Jeff
I've been beating my head against this issue for years and a lot lately. From
the same TDR head I see the two channels move together with very little drift
if any between them. If I put all 4 of my TDR channels up, again they all move
in unison. I think this in in the timebase, not the receivers or TDR pulses.
And it seems to be related to ambient temperature. Could this 1 sec variation
Istvan is seeing related to air conditioners cycling?
Tom Dagostino
971-279-5325
tom@xxxxxxxxxxxxxxxxx
Teraspeed Labs
9999 SW Wilshire Street
Suite 102
Portland, OR 97225
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Loyer, Jeff (Redacted sender "jwloyer" for DMARC)
Sent: Friday, July 07, 2017 3:05 PM
To: tom@xxxxxxxxxxxxxxxxx; 'Istvan Nagy'; 'si-list'
Subject: [SI-LIST] Re: measuring low levels of skew
Hi Tom - good to hear from you. I'm remembering this from a few years ago, but
as I recall I was looking at the two waveforms from adjacent channels of the
same head excited differentially, one flipped mathematically and the skew
calculated automatically. I lined them up exactly but as I watched, they would
drift apart and the skew would become non-zero, even using a lot of averaging.
Of course, you have access to those same scopes and can check, maybe I'm
remembering incorrectly. But, it appears Istvan is seeing something similar.
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Tom Dagostino
Sent: Friday, July 07, 2017 2:54 PM
To: dmarc-noreply@xxxxxxxxxxxxx; 'Istvan Nagy' <buenoshun@xxxxxxxxx>; 'si-list'
<si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: measuring low levels of skew
There tends to be some timebase wander in sampling scopes. If you look at an
open or some other feature over long periods of time you will see its location
drift back and forth on the screen. But generally all of the scope channels
drift at the same rate so making a skew measurement between two channels the
drift does not affect the skew. The drift is random in my experience, I've not
seen anything with periodic drift as you describe.
Regards
Tom Dagostino
971-279-5325
tom@xxxxxxxxxxxxxxxxx
Teraspeed Labs
9999 SW Wilshire Street
Suite 102
Portland, OR 97225
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Loyer, Jeff (Redacted sender "jwloyer" for DMARC)
Sent: Friday, July 07, 2017 2:17 PM
To: Istvan Nagy; si-list
Subject: [SI-LIST] Re: measuring low levels of skew
Ah - that makes sense. And that agrees with my experience when using TDR. It
had nothing to do with probes since I also saw it when using only cables. As I
recall, it was less than a ps, but not by much.
Jeff Loyer
-----Original Message-----
From: Istvan Nagy [mailto:buenoshun@xxxxxxxxx] ;
Sent: Friday, July 07, 2017 1:20 PM
To: si-list <si-list@xxxxxxxxxxxxx>; Loyer, Jeff <jwloyer@xxxxxxxxxx>
Subject: RE: [SI-LIST] measuring low levels of skew
Actually by 1 sec I meant 1 sec.
with the tdr I can view the results real time, but it seems to be modulated by
some noise. 1254ps... 1259ps... 1251ps... varies at about 1 second rate, on the
same trace as I hold the probe.
The vna measurement takes about 30 sec to complete ( not hand held), and each
completion has a different result. At a variance of 8 ps roughly.
Regards, Istvan Nagy
Sent from my BLU smartphone device
On Jul 7, 2017 11:45 AM, "Loyer, Jeff" <jwloyer@xxxxxxxxxx> wrote:
------------------------------------------------------------------
As I recall, this could be a DesignCon paper all by itself. When we did our
work on the Fiberweave effect (DesignCon 2007 paper) measuring TDR skew was a
challenge. Eventually we were able to construct a TDR de-skew method that
got us close so we could do a lot of measurements quickly, but I found TDT
more accurate for final results since a reliable "standard" was easy to
construct (a very short "thru" between transmit and receive). Since we were
interested in differential signaling skew, we only looked at differential
excitation, inverted one of the signals using the math function for skew
measurements.
Like Scott said, measure them at some convenient voltage (I would use a lower
voltage, 1/4 of full swing, especially for microstrip). If you've looked at
microstrip waveforms, you've noticed distortion between the positive and
negative waveforms when there is skew. The slower waveform has a "dip" in it
(FEXT) before it rises, making time comparisons a bit ambiguous. This makes
me think that group delay might be a better method of deducing skew.
If you're looking for sub-ps (I assume by "~1sec" you mean "~1ps")
resolution, that's also going to take a lot of averaging and no
changes/movement between calibration and measurement. It's not hard to
induce a femtosecond or two (or several hundred) of skew.
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Istvan Nagy
Sent: Friday, July 07, 2017 10:08 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] measuring low levels of skew
Hi,
How do you guys measure skew on PCBs in the 2ps range?
We just made a test board, tried to measure skew but the results varied
randomly by +/-8ps. The test board had very good spread glass. So I'm worried
that the measurement inaccuracy was larger than the actual value to be
measured.
Actually the measurement was single-ended delay measurement with TDR (20GHz
TDR scope), then it was redone with a VNA as well. Both had similar results.
With the 65GHz VNA (65GHz probes under microscope) the measurement was for
frequency of dips on S11, then converted to delay. Different traces had
different delay, even at the same length, due to skew, as expected. But the
actual number varied over time (~1sec) too.
Is there a better setup to get lets say 1ps accuracy? What instrument to use,
or what's the secret?
Do you measure delay on each leg, or measure diffpairs for skew directly?
Regards,
Istvan Nagy
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list ;
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List forum is accessible at:
http://tech.groups.yahoo.com/group/si-list ;
List archives are viewable at:
//www.freelists.org/archives/si-list ;
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu ;
