Boonton 4240 RF Power Meter - Manual (Page 116)

Boonton 4240 Series RF Power Meter
Sensor Temperature Coefficient. This term is the error which occurs when the sensor’s temperature has changed
significantly from the temperature at which the sensor was AutoCal’d. Refer to the Boonton Electronics Power
Sensor Manual for the Temperature Coefficient for the sensor being used.
Sensor Noise. For CW measurements it depends on the integration time of the measurement, which is set by the
“AVG” menu setting. In general, increasing averaging reduces measurement noise. Sensor noise is typically
expressed as an absolute power level. The uncertainty due to noise depends upon the ratio of the noise to the signal
d. Th ise:
nal Power (in watts) × 100 %
The noise rating of a particular power sensor may be found on the sensor datasheet, or the Boonton Electronics
insignificant when measuring at high levels (25dB or more
inimu
rm change in the zero-power reading that is not a random, noise
Drift Error = ± Sensor Zero Drift (in watts) / Signal Power (in watts) × 100 %
ro of a part sheet, or the Boonton Electronics
ower S or Ma ual. Z high levels (25dB or more above
rift specification usually indicates a time interval such as one hour. If
the time since performing a sensor Zero or AutoCal is very short, the zero drift is greatly reduced.
Sensor Calibration Factor Uncertainty. Sensor frequency calibration factors (“calfactors”) are used to correct for
sensor frequency response deviations. These calfactors are characterized during factory calibration of each sensor
by measuring its output at a series of test frequencies spanning its full operating range, and storing the ratio of the
actual applied power to the measured power at each frequency. This ratio is called a calfactor. During measurement
operation, the power reading is multiplied by the calfactor for the current measurement frequency to correct the
reading for a flat response.
The sensor calfactor uncertainty is due to uncertainties encountered while performing this frequency calibration (due
to both standards uncertainty, and measurement uncertainty), and is different for each frequency. Both worst case
and RSS uncertainties are provided for the frequency range covered by each sensor, and are listed on the sensor
datasheet and in the Boonton Electronics Power Sensor Manual.
If the measurement frequency is between sensor calfactor entries, the most conservative approach is to use the
higher of the two corresponding uncertainty figures. It is also be possible to estimate the figure by linear
interpolation.
If the measurement frequency is identical to the AutoCal frequency, a calfactor uncertainty of zero should be used,
since any absolute error in the calfactor cancels out during AutoCal. At frequencies that are close to the AutoCal
frequency, the calfactor uncertainty is only partially cancelled out during AutoCal, so it is generally acceptable to
take the uncertainty for the next closest frequency, and scale it down.
power being measure e following expression is used to calculate uncertainty due to no
Noise Error = ± Sensor Noise (in watts) / Sig
Power Sensor Manual. It may be necessary to adjust the sensor noise for more or less averaging, depending upon
the application. As a general rule (within a decade of the datasheet point), noise is inversely proportional to the
filter time or averaging used. Noise error is usually
above the sensor’s m
m power rating).
Sensor Zero Drift. Zero drift is the long-te
component. Increasing averaging will not reduce zero drift. For low-level measurements, this can be controlled by
zeroing the meter just before performing the measurement. Zero drift is typically expressed as an absolute power
level, and its error contribution may be calculated with the following formula:
Zero
The ze d icular power sensor may be found on the sensor data
P ens n ero drift error is usually insignificant when measuring at
rift rating
the sensor’s minimum power rating). The d
Application Notes
6-8