MM0474
However, its effects on power measurement can be corrected for by dividing the
indicated power by the tuner-loss ratio, power out/power in.
(9) Effective Efficiency can be applied as a correction factor to both
conjugate available and Zo available power measurements.
The CALIB FACTOR
switch is set to the Effective Efficiency value, appropriate to the frequency
under test, imprinted on the thermistor mount label. The type of application
of the tuner determines if the power measured is conjugate available or Zo
available.
(10) Conjugate available power is measured when the system consisting of
the RF source, transmission line, tuner and thermistor mount is tuned for a
maximum power level on the 431C.
In this application, the system-mount
combination presents a conjugate match to the source. The power measured is
the actual power that would be delivered by the source to a conjugate load.
(11) Zo available power is measured when a tuner-thermistor mount
combination is tuned for minimum reflection caused by mount mismatch at the
frequency of interest.
The tuner adjustment is made on a reflectometer or
slotted line system, external to the measurement system used for power
measurement.
After the tuner adjustment, the tuner-thermistor mount
combination is connected to the transmission line and RF source on which a
power measurement is made.
g. High Accuracy of Power Measurement Using DC Substitution.
The instrumentation source of error can be reduced by using DC
substitution. With precision instruments used in a DC substitution set up, and
careful procedure, instrument error can be reduced from 1% of full scale to
0.16% of reading, or less. The technique involves: 1) applying the RF power
to be measured to the thermistor mount and noting the power meter reading, 2)
removing the RF power from the thermistor mount and substituting a DC current
from an external DC power source to precisely duplicate the meter reading
obtained in step 1, and 3) calculating the power from the substituted DC
current and thermistor operating resistance.
h. Block Diagram.
(1) The Model 431C Power Meter measures microwave power indirectly using
two bridge circuits (refer to Figure 8). The detection bridge incorporates a
10 kHz oscillator whose amplitude is determined by the amount of microwave
power heating the thermistors in that bridge.
(2) The compensation and metering bridge contains thermistors that are
immersed in the same thermal environment as those of the detection bridge. It
is fed the same 10 kHz bias current that flows in the detection bridge.
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