Istvan
either use insertion loss or a tdr or a tdt That's what you care about and has
higher noise margin. Trust me on this. Don't use group delay. It's a
derivative and sensitive to noise. If you want to use phase, use unwrapped
phase or phase delay.
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________________________________
From: Istvan Nagy <buenoshun@xxxxxxxxx>
Sent: Friday, July 7, 2017 2:32:21 PM
To: Scott McMorrow; si-list
Subject: Re: [SI-LIST] measuring low levels of skew
Scott,
Since the frequency of the dip on the vna measurement result ( s11.Plot) can
vary between each measurement, the input of the tdr simulation already contains
the inaccuracy or noise. So the simulation would be garbage in garbage out. So
instead I just used t [ps]000/f[ghz].
Something might be wrong with my probing or scope setup.
Regards, Istvan Nagy
Sent from my BLU smartphone device
On Jul 7, 2017 10:36 AM, Scott McMorrow <Scott@xxxxxxxxxxxxx> wrote:
1) tdr the measured s-parameter and measure the single ended skew at the middle
of the edge.
2) simulate the differential pair with a 50 ps t-line on one line and a
variable delay t-line on the other line. Sweep a simulation of insertion loss
by stepping the programmable delay from 30ps to 70ps in steps of 500 fs.
Look for the lowest broad band insertion loss. The corresponding delay is your
skew
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________________________________
From: si-list-bounce@xxxxxxxxxxxxx<//freelists.org>
<si-list-bounce@xxxxxxxxxxxxx<//freelists.org>> on behalf of Istvan Nagy
<buenoshun@xxxxxxxxx<http://gmail.com>>
Sent: Friday, July 7, 2017 1:08:17 PM
To: si-list@xxxxxxxxxxxxx<//freelists.org>
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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