" cables and only use them for
> non-phase-critical measurements (or throw them away). It may save you hours
> of consternation.
>
> And yes, you'll see a huge difference between the costly cables (Gore,
> Rosenberger) and cheap ones.
>
> Jeff Loyer
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx<mailto:si-list-bounce@xxxxxxxxxxxxx>
> [mailto:si-list-bounce@xxxxxxxxxxxxx<mailto:si-list-bounce@xxxxxxxxxxxxx>] On
> Behalf Of steve weir
> Sent: Wednesday, January 11, 2012 7:45 AM
> To: si-list@xxxxxxxxxxxxx<mailto:si-list@xxxxxxxxxxxxx>
> Subject: [SI-LIST] Re: Stange behavior in skew measurement
>
> 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
>>
>>
>>
>> ---------------------------------------------------------
>>
>> Absender ist HARTING Electronics GmbH& Co. KG; Sitz der Gesellschaft:
>> Espelkamp; Registergericht: Bad Oeynhausen; Register-Nr.: HRA 5596;
>> persönlich haftende Gesellschafterin: HARTING Electronics Management
>> GmbH; Sitz der Komplementär-GmbH: Espelkamp; Registergericht der
>> Komplementär-GmbH: Bad Oeynhausen; Register-Nr. der Komplementär-GmbH:
>> HRB 8808; Geschäftsführer: Edgar-Peter Duening, Torsten Ratzmann, Dr.
>> Alexander Rost
>>
>>
>>
>>
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>
> --
> Steve Weir
> IPBLOX, LLC
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> --
> Scott McMorrow
> Teraspeed Consulting Group LLC
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Steve Weir
IPBLOX, LLC
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Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
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