Phasedetection

Phase detection is the process of determining the phase of a periodic signal relative to a reference signal or another signal. It is a fundamental operation in communication systems, timing recovery, and signal demodulation. Accurate phase detection enables carrier recovery, synchronization, and demodulation of phase-modulated schemes such as BPSK and QPSK. In radio receivers and coherent optical systems, phase information is essential for proper demodulation and timing alignment.

Common phase detectors include analog mixer-based detectors, which output an error proportional to the phase difference between inputs; digital detectors that use XOR or edge-detection for square-wave signals; and phase-frequency detectors (PFDs) that sense both phase and frequency differences to improve lock range. There are also specialized detectors used in digital communication demodulation, such as Alexander- and Hogge-type detectors, which implement specific phase-error estimation strategies. Each detector type has different linear ranges, response times, and susceptibility to noise and jitter.

In practice, phase detection is implemented as part of a phase-locked loop (PLL), together with a loop filter and a voltage-controlled oscillator. The phase detector generates an error signal that the loop filter converts into a control voltage to align the VCO phase with the reference. Key performance characteristics include phase error, linearity, capture range, lock range, and phase noise.

Other applications of phase detection include imaging systems, where phase-detection autofocus uses phase information to assess focus, and measurement of phase noise and timing jitter in RF and optical systems.