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Phase Locked Loop Phase Detector

3.2 Phase Detector

In the National Semiconductor LM565 Phase-Locked-Loop seen here in figure 3.2.1, the phase detector is defined as the first subsection in the schematic. Pins 2 and 3 provide an input path for the differential input signal, where pin 5, the phase detector VCO input is connected to pin 4, the VCO output. A Widlar current source integrated throughout the schematic provides the biasing for the phase detector.

Figure 3.2.1 LM565 Phase-Locked-Loop

A phase detector or also commonly referred to as a phase comparator is a 5 terminal device having a differential input, differential output, and feedback control signal from a voltage controlled oscillator (VCO) (see figure 3.2.2). The purpose of the phase detector is to compare the phase of the differential input signal to the feedback control signal from the VCO. As the phase of the of the incoming signal changes with relation to the VCO's phase a differential output voltage waveform is generated with an amplitude proportional to the phase difference of the input and VCO's signal. Another words, the phase detector acts similar to a mixer where the product of the differential input signal and the VCO's feedback signal produces their sum and difference frequencies as shown in figure 3.2.2.

Figure 3.2.2 Phase Detector Schematic

Figure 3.2.3 Differential Input Signal vs. VCO Feedback Signal of Phase Detector

Figure 3.2.4 shows the isolated subsection of the phase detector in the LM565 Phase-Locked-Loop. Transistors Q3, Q4, Q5, Q6 make up a "current steering network" that routes current through the resistors R1 and R2. This current steering transistor network controls the differential output to the amplifier stage of the circuit by turning these transistors on and off allowing current to flow through R1 or R2. As current is pulled down a specific path the output to the amplifier on that path is pulled "low" driving the path not take by the current "high" (see figure 3.2.4). This switching characteristic of the current steering transistors is how the detector compares the differential input waveform to the VCO feedback waveform.

Figure 3.2.4 Simplified phase detector showing current path when input (pin 3) is in-phase with VCO

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