SL74HC4046

488

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 SL74HC input levels are required. Both input

structures are shown in Figure 6. The outputs of these

comparators are essentially standard SL74HC outputs

(comparator

2

is

TRI-STATEABLE).

In

normal

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 SL74HC86. 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 increase and the phase

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

be 180 degrees out of phase.

The XOR is more susceptible to locking onto

2. 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.