MM0474
impedance of 1 megohm introduces a measurement error of 0.1% on ranges below
1.0 mW. Twice this error is the power measurement error: 0.2%. This error
must be added to the power measurement to correct the reading.
f. Calibrator accuracy specifications are 0.05%.
If the measurement
uncertainty is determined to 0.02%, a 0.03% leeway remains in the calibration
of the instrument. The closer the instrument can calibrate to a standard, the
better the calibration of power meters may be performed. Calibration of the
precision resistors in the RESISTANCE STANDARD switch circuitry is best done
using standards laboratory techniques to ensure minimization of measurement
uncertainty.
g. Power Meter DVM Output Measurement.
A digital voltmeter can be
connected to the 431C Power Meter DVM jack to increase resolution of a power
meter reading and allows an easy method of repeating a precise measurement
readout value. Measurement error corrections for voltmeter impedance loading
must be made when using a voltmeter to measure the voltage output of the 431C
Power Meter.
The DC voltage at the DVM connector on the rear panel is
developed across a 1 k ohm resistor.
Therefore, a voltage measurement made
with a digital voltmeter having an input impedance of 500 k ohms will introduce
an error of 0.2%. A digital voltmeter with an input impedance of 10 megohms
will introduce a much smaller error of 0.01%. Correction percentages should be
added to voltmeter readings. If the .01, .1, 1 or 10 mW power meter ranges are
used, the digital voltmeter reads directly.
If the .03, .3, or 3 mW power
meter ranges are used, the digital voltmeter reading must be multiplied by
.0316, .316, or 3.16, respectively.
h. Detection Thermistor Resistance. Steps 8 through 11 of Figure 3 list a
procedure to determine the operating resistance of the 431C Power Meter RF
detection bridge at balance, and thus measure the operating resistance of the
detection thermistor element (Rd) during a power measurement.
NOTE
The
actual
operating
resistance
of
detection
thermistors may deviate as much as 0.5% from their
nominal values.
For this reason, the actual operating
resistance should be checked so that the substituted DC
power in a DC substitution measurement can be calculated
accurately.
i. The 8402B provides a convenient method of determining the detection
thermistor operating resistance.
The thermistor mount cable is connected
between the 431C Power Meter THERMISTOR MOUNT and 8402B Calibrator RESISTANCE
STANDARD connectors. By the THERMISTOR RESISTANCE switch, the 8402B Calibrator
substitutes precision resistance values in place of the thermistor elements
normally in the 431C bridge circuits.
The switched resistances provide a
method of determining oscillator/non-oscillation states of the 431C Power
Meter.
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