HP 54721A User Reference page 15

Channel Setup Menu
Probe
Units
Units lets you select Volt, Ampere, Watt, or unknown as the unit of measure
for the channel scale, offset, and vertical measurement values. The unit you
select is appended to these numeric values. You would use Volt for voltage
probes, Ampere for current probes. Watt for optical-to-electrical (0/E)
converters, and unknown when there is no unit of measure or when the unit
of measure is not one of the available choices.
Ext gain and Ext offset
When you select Ampere, Watt, or unknovm, two additional softkeys are
available, external gain and external offset. These two additional softkeys
allow you to compensate for a transducer's actual characteristics as
compared to its ideal characteristics. For example, you might have an
amplified lightwave converter whose ideal characteristics are 300 V/W vrith
0 V of output offset. But, its actual characteristics are 324 V/W with 1 mV of
output offset. Set the external gain to 324 V/W and the external offset to
1 mV.
Example Because the mainframe's CAL signal is a voltage source, you can let the
Voltage probes oscilloscope compensate for the actual characteristics of your probe by
letting the oscilloscope calibrate to the tip of the probe. The following steps
illustrate how to calibrate a voltage probe to the probe tip.
1 SettheAtten units softkey to ratio.
2 Set the Units softkey to Voltage.
3 Cormect the probe to the front-panel CAL signal
4 Press the Calibrate Probe softkey.
The oscilloscope automatically caEbrates to the tip of the probe, setting the
probe attenuation and compensating for any probe offset.
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