Page 22 - Phase-Locked Loops Design, Simulation, and Applications

MIXED-SIGNAL PLL BUILDING BLOCKS Ronald E. Best 16

Figure 2.5 The EXOR phase detector.

pull-in process is quite slow. It will be demonstrated later in Sec. 2.4.4 that another type of
phase detector (the so-called phase-frequency detector) enables much faster acquisition,
because its output signal is not only phase sensitive, but also frequency sensitive (in the
unlocked state).
We continue the discussion of phase detectors with the EXOR (exclusive-OR) gate.

Type 2: EXOR phase detectors
The operation of the EXOR phase detector (Fig. 2.5) is similar to that of the linear multiplier.
The signals in DPLLs are always binary signals (for instance, square waves). We assume for
the moment that both signals u and u ′ are symmetrical square waves. Figure 2.6 depicts
1 2
the waveforms of the EXOR phase detector