Understand quadrature encoders with a quick technical recap

An unexpected revisit to my earlier post on mouse encoder hacking sparked a timely opportunity to reexamine quadrature encoders, this time with a clearer lens and a more targeted focus on their signal dynamics and practical integration. So, let’s get a fresh restart and dive straight into the quadrature signal magic.

Starting with a flake of theory, a quadrature signal refers to a pair of sinusoidal waveforms—typically labeled I (in-phase) and Q (quadrature)—that share the same frequency but are offset by 90° in phase. These orthogonal signals do not interfere with each other and together form the foundation for representing complex signals in systems ranging from communications to control.

Figure 1 A visualization illustrates the idealized output from a quadrature encoder, highlighting the phase relationship. Source: Author

In the context of quadrature encoders, the term describes two square wave signals, known as A and B channels, which are also 90° out of phase. This phase offset enables the system to detect the direction of rotation, count discrete steps or pulses for accurate position tracking, and enhance resolution through edge detection techniques.

As you may already be aware, encoders are essential components in motion control systems and are generally classified into two primary types: incremental and absolute. A common configuration within incremental encoders is the quadrature encoder, which uses two output channels offset in phase to detect both direction and position with greater precision, making it ideal for tracking relative motion.

Standard incremental encoders also generate pulses as the shaft rotates, providing movement data; however, they lose positional reference when power is interrupted. In contrast, absolute encoders assign a unique digital code to each shaft position, allowing them to retain exact location information even after a power loss—making them well-suited for applications that demand high reliability and accuracy.

Note that while quadrature encoders are often mentioned alongside incremental and absolute types, they are technically a subtype of incremental encoders rather than a separate category.

Oh, I almost forgot: The Z output of an ABZ incremental encoder plays a crucial role in precision positioning. Unlike the A and B channels, which continuously pulse to indicate movement and direction, the Z channel—also known as the index or marker pulse—triggers just once per revolution.

This single pulse serves as a reference point, especially useful during initialization or calibration, allowing systems to accurately identify a home or zero position. That is to say, the index pulse lets you reset to a known position and count full rotations; it’s handy for multi-turn setups or recovery after power loss.

Figure 2 A sample drawing depicts the encoder signals, with the index pulse clearly marked. Source: Author

Hands-on with a real-world quadrature rotary encoder

A quadrature rotary encoder detects rotation and direction via two offset signals; it’s used in motors, knobs, and machines for fine-tuned control. Below is the circuit diagram of a quadrature encoder I designed for a recent project using a couple of optical sensors.

Figure 3 Circuit diagram shows a simple quadrature encoder setup that employs optical sensors. Source: Author

Before we proceed, it’s worth taking a moment to reflect on a few essential points.

• A rotary encoder is an electromechanical device used to measure the rotational motion of a motor shaft or the position of a dial or knob. It commonly utilizes quadrature encoding, an incremental signaling technique that conveys both positional changes and the direction of rotation. On the other hand, linear encoder measures displacement along a straight path and is commonly used in applications requiring high-precision linear motion.
• Quadrature encoders feature two output channels, typically designated as channel A and channel B. By monitoring the pulse count and identifying which channel leads, the encoder interface can determine both the distance and direction of rotation.
• Many encoders also incorporate a third channel, known as the index channel (or Z channel), which emits a single pulse per full revolution. This pulse serves as a reference point, enabling the system to identify the encoder’s absolute position in addition to its relative movement.
• Each complete cycle of the A and B channels in a quadrature encoder generates square wave signals that are offset by 90 degrees in phase. This cycle produces four distinct signal transitions—A rising, B rising, A falling, and B falling—allowing for higher resolution in position tracking. The direction of rotation is determined by the phase relationship between the channels: if channel A leads channel B, the rotation is typically clockwise; if B leads A, it indicates counterclockwise motion.
• To interpret the pulse data generated by a quadrature encoder, it must be connected to an encoder interface. This interface translates the encoder’s output signals into a series of counts or cycles, which can then be converted into a number of rotations based on the encoder’s cycles per revolution (CPR) counts. Some manufacturers also specify pulses per revolution (PPR), which typically refers to the number of electrical pulses generated on a single channel per full rotation and may differ from CPR depending on the decoding method used.

Figure 4 The above diagram offers a concise summary of quadrature encoding basics. Source: Author

That’s all; now, back to the schematic diagram.

In the previously illustrated quadrature rotary encoder design, transmissive (through-beam) sensors work in tandem with a precisely engineered shaft encoder wheel to detect rotational movement. Once everything is correctly wired and tuned, your quadrature rotary encoder is ready for use. It outputs two phase-shifted signals, enabling direction and speed detection.

In practice, most quadrature encoders rely on one of three sensor technologies: optical, magnetic, or capacitive. Among these, optical encoders are the most commonly used. They operate by utilizing a light source and a photodetector array to detect the passage or reflection of light through an encoder disk.

A note for custom-built encoder wheels: When designing your own encoder wheel, precision is everything. Ensure the slot spacing and width are consistent and suited to your sensor’s resolution requirements. And do not overlook alignment; accurate positioning with the beam path is essential for generating clean, reliable signals.

Layers beneath the spin

So, once again we circled back to quadrature encoders—this time with a bit more intent and (hopefully) a deeper dive. Whether you are just starting to explore them or already knee-deep in decoding signals, it’s clear these seemingly simple components carry a surprising amount of complexity.

From pulse counting and direction sensing to the quirks of noisy environments, there is a whole layer of subtleties that often go unnoticed. And let us be honest—how often do we really consider debounce logic or phase shift errors until they show up mid-debug and throw everything off?

That is the beauty of it: the deeper you dig, the more layers you uncover.

If this stirred up curiosity or left you with more questions than answers, let us keep the momentum going. Share your thoughts, drop your toughest questions, or suggest what you would like to explore next. Whether it’s hardware oddities, decoding strategies, or real-world implementation hacks—we are all here to learn from each other.

Leave a comment below or reach out with your own encoder war stories. The conversation—and the learning—is far from over.

Let us keep pushing the boundaries of what we think we know, together.

T. K. Hareendran is a self-taught electronics enthusiast with a strong passion for innovative circuit design and hands-on technology. He develops both experimental and practical electronic projects, documenting and sharing his work to support fellow tinkerers and learners. Beyond the workbench, he dedicates time to technical writing and hardware evaluations to contribute meaningfully to the maker community.

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