Quadrature encoders are a specific type of relative encoder that have four different output states. If the root quad-, means four, but there are only two sensors in this encoder, where does the name come from? The output from the two Hall effect sensors are called “Channel A” and Channel B” respectively; an example of the output is shown below. In a single period (T), defined as the duration of time of one complete cycle in a repeating pattern, the timing diagram has four distinct states (see a, b, c, and d below), hence a quadrature encoder.