Sensors and Signal Conditioning, 2nd Edition

8.5.1   Frequency Measurement

Frequency is usually measured by counting cycles during a known time interval. Figure 8.37 shows that this interval (gate time) is usually obtained from a precision clock by frequency dividers. The result is

Figure 8.37   Basic block diagram of a frequency meter.

Because the input signal and the internal clock are asynchronous, we may stop counting just before the next input transition arrived or just after it arrived. This implies an uncertainty of 1 count, usually described by saying that the actual result is N ± 1 counts.

Because the resolution of this measuring method is 1 count, the relative resolution is 1/N, which improves for large N. However, a large N implies a long measurement time, particularly for low frequencies. For example, measuring 10 kHz with uncertainty below 0.1% requires N = 1000 and, because each input cycle lasts 100 μs, the measurement time will be 100 ms.

Example 8.3 A common sensor for traffic control is a buried flat coil supplied by a current from 20 kHz to 150 kHz. As a car enters the coil, its inductance decreases because of eddy currents induced in the car, and when the car leaves the coil the inductance returns to its original value. If the coil is included in a harmonic oscillator, the change in oscillation frequency ...