Phase detector

Detecting phase difference is important in other applications, such as motor control, radar and telecommunication systems, servo mechanisms, and demodulators.

The Type I detector always produces an output waveform, which must be filtered to control the phase-locked loop voltage-controlled oscillator (VCO).

A type II detector is sensitive only to the relative timing of the edges of the input and reference pulses and produces a constant output proportional to phase difference when both signals are at the same frequency.

A mixer-based detector (e.g., a Schottky diode-based double-balanced mixer) provides "the ultimate in phase noise floor performance" and "in system sensitivity."

A phase detector suitable for square wave signals can be made from an exclusive-OR (XOR) logic gate.

The XOR detector compares well to the analog mixer in that it locks near a 90° phase difference and has a pulse wave output at twice the reference frequency.

Applying the XOR gate's output to a low-pass filter results in an analog voltage that is proportional to the phase difference between the two signals.

The remainder of its characteristics are very similar to the analog mixer for capture range, lock time, reference spurious and low-pass filter requirements.

The PFD improves the pull-in range and lock time over simpler phase detector designs such as multipliers or XOR gates.

Bang-bang phase detectors are simple but are associated with significant minimum peak-to-peak jitter, because of drift within the dead band.

[citation needed] For other types of phase-frequency detectors other, though possibly less-elegant, solutions exist to the dead zone phenomenon.

Phase detectors with this kind of control system don't exhibit a dead band and typically have lower minimum peak-to-peak jitter when used in PLLs.

It is also possible to measure the delay between the envelopes of two short optical pulses by means of cross correlation in a nonlinear crystal.

Four phase detectors. Signal flow is from left to right. In the upper left is a Gilbert cell , which works well for sine waves and square waves , but less well for pulses. In the case of square waves it acts as an XOR gate, which can also be made from NAND gates. On the middle left are two phase detectors: adding feedback and removing one NAND gate produces a time frequency detector. The delay line avoids a dead band. On the right is a charge pump with a filter at its output.
An example CMOS digital phase frequency detector. Inputs are R and V while the outputs U p and D n feed to a charge pump.