2.7 Self-Calibration and Auto-Zeroing
The purpose of the PPM’s Self-Calibration function is to ensure that measurement accuracy will be
maintained despite changes in detectors, changes in detector diode temperature, or other changes that
might occur in the PPM’s analog circuits.
During the time the instrument is performing its Self-Calibration routine, the RF output of the 1 GHz
calibrator is accurately stepped from +20 to -30 dBm. Each step is measured by an internal standard and,
at each point, the calibrator output power as measured by the RF detector is compared to the standard.
The power readings are then digitally adjusted by the PPM microprocessor and stored in memory. For
power levels below -30 dBm, the system relies on the Auto-Zero function as well as the inherent
repeatability of the detector voltage output versus input power at these levels. (Refer to Section 4.2.7 for
a complete description of the automatic calibration system.)
To make any type of power measurements with the PPM it will be necessary to use the appropriate
calibration and zeroing procedures to ensure the highest accuracy.
After the instrument has warmed up for 30 minutes and the mode of operation has been selected, a
detector self-calibration should be performed. In addition, if the ambient room temperature changes more
than 5 °C from the temperature at which the system was last calibrated, self-calibration should also be
done. If there has been a change in the temperature of the detector of more than 5 °C since it was last
calibrated, the difference will be shown.
2.7.1 Self-Calibration Procedure
Connect the channel A detector to the 1 GHz Calibrator output. Press [MENU] [F3] to call up the Cal or
Zero Detectors menu. The menu offers the following choices:
• F1 to Calibrate the Detectors
• F2 to Auto-Zero the Detectors
• F3 to Return to Data Display
Select F1 for the Detector Self-Calibration.
☛ NOTE:
The CLEAR key as well as F3, can also be used to return to data display.
The display will show the last date and time the detector was calibrated. To proceed with the calibration,
press any Units key. The Self-Cal process takes about 50 seconds, during which time the statement
A (or B) CALIBRATION IN PROGRESS
will be displayed.
If a Model 8501A is in use, press [CLEAR] to return the routine to the data display (the last mode of
operation). If the Model 8502A is being used, press [CLEAR] to bypass the channel A calibration and go
to the channel B calibration.
After successful completion of the Self-Cal function, the instrument will go back to displaying data in the
last mode of operation. With the Model 8502A, either bypassing or successfully completing the channel
A calibration will take the routine to the channel B calibration.
Channel B calibration is done in the same manner as the channel A calibration. The date and time of the
last B channel calibration will be presented, and the option will be given to either press any Units key to
start the cal procedure or press [CLEAR] to return to the data display.
Series 8500A Peak Power Meters
2-14
Manual No. 20790, Rev C, November 1998
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