176 Frequency measurement

Low frequencies can be measured using an electromechanical moving-coil instrument. Current from the supply flows through two parallel fixed coils and then through a moving coil mounted between them (Figure 17.14). The two fixed coils are tuned to slightly different frequencies and the resulting fields set up a torque which is proportional to frequency.

The nominal frequency at which the instrument can be used depends on the tuning of the coils and the instrument is only accurate to within plus or minus a few per cent of this frequency. A ratiometer (Figure 17.15) can be used at higher frequencies up to 5 kHz or so. This consists of two moving coils arranged at right angles and mounted between the poles of a permanent magnet. The system current is fed into the two coils through separate phase-shifting networks and the result is to produce a torque proportional to frequency.

At frequencies higher than this, a solid-state counter must be used. This is based on a stable oscillator and, in effect, counts the pulses generated during one cycle of the supply frequency. The range and accuracy of the instrument depends on the master oscillator frequency, but units capable of use over the whole range up to 600 MHz

Fixed coils

Supply

Moving coil

Moving coil

Supply

Fixed coils.

Tuning inductors

Figure 17.13 Schematic diagram of electrodynamics instrument

Figure 17.14 Frequency meter

Supply

Permanent magnet

_ Moving coils r

Figure 17.15 Frequency ratio meter are available, although some discrimination is likely to be lost at the lower end.

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