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.

Integrated Publishing, Inc. |