Adv a nc e Inform ation

FS8308

Page 9

April 2003

Phase/Frequency Detector (PFD)

The PFD compares an internal input frequency divider output signal, f

reference frequency divider output signal, f

proportional to the phase error between f

a passive filter as shown in Fig. 2.

Lock Detector (LD)

When phase comparator detects phase difference, LD terminal outputs “L”. When phase

comparator locks, LD terminal outputs “H”. On standby, outputs “H”. The criteria for lock

condition is that the phase difference between f

more than three consecutive times.

The input/output waveforms for the PFD and LD are shown in Fig. 3.

Fig. 2 – Passive low-pass filter circuit

Fig. 3 – PFD input/output waveforms

DO

to VCO

R1

C1

C2

high-Z

high-Z

high-Z

f

R

f

V

DO

LD

< 2/xin

< 2/xin

< 2/xin

2/xin