MM0474
(4) The CALIB FACTOR switch on the front panel allows rapid power
measurements to be made with improved accuracy.
The switch is set to the
Calibration Factor value, appropriate to the frequency of measurement,
imprinted on the thermistor mount label.
With the proper setting, the 431C
compensates for the Calibration Factor of the thermistor mount.
(5) Calibration Factor is applied as a correction factor to all
measurements made without a tuner. Under this condition, the power indicated
is the power that would be delivered by the source to a load impedance equal to
Zo. This measured power is called Zo available power.
(6) Calibration Factor correction ensures that a power measurement
uncertainty range is centered on the Zo available power level instead of on the
power delivered to the thermistor mount impedance.
Total measurement
uncertainty limits for a given power measurement using Calibration Factor are
the sum of the uncertainties contributed by: 1) Mismatch loss, 2) Calibration
Factor uncertainty, and 3) Instrumentation error.
Before correction, the
maximum possible error is 17.2% and the statistically most probable error is -
7.35%.
(7) Figure 7 shows the total power measurement uncertainty after
Calibration Factor correction. Note that the range of uncertainty, 19.7%, is
the same as it was before correction.
However, the measurement uncertainty
range has shifted, and is now more symmetrical about the Zo available power-
level.*
The total uncertainty after correction is the algebraic sum of the
instrumentation error (1%), the accuracy to which the Calibration Factor is
determined (2%), and the uncertainty on the power actually entering the
thermistor mount. After correction, the power measurement uncertainty on the
Zo available power is +10.2% to -9.5%.
The maximum possible error is 10.2%
(was 17.2%) and the statistically most probable error is +0.35% (was -7.35%).
This is a typical example showing how the use of Calibration Factor correction
to a measurement to Zo available power not only reduces the maximum possible
error, but more importantly, the magnitude of the statistically most probable
error is reduced to very near the Zo available power level.
*The relationship between indicated power on the 431C and the Zo available
power is given by the following equation:
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