MM0474
f. Calibration Factor and Effective Efficiency.
(1) Calibration factor and effective efficiency are correction factors
for improving power measurement accuracy.
Both factors are marked on every
thermistor mount. Calibration factor compensates for thermistor mount VSWR and
RF losses whenever the thermistor mount is connected to an RF source without a
tuner. Effective efficiency compensates for thermistor mount RF losses when a
tuner is used in the measurement system.
(2) When the 432A CALIBRATION FACTOR selector is set to the appropriate
factor indicated on the thermistor mount, the power indicated by the meter is
the power that would be delivered by the source to a load impedance equal to
Zo.
More accurately, the relationship between indicated power and the power
available to a Zo load is given by the following equation:
Calibration factor does not compensate for source VSWR,
or
for
multiple
reflections between the source and the thermistor mount.
(3) To minimize mismatch between the source and the thermistor mount
without a tuner, insert a low SWR precision attenuator in the transmission line
between the thermistor mount and the source. Since the mount impedance (and
corresponding SWR) deviates significantly only at the high and low ends of a
microwave band, it is generally unnecessary to use a tuner. A tuner or other
effective means of reducing mismatch or when more accuracy is required.
g. Precision Power Measurement.
(1) General.
Using precision instruments and careful procedures,
measurement error can be reduced to 0.2% of reading to +0.5 W. The technique
involves: 1) zeroing the bridge circuits and measuring the bridge amplifier
output voltage difference with a digital voltmeter, then 2) connecting RF power
to the thermistor mount and then measuring the bridge amplifier output voltage
difference again, and 3) calculating
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