Boonton Power Sensor - Manual (Page 45)

use. Sensor temperature drift uncertainty may be assumed to be zero for sensors operating
exactly at the calibration temperature.
Sensor Noise. The noise contribution to pulse measurements depends on the number of
samples averaged to produce the power reading, which is set by the "averaging" menu setting.
For continuous measurements with CW sensors, or peak sensors in modulated mode, it
depends on the integration time of the measurement, which is set by the "filter" menu setting.
In general, increasing filtering or 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 power being measured. The following expression is used to
calculate uncertainty due to noise:
Noise Error = ± Sensor Noise (in watts) / Signal Power (in watts) * 100 %
The noise rating of a particular power sensor may be found in Section 2 of this manual. It may
be necessary to adjust the sensor noise for more or less filtering or 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 insignificant when
measuring at high levels (25dB or more above the sensor's minimum power rating).
Sensor Zero Drift. Zero drift is the long-term change in the zero-power reading that is not a
random, noise component. Increasing filter or 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 Drift Error = ± Sensor Zero Drift (in watts) / Signal Power (in watts) *100 %
The zero drift rating of a particular power sensor may be found in Section 2 of this manual.
Zero drift error is usually insignificant when measuring at high levels (25dB or more above the
sensor's minimum power rating). The drift 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 character-
ized 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 in Section 3 of this 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.
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