All,
I have a hypothesis for some of the skew you report. Back when we were
measuring BER (bit error rate) vs delay for serial phys using a BER
tester we noticed a variation with temperature of about 2ps/C due to
changes in permittivity of the cable dielectric. I've attached a paper
which explains this effect further. Perhaps this can help you explain
your observations.
Regards,
Chuck Hill, SATAIO Phy WG chair
On 07/08/2017 03:28 PM, Istvan Nagy wrote:
Hi,
I sent this email yesterday, but it might have been scrambled. It looks
scrambled in the si-list archive page. resending...
https://www.freelists.org/post/si-list/measuring-low-levels-of-skew,14
Its not really periodic, its random wander and schizophrenic shaking. In 1
sec it drifts by several ps on the screen.
The problem is, I need to measure single ended traces, 12" each, to find min
and max tpd to calculate max worst case skew. With diffpairs I would have to
measure too many boards and the glass pitch and its variation would
influence it too.
So I might be seeing 2-3 effects:
- Tdr measurement real time wander and random noise, while probe steady.
- vna micro probe lift up, release down on same trace (viewing in
microscope) the the resonant freq changes.
- Even without lifting up, the next vna measurement changes.
The tdr aas hand held, the vna was solid heavy fixed microprobe.
vna Settings. First I checked where the 10 th harmonic was, around 6 ghz.
Then zoom into 5.8...6.1 ghz and scan with 30...100k points. To get accurate
distinction of frequency measurement. That takes some time. 10...30 sec
Regards, Istvan Nagy
-----Original Message-----
From: Tom Dagostino
Sent: Friday, July 07, 2017 3:33 PM
To: dmarc-noreply@xxxxxxxxxxxxx ; 'Istvan Nagy' ; 'si-list'
Subject: RE: [SI-LIST] Re: measuring low levels of skew
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 littledrift 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
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