A phase detector is an electronic device that compares the phase of two signals. It is commonly used in systems like phase-locked loops (PLLs) to synchronize the frequency of an output signal with a reference signal. By measuring the phase difference, the phase detector can generate an error signal that helps adjust the output to match the reference. There are various types of phase detectors, including analog and digital versions. Analog phase detectors typically use diodes or mixers, while digital phase detectors often rely on flip-flops or counters. These devices are essential in applications such as communication systems and signal processing.