Understanding Quadrature Signals

The output from incremental optical encoders and from many other types of measurement devices is known as a “quadrature” signal. A quadrature signal consists of two square waves 90° out of phase. The input channel on our encoder interface board counts the square wave transitions and determines direction by comparing whether channel A is leading channel B or vice-versa. The number of revolutions is determined by dividing the count by the number of pulses per revolution: e.g., 100 counts with a 1000 PPR (Pulses Per Revolution) encoder is equal to 36 degrees. Velocity is computed the same way by counting the number of pulses per second.

A quadrature signal may be contrasted with a tachometer signal, which has only one square wave output and therefore does not provide any directional information. Tachometer signals can be misleading because if the tachometer stops on a transitional edge, vibration will cause your counter to continue incrementing.

In addition to the two quadrature signals, most encoders supply an index signal with one pulse per revolution. Our ESH 250 series encoder interface boards can use the index signal to reset the counter, allowing you to monitor your position within the current revolution.

Differential encoders provide a complimentary signal for each of the three standard signals. Differential transmission prevents signal degradation in applications where the signal is to be sent over long distances. Each channel of our encoder interface boards can be programmed to accept differential or single-ended quadrature signals.

Our encoder interface boards can accept another type of encoder output as well: direction-sensing pulsed output. In this mode, the encoder generates a pulse for each of the four transitional edges of one quadrature cycle. Pulse output encoders will have clockwise and counterclockwise signals in place of the A and B signals of the quadrature signal. Pulses will be generated on the CW line when A is leading, and on the CCW line when B is leading.