Teledyne-lecroy QPHY-SATA Manuel d'utilisateur télécharger pdf (Page 44)

44 915745 Rev G

Cable Deskewing without the Fast Edge Output

The following procedure demonstrates how to 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 a half-hour before proceeding. This procedure should be run again if the temperature of the oscilloscope

changes by more than a few degree.

1. Connect a differential data signal to C2 and C3 using two approximately matching cables. Set up the

oscilloscope 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 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

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

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.

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.

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Teledyne-lecroy QPHY-SATA Manuel d'utilisateur Page 44