Measuring High-Frequency Signals with Analogue Meters
One major limitation in using analogue meters for a.c. voltage measurement is that the maximum frequency measurable directly is low—2 kHz for the dynamometer voltmeter and only 100 Hz in the case of the moving iron instrument. A partial solution to this limitation is to rectify the voltage signal and then apply it to a moving coil meter, as shown in Figure 7.7. This extends the upper measurable frequency limit to 20 kHz. However, inclusion of the bridge rectifier makes the measurement system particularly sensitive to environmental temperature
Figure 7.6. Circuitry of an analogue multimeter.
Figure 7.7. Measurement of high-frequency voltage signals.
changes, and nonlinearities significantly affect measurement accuracy for voltages that are small relative to the full-scale value.
Calculation of Meter Outputs for Nonstandard Waveforms
The two examples given here provide an exercise in calculating the output reading from various types of analogue voltmeters. These examples also serve as a useful reminder of the mode of operation of each type of meter and the form that the output takes.
Calculate the reading that would be observed on a moving coil ammeter when it is measuring the current in the circuit shown in Figure 7.8.
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A moving coil meter measures mean current.
Figure 7.8. Circuit for Examples 7.1 and 7.2.
Calculate the reading that would be observed on a moving iron ammeter when it is measuring the current in the circuit shown in Figure 7.8.
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