Teledyne LeCroy TF-DSQ - Manual (Page 20)

Operator's Manual
20 TF-DSQ-OM-E RevC
Two channels are properly deskewed relative to each other when the difference between the
deskew values entered for each channel aligns an edge occurring at the same time and applied
to both channels. A channel is properly deskewed in an absolute sense when an edge is applied
to that channel, the oscilloscope is triggered on that edge, and that edge appears such that the
trigger threshold crossing appears at the trigger delay, which is the zero time reference on the
oscilloscope screen.
Probes are deskewed one at a time relative to a reference such that the resulting calibration
deskews each probe relative to every other probe.
The TF-DSQ fixture has a built-in step generator. The edge generated is driven out of a cable into
an oscilloscope channel or external input where the fixture is connected. The edge is
simultaneously driven onto a transmission line containing the probing pads. When a probe is
connected to the probing pads, the edge appears at the fixture connection to the oscilloscope
and on the channel where the probe is connected. Generally, there is a difference in time
between the edges due to propagation delay variations between the fixture cabling and the
probe.
During the deskew procedure, the oscilloscope triggers on the channel or external input used for
the fixture connection and the time from this trigger to the edge on the probe channel being
deskewed is calculated. This time becomes the skew time for the probe. This process is repeated
for all of the other probes being utilized.
During oscilloscope operation, there is a small dilemma. Each probe that is deskewed causes the
waveform to be delayed or advanced due to the deskew time calculated for the channel. This
causes the trigger to be misaligned because the trigger point is where the edge reaches the
internal oscilloscope trigger circuitry, not when the edge appears at the probe tips. The
oscilloscope accounts for this by subtracting the deskew correction for the channel used as the
trigger source from the deskew correction on all channels. In essence, the channel used as the
trigger source has zero deskew correction applied, while the relative deskew difference between
all channels is maintained. This trigger compensation is hidden from the user. If better trigger
alignment is desired or if there is some need to shift all of the waveforms, provide a Deskew All
field value on the Advanced dialog to shift all traces together.