FS8170

Page 9

July 2005

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 (a). The polarity of DO is selectable by setting the bit FC

to high or low. The setting should depend on the frequency-voltage characteristic of exter-

nal VCO as depicted in Fig. 2 (b).

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

Fig. 2 – Low-pass filter and external VCO frequency-voltage characteristic

Table 6: Data format for FC bit (LDS = HIGH)

FC = HIGH

FC = LOW

DO

φR

φPFOLD

DO

φR

φPFOLD

f

HL

L

FOLD =

f

R

LH

FOLD =

f

V

f

LHZ

H

L

L

f

Z

L

ZZ

L

Z

a. Z denotes high impedance state

DO

VCO

(a) Passive low-pass filter

(1)

(2)

f

VCO

V

DO

(b) VCO frequency-voltage characteristic

Note: If VCO has a positive tuning curve similiar to trace

(1), set FC = “H,” otherwise if the VCO has a negative

tuning curve similar to trace (2), set FC = “L.”