TLF5111

June 1990

DS8908B AMFM Digital Phase-Locked

Loop Frequency Synthesizer

General Description

The DS8908B is a PLL synthesizer designed specifically for

use in AMFM radios It contains the reference oscillator a

phase comparator a charge pump an operational amplifier

and a 19-bit shift registerlatch for serial data entry The

device is designed to operate with a serial data controller

generating the necesary division codes for each frequency

and logic state information for radio function inputsoutputs

A 396 MHz pierce oscillator and divider chain generate a

198 MHz external controller clock a 20 kHz 10 kHz 9 kHz

and a 1 kHz reference signals and a 50 Hz time-of-day

signal The oscillator and divider chain are sourced by the

and time-of-day indication when the balance of the PLL is

powered down

The 21-bit serial data steram is transferred between the fre-

quency synthesizer and the controller via a 3-wire bus sys-

tem comprised of a data line a clock line and an enable

line

The first 2 bits in the serial data stream address the synthe-

sizer thus permitting other devices such as display drivers to

share the same bus The next 14 bits are used for the

select the AM or FM local oscillator input A high level on

this bit enables the FM input and a low level enables the AM

input The 16th and 17th bits are used to select one of the 4

reference frequencies The 18th and 19th bits are connect-

ed via latches to open collector outputs These outputs can

be used to drive radio functions such as gain mute AM

FM or charge pump current source levels

scaler a high speed charge pump and an operational am-

the number loaded into the shift register The programmable

prescaler by the AM input

prescaler by the FM input The AM input

will work at frequencies up to 15 MHz while the FM input

works up to 120 MHz The VCO can be tuned with a fre-

quency resolution of either 1 kHz 9 kHz 10 kHz or 20 kHz

The buffered AM and FM inputs are self-biased and can be

driven directly by the VCO through a capacitor The ECL

phase comparator produces very accurate resolution of the

phase difference between the input signal and the reference

oscillator The high speed charge pump consists of a

switchable constant current source and sink The charge

pump can be programmed to deliver from 75 mA to 750 mA

of constant current by connection of an external resistor

puts Connection of programming resistors to the bit outputs

enables the controller to adjust the loop gain for the particu-

lar reference frequency selected The charge pump will

source current if the VCO frequency is high and sink

current if the VCO frequency is low The low noise opera-

tional amplifier provided has a high impedance JFET input

and a large output voltage range The op amp’s negative

input is common with the charge pump output and its posi-

tive input is internally biased

Features

Y

Uses inexpensive 396 MHz reference crystal

Y

thesis at FM frequencies

Y

FM resolution of either 10 kHz or 20 kHz allows usage

of 107 MHz ceramic filter distribution

Y

Serial data entry for simplified control

Y

50 Hz output for time-of-day reference driven from sep-

Y

2 open collector buffered outputs for controlling various

radio functions or loop gain

Y

Separate AM and FM inputs AM input has 15 mV (typi-

cal) hysteresis

Y

Programmable charge pump current sources enable ad-

justment of system loop gain

Y

Operational amplifier provides high impedance load to

charge pump output and a wide voltage range for the

VCO input

Connection Diagram

Dual-In-Line Package

TLF5111 – 1

Top View

Order Number DS8908BN

See NS Package Number N20A

TRI-STATE

is a registered trademark of National Semiconductor Corp

C1995 National Semiconductor Corporation

RRD-B30M105Printed in U S A