Phase detector

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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.

A phase detector is a frequency mixer or analog multiplier circuit that generates a voltage signal which represents the difference in phase between two signal inputs. It is an essential element of the phase-locked loop (PLL).

Detecting phase differences is very important in many applications, such as motor control, radar and telecommunication systems, servo mechanisms, and demodulators.

[edit] Electronic phase detector

Some signal processing techniques such as those used in radar may require both the amplitude and the phase of a signal, to recover all the information encoded in that signal. One technique is to feed an amplitude-limited signal into one port of a product detector and a reference signal into the other port; the output of the detector will represent the phase difference between the signals. If the signal is different in frequency from the reference, the detector output will be periodic at the difference frequency. ^[1]

Phase detectors for phase-locked loop circuits may be classified in two types. ^[2] A Type I detector is designed to be driven by analog signals or square-wave digital signals and produces an output pulse at the difference frequency. The Type I detector always produces an output waveform, which must be filtered to control the phase-locked loop variable frequency 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. This output will tend not to produce ripple in the control voltage of the VCO.

[edit] Types

The designs of phase detectors range from very simple to complex. An exclusive-OR (XOR) logic gate makes a passable phase detector. When the two signals being compared are completely in-phase, the XOR gate's output will have a constant level of zero. When the two signals differ in phase by 1°, the XOR gate's output will be high for 1/180th of each cycle — the fraction of a cycle during which the two signals differ in value. When the signals differ by 180° — that is, one signal is high when the other is low, and vice versa — the XOR gate's output remains high throughout each cycle. 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.

A phase detector can also be made from an analog multiplier (which is more suitable for sinusoidal signals), a sample and hold circuit, a charge pump, or a logic circuit consisting of flip-flops. (A phase detector based on logic gates is shown in the figure.) When a phase detector that's based on logic gates is used in a PLL, it can quickly force the VCO to synchronize with an input signal, even when the frequency of the input signal differs substantially from the initial frequency of the VCO. Such phase detectors also have other desirable properties, such as better accuracy when there are only small phase differences between the two signals being compared.

This is because a digital phase detector has a nearly infinite pull-in range in comparison to an analog mixer-based detector. On the other hand, 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." since it does not create finite pulse widths at the phase detector output.^[3] Another advantage of a mixer-based PD is its relative simplicity. ^[3]

[edit] Phase-frequency detector

A phase-frequency detector is an asynchronous sequential logic circuit originally made of four flip-flops (e.g., the phase-frequency detectors found in both the RCA CD4046 and the motorola MC4344 ICs introduced in the 1970s). Such a detector has the advantage of producing an output even when the two signals being compared differ not only in phase but in frequency. A phase-frequency detector prevents a "false lock" condition, in which the PLL's VCO synchronizes with the wrong phase of the input signal or with the wrong frequency (e.g., a harmonic of the input signal). ^[4]

Phase-frequency detectors can however suffer from a phenomenon known as the zero phase error dead zone where the device either does not respond at all or as a typical negative feedback controller when the phase difference is near zero. In 1976 it was shown <citation needed> that by using a three-state phase detector configuration (using only two flip-flops) instead of the original RCA/Motorola twelve-state configurations, this problem could be elegantly overcome. For other types of phase-frequency detectors other, though possibly less-elegant, solutions exist to the dead zone phenomenon. ^[4] Other solutions are necessary since the three-state phase-frequency detector does not work for certain applications involving randomized signal degradation, which can be found on the inputs to some signal regeneration systems (e.g., clock recovery designs). ^[5]

[edit] Optical phase detectors

Phase detectors are also known in optics as interferometers. For pulsed (amplitude modulated) light, it is said to measure the phase between the carriers. 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. And it is possible to measure the phase between the envelope and the carrier of an optical pulse, by sending a pulse into an nonlinear crystal. There the spectrum gets wider and at the edges the shape depends significantly on the phase.

[edit] Further reading

• * Crawford, James A. 1994. Frequency Synthesizer Design Handbook, Artech House, ISBN 0-89006-440-7
• * Wolaver, Dan H. 1991. Phase-Locked Loop Circuit Design, Prentice Hall, ISBN 0-13-662743-9
• * Egan, William F. 2000. Frequency Synthesis by Phase-lock, 2nd Ed., John Wiley & Sons, ISBN 0-471-32104-4

[edit] See also

• Carrier recovery
• Differential amplifier

[edit] External links

• Analog.com
• Chapter 8 Modulators and Demodulators
• [1]
• Linearized three state phase detector by Steven F. Gillig (patent filed 1990, awarded 1990, assigned to Motorola) ^[6]
• Linearized digital phase and frequency detector by John D. Hatchett and Andrew S. Olesin (patent filed 1980, awarded 1983, assigned to Motorola) ^[7]

[edit] References

1. ^ Donald G. Fink (ed), Electronic Engineers Handbook, McGraw Hill, New York 1975, ISBN 0-07-02980-4 pg. 25-76
2. ^ Paul Horowitz and Winfield Hill, The Art of Electronics 2nd Ed. Cambridge University Press, Cambridge, 1989 ISBN 0-521370957 pg. 644
3. ^ ^{a b} Crawford, 1994, p. 9, 19
4. ^ ^{a b} Crawford, 1994, p. 17-23, 153, and several other pages
5. ^ Wolaver, 1991, p.211
6. ^ Egan, 2000, p. 239
7. ^ Egan, 2000, p. 240