Boonton Power Sensor - Manual (Page 43)

The sensor reflection coefficient, D
SNSR
is frequency dependent, and can be referenced in
Section 2 of this manual. For most measurements, this is the single largest error term, and care
should be used to ensure the best possible match between source and sensor. Figure 6-1. plots
Mismatch Uncertainty based on known values of both source and sensor SWR.
Sensor Shaping Error. This term is sometimes called "linearity error", and is the residual
non-linearity in the measurement after an AutoCal has been performed to characterize the
"transfer function" of the sensor (the relationship between applied RF power, and sensor
output, or shaping). Calibration is performed at discrete level steps and is extended to all
levels. Generally, sensor shaping error is close to zero at the autocal points, and increases in
between due to imperfections in the curve-fitting algorithm.
An additional component of sensor shaping error is due to the fact that the sensor's transfer
function may not be identical at all frequencies. The published shaping error includes terms
to account for these deviations. If your measurement frequency is close to your AutoCal
frequency, it is probably acceptable to use a value lower than the published uncertainty in your
calculations.
For CW sensors using the fixed-cal method of calibrating, the shaping error is higher because
it relies upon stored "shaping coefficients" from a factory calibration to describe the shape of
the transfer function, rather than a transfer calibration using a precision power reference at the
current time and temperature. For this reason, use of the AutoCal method is recommended for
CW sensors rather than simply performing a FixedCal. The shaping error for CW sensors
using the FixedCal calibration method is listed as part of the "Sensor Characteristics"
outlined in Section 2 of this manual. If the AutoCal calibration method is used with a CW
sensor, a fixed value of 1.0% may be used for all signal levels.
All peak power sensors use the AutoCal method only. The sensor shaping error for peak
sensors is also listed in Section 2 of this manual.
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.
This condition is detected by the Model 4530 and a "temperature drift" message warns the
operator to recalibrate the sensor for drift exceeding ± 4 °C on non-temperature compensated
peak sensors.
Temperature compensated peak sensors have a much smaller temperature coefficient, and a
much larger temperature deviation, ± 30 °C is permitted before a warning is issued. For these
sensors, the maximum uncertainty due to temperature drift from the autocal temperature is:
Temperature Error = ± 0.04dB (0.93%) + 0.003dB (0.069%) / °C
Note that the first term of this equation is constant, while the second term (0.069%) must be
multiplied by the number of degrees that the sensor temperature has drifted from the AutoCal
temperature.
CW sensors have no built-in temperature detectors, so it is up to the user to determine the
temperature change from AutoCal temperature. Temperature drift for CW sensors is
determined by the temperature coefficient of the sensor. This figure is 0.01dB (0.23%) per
degreeC for the 51075 and many other CW sensors. Refer to Section 2 for the exact figure to
36 Power Sensor Manual 38