Boonton 4530 Peak Power Meter User Manual - Manual (Page 39)

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Boonton Electronics Chapter 3
4530 Series RF Power Meter Operation
Since each sensor is different, the power meter must know the precise relationship between RF input amplitude and the
sensor’s detected output. Information about this relationship can be characterized at the factory, and stored in the
sensor’s EEPROM, then used by the power meter to calculate the input power from the sensor’s output. This technique
is known as shaping, because it corrects the shape of the sensor’s nonlinear transfer function into a linear function of
power. Because the sensor’s output curve is not perfectly stable with time and temperature, greater absolute accuracy
may be gained by calibrating points on the transfer function at the time of measurement, and including these factors in
the shaping calculation.
The most basic field calibration consists of two reference points on the curve. By correcting these two points so they
read the expected values, the accuracy of the entire curve is increased. Generally, 0mW and 1mW are chosen as
calibration points. The 0mW calibration is known as a Zero adjustment, and the 1mW (0dBm) calibration is know as a
FixedCal. These power reference levels are generated by a precision RF calibrator that is built in to the power meter.
The accuracy of the shaping technique can be further improved by increasing the number of power calibration points.
For this purpose, the 4530 is equipped with a programmable step calibrator, which generates precise RF power levels
between +20dBm and -60dBm. By stepping through the entire power range of the sensor (including zero power), the
basic accuracy of the shaping technique is significantly increased over a simple, two-point calibration. This technique
is known as an AutoCal.
All peak sensors and most CW sensors are calibrated using a precision step calibrator. The Model 4530 has a built-in
50 MHz step calibrator, and can program an optional Model 4530 1 GHz Calibrator Accessory (see Appendix B). All
57000 series peak sensors can be calibrated using either calibrator. Most 56000 series peak sensors require the 1 GHz
calibrator. All CW sensors except waveguide types, and sensors with more than 20 dB attenuation, can also use the
FixedCal method. This method uses either calibrator at a fixed level in combination with shaping curves to produce a
correct reading, but offers less accuracy than a full step calibration. Waveguide sensors and a few other models must
use the FreqCal method. An external 0 dBm source at the calibration frequency is required. All calibration data is saved
in non-volatile memory. No calibration is required for voltage probe/sensors; only zero offset adjustment is available.
When a peak or CW sensor is step calibrated (AutoCal) a zeroing procedure is performed followed by a power step
calibration in small increments over the entire dynamic range of the sensor. The resulting calibration table is saved in
non-volatile memory. If a new peak sensor, which has not been AutoCal’ed, is plugged in, the AutoCal message will
appear in the graphics and text headers indicating that a calibration must be performed before any measurements can
be made, since there is not yet a valid calibration table for the peak sensor in use. When a new calibration has
successfully completed, the previous one will be overwritten.
Occasionally, a zero or calibration procedure may not complete successfully. In most cases, this can be traced to the
sensor not being connected to the active calibrator. Zeroing can be performed any time the signal source is turned
completely off or the sensor RF port is disconnected. Fixed or autocal must be performed with the sensor connected
to the instrument’s internal calibrator port, or the port of a Model 2530 1GHz Calibrator. In either case, the active
calibration source must be set to match the calibrator being used in the
Zero/Cal > CalSource menu. If zeroing ro
calibration fails, a status code is reported on the display. See table 3-8 for a list of calibration status codes.
3.10.1 Sensor Connection. Connect the sensor to the 4530 by plugging one end of a sensor cable into the power
sensor and the other end into the sensor input on the instrument’s front (or rear) panel. Peak power sensor
cables are the same on both ends, so it does not matter which end of the cable is inserted into the sensor. CW
power sensors and RF voltage probes use a two-pin connector on the sensor, and the cable has a multi-pin
smart adapter on the instrument end. This adapter contains the EEPROM that holds the sensor’s character-
istics and calibration information, so the cable/adapter assembly must be matched to the sensor. Serial
number labels on each should be used to identify matching assemblies.
When the sensor is connected to the 4530, message is displayed indicating the type and model of the sensor,
and will download its calibration information. At this point, CW and voltage sensors may be used to take
measurements using the default shaping calibration technique. For best accuracy, however, a sensor zero
and/or calibration should be performed. Peak sensors require a multi-point calibration (autocal) before mea-
surements can be taken for the first time.