Datasheet
Application Note
www.tektronix.com/power4
Figure 5. Default Timing Skew between Voltage and Current Probes. Figure 6. Nominal Correction of Timing Skew between Voltage and Current Probes.
Eliminating Skew between Voltage and
Current Probes
To make power measurements with a digital oscilloscope, it
is necessary to measure voltage across and current through
the device under test. This task requires two separate probes:
a voltage probe (often a high voltage differential probe) and
a current probe. Each voltage and current probe has its own
characteristic propagation delay and the edges produced in
these waveforms more than likely will not be automatically
aligned. The difference in the delays between the current
probe and the voltage probe, known as skew, causes
inaccurate amplitude and timing measurements. It is important
to understand the impact of the probe’s propagation delay on
maximum peak power and area measurements as power is
the product of voltage and current. If the voltage and current
signals are not perfectly aligned, results will be incorrect.
Tektronix MDO/MSO/DPO4000 and MDO/MSO/DPO3000
Series oscilloscopes offer a “Deskew” feature to remove
the skew between the probes. When the Deskew menu is
selected, an information box is displayed that describes the
Probe Model, Nominal Propagation Delay, Recommended
Deskew and Actual Deskew for each channel. The voltage
and current waveforms in Figure 5 have approximately 8 ns
of skew and the propagation delay for each probe is shown
in the information box. The TDP1000 (Tektronix differential
voltage probe) has a nominal propagation delay of 6.5 ns
whereas the TCP0030A (Tektronix current probe) has a
nominal propagation delay of 14.5 ns. The difference in
propagation delays is 8 ns.
Correcting for the skew between the probes is as simple
as selecting the “Set all deskews to recommended values”
side bezel button as shown in Figure 6. Choosing this
option adjusts the probes’ “Actual Deskew” values to the
“Recommended Deskew” values. The “Recommended
Deskew” value is based upon the probe’s nominal propagation
delay which is stored in the probe’s internal memory, assuming
the probe is TekVPI
®
enabled or generally supports automated
probe deskew.