LeCroy DDA 7 Zi series, Руководство оператора

Страница: 516 / 544

WavePro 700Zi
515
WP700Zi-OM-E-RevA
between channel 4 and channel 3 is calculated as 40 ps and -40 ps is entered in channel 4's skew entry. When
triggering on channel 4, channel 2 is advanced 30 ps in time and channel 3 is advanced 40 ps in time such that all
of the edges are aligned with each other and with the trigger point.
When triggering on channel 2, these times are adjusted by a common 30 ps. So, 30 ps is added to channel 2 to
cause 0 deskew amount, and 30 ps is added to channel 4 to cause -10 ps deskew. Without this common addition
(or delay) of all waveforms by 30 ps, all edges would remain aligned, but channel 2's trigger position would be off
by 30 ps. So, you can see that this 30 ps commonly, added to all waveforms, keeps the trigger points aligned.
Deskew Risetime Adjustment Theory
There are two situations that require adjustment of the deskew values to account for risetime:
x
Two probes are used for relative time measurements, but each probe has a different risetime.
x
Two probes are used for relative time measurements, but each signal has a different risetime.
The first case will be explained with obvious analogy to the second case.
Often, two probes are utilized for measurements where each probe has a different risetime. This might occur
because you are making a measurement where a reasonably low bandwidth probe can be used, but you do not
have two of them, so you substitute a higher speed probe.
When the probes are deskewed, the high-speed edge from the TF-DSQ fixture is utilized. The deskewed probes
might show signals that look as follows when the high-speed edge supplied by the TF-DSQ fixture is observed:
In the above picture, the green trace edge is the edge actually supplied by the TF-DSQ fixture, but because of
risetime limitations of the probe and channel, the edges appear as the red and blue traces. These traces are
shown aligned at the 50% crossing point at the end of the deskew calibration.
Now, lets assume that these probes are applied to signals with the same risetime, but different from the risetime
applied by the deskew fixture: