N2808A Datasheet, PDF (3/13 Pages) Keysight Technologies – PrecisionProbe for the 9000 Series

03 | Keysight | N2808A PrecisionProbe for the 9000 Series - Data Sheet
Background – PrecisionProbe (Probe correction)
Probes and cables are inherently lossy and rarely identical in
their characteristics. The loss at times can be substantial, or
enough from an ideal flat frequency response to cause variation in
measurements and the loss of valuable margins. To compensate
for the inherent loss, oscilloscope vendors use DSP correction
to compensate for loss caused by probes. The vendor uses a
“golden” model and base, all compensation/ correction on the
single model. While this strategy mitigates the inherent loss of
the probe it can’t solve probe to probe variation.
If a probe’s characteristics have changed/drifted or were not
close to the model to begin with, the compensation may cause
the probe to make worse measurements. There are also a myriad
of probe heads to attach to probe amplifiers, to achieve maximum
accuracy every combination must be measured down to the
indiviual probe head, tip, and amplifier. Oscilloscope vendors
are unable to measure to this level of accuracy and the end
result is that you get unwanted inaccuracies and probe to probe
variability.
Figure 3. Probe browser with a non-standard pitch
You also use custom probes and probe heads. While this provides
great convenience for you it means that the oscilloscope vendor
no longer can even provide a “golden” compensation for your
probe configuration. As a result, custom probes are uncorrected
and inaccurate, but convenient.
You may also want to add something between the probe amplifier
(such as Keysight’s 3.5 GHz 1131A) probe amplifier and (such
as Keysight’s 6 GHz E2675A browser) probe head, including a
cable to add length or a switch matrix. Adding a new element
in the probe system adds inaccuracies as as the probe amplifier
and browser head are compensated to the model, but the
newly created probe system now has no model. The result is
that you must except 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 actually achieved in actual
measurements.
Figure 4. Image of custom probe
Figure 5. Image using LW ZIF Head