Phase detector and its main characteristics

Phase Detector (PD) - an electronic device, comparing the phases of two input signals of equal or nearly equal frequencies.

PD supplied to the input two signals whose phases are needed to compare the output signal PD is formed, typically a voltage proportional to the phase difference of the input signals.

Systems used in the phase locked loop frequency synthesizer, radio receivers apparatus stabilizers spindle speed, for example, hard disk drives, and others.

Figure 3.1 Phase Detector UNI-T UT261B.

Phase Detector Types

Exclusive OR

The simplest PD - XOR gate. When applied to the input of the element two equal rectangular oscillation frequency with zero phase shift in the output voltage is zero (logic 0). When a nonzero phase shift to the output element are formed pulses whose average value is directly proportional to the phase shift and reaches a maximum (output logical 1) Shear equal π. For averaging the pulse signal output at the output of this filter is set low pass filter (LPF).

Balanced mixer

Another type PD - a four-quadrant multipliers of the two input signals, which are often called balanced mixers. The output of the balanced mixer contains twice the frequency of the input signal and a constant component, which is proportional to the phase difference, which follows from the expression:

(3.1)

Sinus small angle is approximately replaced by the angle. Component at twice the frequency can be easily filtered by the LPF.

Circuitry balanced mixers are usually built under the scheme Gilbert.

PD, triggered by the fronts of the input signals

FD this type are sensitive to the relative position of the fronts of the input signals. For example, if signal A leads signal B, the output FD of the positive polarity pulses are formed with a duration proportional to the phase difference, and a repetition frequency equal to the frequency of the input signal. If the B signal is ahead of the signal A, the output pulses of negative polarity are formed. To obtain an output voltage proportional to the phase difference PD is used at the output LPF.

Application of PD

Traditional use of the PD - in servo systems, frequency locked loop, where PD, together with the variable frequency generator, a voltage controlled oscillator (VCO) included in the negative feedback loop. The signal assignment for the system of automatic control is the frequency of the input signal, and PD is the comparator. The transfer function of lowpass filter mounted on the PD output to the VCO is further added a zero, to ensure stability of the phase margin. In the simplest case, if the LPF is RC-filter LF, the zero in the transfer function can be obtained by including with the required resistance resistor in series with the filter capacitor.

PD also used in synthesizers, multipliers and frequency dividers. In these systems, the input of the PD signals do not supplied, and the signals obtained from the multiplication, division, and necessary amounts of frequency differences.In radio communication systems used in the FD-locked oscillator frequency superheterodyne receiver.

Upon stabilization speed spindles and shafts to one of the inputs of the PD is supplied from the reference signal generator, a second - pulses from the speed sensor frequency tags, and the output signal PD VCO is not controlled, and the electric drive shaft.