KK74HC4046A

Phase Comparators

All three phase comparators have two inputs,

special DC bias network that enables AC coupling of

input signals. If the signals are not AC coupled, standard

IN74HC input levels are required. Both input structures

are shown in Figure 6. The outputs of these comparators

are essentially standard IN74HC outputs (comparator 2 is

voltage levels are fed to the loop filter. This differs from

some phase detectors which supply a current to the loop

filter and should be considered in the design.

Figure 6. Logic Diagram for Phase Comparators

Phase Comparator 1

This comparator is a simple XOR gate similar to

the IN74HC86. Its operation is similar to an overdriven

balanced modulator. To maximize lock range the input

frequencies must have a 50% duty cycle. Typical input

and output waveforms are shown in Figure 7. The output

of the phase detector feeds the loop filter which averages

the output voltage. The frequency range upon which the

PLL will lock onto if initially out of lock is defined as the

capture range.The capture range for phase detector 1 is

dependent on the loop filter design. The capture range can

be as large as the lock range, which is equal to the VCO

frequency range.

To see how the detector operates, refer to Figure 7.

When two square wave signals are applied to this

comparator, an output waveform (whose duty cycle is

dependent on the phase difference between the two

signals) results. As the phase difference increases, the

output duty cycle increases and the voltage after the loop

filter increases. In order to achieve lock when the PLL

input frequency increases, the VCO input voltage must

the VCO input is at 0 V

Figure 7. Typical Waveforms for PLL Using

Phase Comparator 1

This requires the phase detector output to be

grounded; hence, the two input signals must be in phase.

out of phase.

The XOR is more susceptible to locking onto

For instance, a signal 2 times the VCO frequency results

in the same output duty cycle as a signal equal to the

VCO frequency. The difference is that the output

frequency of the 2f example is twice that of the other

example. The loop filter and VCO range should be

designed to prevent locking on to harmonics.

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