4-17
Figure 4-14.—Ac/dc differential voltmeter.
Q-8. What is the general function of the differential voltmeter?
The differential voltmeter can be used as a conventional TRANSISTORIZED ELECTRONIC
VOLTMETER (TVM) and a DIFFERENTIAL NULL VOLTMETER. It can also be used to measure
variations of a voltage near some known value (NULL DETECTOR), high resistance values
(MEGGOMETER), and for dBm measurements.
METER DESIGN CHARACTERISTICS
The differential voltmeter is a solid-state instrument that provides the capability of making dc
voltage measurements from +/- 10 microvolts to +/- 1,100 volts. Ac voltages from 0.001 to 1,100 volts
can be measured over a frequency range from 5 hertz to 100 kilohertz. Both of these measurements can be
made without concern for loading the circuit. The differential voltmeter has four voltage readout dials that
vary the resistance of the divider assembly as described above.
The differential voltmeter uses a built-in NULL DETECTOR to measure an unknown voltage. The
meter circuitry compares the unknown voltage to a known, adjustable reference voltage supplied by the
meter. The reference voltage is provided by a high-voltage dc power supply and decade resistor divider
assembly strings that are set by voltage readout dials. In this way, the output from the high-voltage power
supply can be precisely divided into increments as small as 10 microvolts. The readout dials are used to
adjust the meter pointer to 0 and the unknown voltage is then read from the voltage dials.
A primary feature of the differential voltmeter is that it does not draw current from the unknown
source for dc measurements when the measurement is obtained. Therefore, the determination of the
unknown dc potential is independent of its source.
FRONT PANEL CONTROLS
The front panel of a typical differential voltmeter is shown in figure 4-15. With a few differences, the
controls and terminals are similar to those used on other differential voltmeters. The NULL SENSE
