Custom probe heads provide great convenience, but prevent Oscilloscope vendors from
providing a “golden” model to correct for probe loss. The result is uncorrected and
inaccurate probes in your measurement system.
You may also want to add something between the probe amplifier (such as Keysight’s
30 GHz N2803A) probe amplifier and probe head (such as Keysight’s 30 GHz N5444A
browser probe head), including a cable to add length or a switch matrix. This adds
inaccuracies as the probe amplifier and browser head are compensated to the model,
the newly created probe now has no model. The result is that you must accept the
inaccuracies that have been added or try to characterize the additional element in
the probe link. While accepting both of these trade-offs can be sufficient, it is time
consuming to evaluate the element every time and not characterizing the element
causes loss of margins (including higher jitter, smaller eyes, and slower rise times).
This can also be the cause of differences between numbers measured in simulation
and the number achieved in actual measurements.
Figure 3. Image using LW ZIF Head
Cables pose many of the same problems as probes with cable-to-cable variability.
Oscilloscope vendors now mitigate this problem with the use of de-embedding
software (Keysight’s InfiniiSim N5465A software). Typically for cables, you must
characterize the cable using either a TDR or a VNA. Both of these methods
provide characterization and s-parameters, but take time. It is the time required
that typically will mean that you will characterize only one or two cables, and use
that characterization (s-parameter file) to do the measurements of every similar
cable they own, causing cable-to-cable variability if the characteristics vary from
the “golden” cable.
PrecisionProbe
PrecisionProbe can solve the problems outlined in the background information by
allowing quick characterization and switches).
The innovative software takes advantage of the fast “cal output” signal on the
90000 Q-Series, 90000 X-Series, and 90000A Series oscilloscopes to properly
characterize cables and probes out to 33 GHz of bandwidth. For measurements
greater than 33 GHz, the PrecisionProbe Advanced Kit includes Keysight’s N2806A
Calibration Pulse Generator to accelerate the “cal output” edge to sub-7 ps rise
times. This edge speed allows for PrecisionProbe characterization out to 63 GHz.
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