HP 8901B Operation And Calibration Manual page 110

4-2 Calibration
Calibration
RF Power Calibration The RF Power calibration procedure is shown in Chapter 1 of this
guide. Once the procedure has been completed it does not need to be
repeated again unless you change Sensor Modules or the calibration
factors for the Sensor Module you are using change (due to repair
or recalibration of the Sensor Module). (If the Sensor Module or
the calibration factors are changed, you
will
need to complete the
calibration factor entry procedure provided in Appendix B before
proceeding with the calibration procedure shown
in
Chapter
1.)
To Maintain Accuracy
To maintain calibration accuracy, it is necessary to periodically press the
ZERO key (approximately once every ten minutes while making RF Power
measurements is recommended and each time you connect the sensor to
a different output port). Pressing the ZERO key enables the HP 8901B
to compensate for any inherent, dc-offset error in the measurement
system caused by changes in the ambient operating temperature.
Tuning
You may remember that in Chapter
1
you were told that the HP 8901B
should be tuned to the frequency of the input signal when making
RF Power measurements. Even though the RF Power measurement is
not a tuned measurement, the instrument uses the frequency setting to
determine the appropriate calibration factor to select from the table of
power sensor calibration factors stored in its memory. (If a calibration factor
is not stored for the specific frequency being measured, the HP 8901B
derives a calibration factor by linear interpolation between the closest cal
factors in the storage table on both sides of the measurement frequency.)
Extending
Operation
Tuning
The RF Power calibration technique requires that the HP 8901B
is
tuned
to the frequency of the input signal. As shown in Chapter
1,
there are
two ways to tune to the input signal. If you know the frequency of your
input signal, the most direct way to tune is by keying in the frequency of
the input signal and then pressing the
MHz
key. If you do not know the
input frequency, set the input signal level to between 0 and -20 dBm, then
press the FREQ
key, the Blue
Shift key, and the
AUTO TUNING
key on
the HP 8901B. When the HP 89018 displays the frequency of the input
signal, press the
MHz
key to select Manual Tuning, and then re-enter
the RF Power measurement mode and proceed with your measurement.