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MECHANICAL ROTATION frequency is measured using a device called a TACHOMETER.
Three basic tachometers are used for measuring mechanical rotation frequency—the CENTRIFUGAL
tachometer, the CHRONOMETRIC tachometer, and the STROBOSCOPIC tachometer.
ELECTRICAL-OUTPUT frequencies of ac generators can be measured by VIBRATING-REED
devices or TUNED CIRCUITS.
AUDIO FREQUENCIES can be measured by a process known as ZERO BEATING. This is done
by matching an unknown signal with a locally generated signal of the same frequency obtained from a
calibrated high-precision oscillator. As the two frequencies are brought closer to the same value, they
reach a point of zero beat. This is when the frequency generated in the oscillator is equal to the frequency
of the unknown signal being measured. Another term for zero beating is HETERODYNING.
WAVEMETERS are calibrated resonant circuits used to measure frequency. Any type of resonant
circuit can be used in wavemeter applications. The type used depends on the frequency range for which
the meter is intended.
For measuring frequencies in the microwave range, the CAVITY WAVEMETER is the type most
commonly used.
