MM0474
setup, and careful procedure, 431C instrument error can be reduced from 1% of
full scale to 0.16% of reading, or less. The technique involves: applying the
RF power to be measured to the thermistor mount and noting the power meter
reading removing the RF power from the thermistor mount and substituting a DC
current from an external DC power source to duplicate precisely the power from
the substituted DC current and thermistor operating resistance.
b. Equipment Used for DC Substitution.
Figure 3 shows the instrument
setup for a DC substitution measurement.
The 8402B Calibrator provides DC
power and appropriate switching to perform DC substitution measurements with
the Model 431C.
c. Although the DC substitution technique is the most accurate method of
measuring RF power, there are sources of error that must be considered. The
accuracy of DC substitution depends largely upon: substitution voltage
accuracy, how precisely the power meter reading is duplicated, the actual
operating resistance of the thermistor.
d. DC Substitution Accuracy. The DIFFERENTIAL VOLTMETER jack on the rear
panel is used to monitor the calibrator output current. Voltage present at the
jack is directly proportional to the substituted current.
To determine the
uncertainty of substitution measurements, the following factors must be
accounted for:
(1) Uncertainty of the thermistor mount operating resistance: can be
measured to 0.06% with the calibrator internal resistance standard.
(2) Tolerance of resistor across which the voltage is measured: 0.01%.
(3) Accuracy of the voltmeter used for the measurement: 0.005% of
reading, +0.0004% of range, +1 V when using a DC VOLTMETER STANDARD.
(4) Differential voltmeters do not load the output voltage; loading
effect on the output voltage when using a digital voltmeter must be accounted
for. Input impedance mast be 10 megohms or greater on the 1.0 to 10 mW ranges,
and 100 megohms or greater on the lower ranges of the calibrator for negligible
loading of less than 0.001%.
(5) Twice the voltage
uncertainty
equals
the
power
uncertainty
introduced by the voltmeter.
The
impedance is 100 ohms on the 1.0 through 10 mW ranges, and 1 kilohm on the
lower ranges.
At null, a differential voltmeter draws no current from the
calibrator voltage output and therefore does not cause a loading error. When
using other types of voltmeters, the effect of loading must be accounted for.
For example, a digital voltmeter with an input
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