Teledyne LeCroy QPHY-PCIe3-Tx-Rx - Manual (Page 30)

26 923662 Rev B
Cable Deskewing Without Using the Fast Edge Output
The following procedure demonstrates how to manually deskew two oscilloscope channels and
cables using the differential data signal, with no need for any T connector or adapters.
This can be done once the temperature of the oscilloscope is stable. The oscilloscope must be
warmed up for at least 20 min. before proceeding. This procedure should be run again if the
temperature of the oscilloscope changes by more than a few degrees.
1. Connect a differential data signal to C1 and C2 using two approximately matching
cables. Set up the oscilloscope to use the maximum sample rate. Set the timebase for a
few repetitions of the pattern (at least a few dozen edges).
2. On the C3 menu, check Invert. Now C1 and C2 should look the same.
3. Using the Measure Setup, set P1 to measure the Skew of C1, C2. Turn on Statistics
(Measure menu). 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:
UU
[Data skew + cable skew + channel skew] + [ (-Data skew) + cable skew + channel skew]
UU
2
The above formula simplifies to:
[cable skew + channel skew]
7. Set the resulting value as the Deskew value in C1 menu.
8. Restore the cable connections to their Step 1 settings (previous). Press the Clear
Sweeps button on the oscilloscope. The mean skew value should be approximately zero
- that is the data skew. Typically, results are <1ps given a test fixture meant to minimize
skew on the differential pair.
9. On the C2 menu, clear the Invert checkbox and turn off the parameters.