Teledyne LeCroy QPHY-SAS2 - Manual (Page 43)

QPHY-SAS2 Software Option
917718 Rev A
43
Cable Deskewing without using the Fast Edge Output
The following procedure demonstrates how to deskew two oscillo scope 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
oscillo scope must be warmed up for at
least a half-hour 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 C2 and C3 using two approximately matching cables. Set up the
oscillo scope to use the maximum sample rate (e.g. for WaveRunner 204Xi: two channel mode; on the
Smart Memory menu set Fixed Sample Rate 10GS/s). Set the timebase for a few repetitions of the
pattern (at least a few dozen edges).
2. On the C3 menu, check Invert . Now C2 and C3 should look the same.
3. Using the Measure Setup , set P1 to measure the Skew of C2, C3. 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 oscillo scope (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
7. The above formula simplifies to:
[cable skew + channel skew]
8. Set the resulting value as the Deskew value in C2 menu.
9. Restore the cable connections to their Step 1 settings (previous). Press the Clear Sweeps button on the
oscillo scope. 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.
10. On the C3 menu, un-check the Invert checkbox and turn off the parameters.
In the previous procedure, we used the default setup of the Skew parameter (which is detecting positive edges on
both signals at 50%). We also inverted C3 in order to make C2 and C3 both have positive edges at the same
time.