Technician Handbook ! 673 Electronic & Computer Controlled Systems Frequency When a waveform has a recurring pattern, the pattern repeats a certain number of times per second. The number of times it repeats in a second is its frequency in hertz. • One hertz (Hz) equals one cycle per second. • One megahertz (MHz) equals one million cycles per second. In some applications, it is the frequency of a signal that contains the information. A wheel speed sensor is an example. A passive wheel speed sensor creates a sine wave as the wheel rotates. As the wheel rotates faster, the sine wave’s frequency increases proportionately. An active wheel speed sensor, on the other hand, creates a square wave.
As the wheel turns faster, the square wave’s frequency also increases. In both cases, the ECU uses the signal’s frequency to calculate speed. NOTE Revolutions per minute (RPM) is a type of frequency. 6000 RPM is equivalent to 100 cycles per second (100 Hz). 26 Technical Training