Teledyne LeCroy QPHY-HDMI - Manual (Page 34)

Cable Deskewing Using the DUT Signal
This procedure does not require a Fast Edge output. Its other advantages are:
• It does not require adapters.
• You can de-skew at the same V/div settings you'll use to capture your data (because you
are capturing your data for this procedure).
• If the differential data signal has higher speed edges than the oscilloscope's AUX OUT, it
is easier to get good timing measurements with the faster edges. This is definitely the
case for HDMI.
Note: This procedure demonstrates how to deskew channels 2 and 3. For optimal results, this
method should be followed for channels 2 and 3, and then channels 1 and 2, followed by
channels 4 and 3 to insure that all channels are deskew appropriately.
1. Connect a differential data signal to C2 and C3 using two approximately matching
cables. Set up the oscilloscope as you plan to use it. Set the timebase to capture a few
repetitions of the compliance test pattern (at least a few dozen edges). Press Auto to
start acquisition.
2. On the C3 dialog, check Invert. Now C2 and C3 should look the same.
3. On the Measure dialog, set P1 to measure the Skew of C2, C3. Turn on Statistics.
Write down the mean skew value after it stabilizes. This mean skew value is the addition
of Data skew + cable skew + channel skew.
4. Swap the cable connections on the Data source side (on the test fixture), and then press
the Clear Sweeps button on the oscilloscope (to clear the accumulated statistics; since
we changed the input).
5. Write down the mean skew value after it stabilizes. This mean skew value is the addition
of (-Data skew) + cable skew + channel skew.
6. Add the two mean skew values and divide the sum in half:
The above formula simplifies to:
[cable skew + channel skew]
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