MM0474
ranges, the effect is negligible. For most mounts, maximum error introduced by
thermoelectric effect is about 0.3 W and is typically 0.1 W on the 0.01 mW
range.
(11) Instrumentation Error. The degree of inability of the instrument to
measure the true substitution audio bias or DC power supplied to the thermistor
mount is called power meter accuracy or instrumentation error. Instrumentation
error of the Model 431C is 2% of full scale, +20C to +35C. Instrumentation
error can be reduced to 0.16% of reading, or less, by using DC substitution.
f. Calibration Factor and Effective Efficiency.
(1) Calibration Factor and Effective Efficiency are two power ratios
used as correction factors to improve overall accuracy of microwave power
measurements.
The ratios are used under different measurement conditions.
Calibration Factor is used when the thermistor mount is coupled to the RF
source without a tuner.
Calibration Factor corrects for both SWR and
inefficiency of the thermistor mount. Effective Efficiency corrects only for
the inefficiency of the thermistor mount.
(2) Each thermistor mount has a particular impedance. This impedance,
and hence the mount SWR, remain constant over the major portion of the
microwave band for which the mount is designed to operate. For most thermistor
mounts this constant SWR is low; thus the mismatch uncertainty is small. Since
the mount impedance and corresponding SWR deviate significantly only at the
high and low ends of a microwave band, it is generally unnecessary to use a
tuner. However, a tuner or other effective means of reducing mismatch error is
recommended when the source SWR is high or when high accuracy is required. To
minimize mismatch between the source and the thermistor mount without the use
of a toner, a low SWR precision attenuator can be inserted in the transmission
line to isolate the thermistor mount from the source.
Since a tuner is not
often used, Calibration Factor is a more practical term than Effective
Efficiency.
(3) Calibration Factor. Calibration Factor is the ratio of substituted
audio or DC power in the thermistor mount to the microwave RF power incident
upon the mount.
Calibration Factor is a figure of merit assigned to a thermistor mount to
correct for the following sources of error: 1) RF reflected by the mount due to
mismatch, 2) RF loss caused by absorption within the mount but not in the
thermistor element, and 3) DC-to-microwave substitution error.
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