Hi Istvan,
30s is too short for the measurement when the total delay is high. Try a
stepped sweep, an IF of 500Hz, let the signal settle and use averaging for the
VNA measurement.
The best formula for skew calculation from phase is the following:
S2d1=1/SQR(2)*(S21-S23)
S4d1=1/SQR(2)*(S43-S41)
Dt1=-unwrap(phaserad(S2d1),pi)/(2*PI*freq)
Dt2=-unwrap(phaserad(S4d1),pi)/(2*PI*freq)
Skew=Dt1-Dt2
The Accuracy using TDR or TDT suffers strongly from high losses and reflections.
Best Regards
Gert
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Degerstrom, Michael J.
Sent: Monday, July 17, 2017 9:04 PM
To: buenoshun@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: measuring low levels of skew
Istvan,
We've had a lot of success using a VNA and measuring S21 phase to compute delay
(don't use group delay as Scott already has mentioned). The S21 (insertion)
phase is very robust to impedance variation and less-than-perfect launches.
I remember playing with VNA power levels and some other settings to come up
with the best settings, but I don't remember those details.
Since we have a wide frequency sweep we do average the delay using many of the
frequency points (using a matlab script). I thought the averaging was useful.
Our DUTs are have tiny pads for ~250 micron pitch microprobes. We've often
measured < 250 psec (1 -sigma) over low-skew 4" differential pairs. I think
we've had repeatability in the 10's of fs, but don't recall exact numbers. At
those levels you are probably limited by repeatability of probe contact
locations.
Regards,
Mike
Michael Degerstrom | SPPDG | 507-538-5486 | degerstrom.michael@xxxxxxxx
Mayo Clinic | 200 1st St. SW | Rochester, MN 55905 |
http://www.mayo.edu/sppdg
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Istvan Nagy
Sent: Friday, July 07, 2017 12:08 PM
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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