TLF7398

April 1990

DS8911DS8913 AMFMTV Sound

Up-Conversion Frequency Synthesizer

General Description

The DS8911 is a digital Phase-Locked Loop (PLL) frequen-

cy synthesizer intended for use as a Local Oscillator (LO) in

electronically tuned radios The device is used in conjunc-

tion with a serial data controller a loop filter some varactor

diodes and several passive elements to provide the local

oscillator function for both AM and FM tuning

The conventional superheterodyne AM receiver utilizes a

low IF or down conversion tuning approach whereby the IF

is chosen to be below the frequencies to be received The

DS8911 PLL on the other hand utilizes an up-conversion

technique in the AM mode whereby the first IF frequency is

chosen to be well above the RF frequency range to be

tuned This approach eliminates the need for tuned circuits

in the AM frontend since the image half IF and other spuri-

ous responses occur far beyond the range of frequencies to

be tuned Sufficient selectivity and second IF image protec-

tion is provided by a crystal filter at the output of the first

mixer

A significant cost savings can be realized utilizing this up-

conversion approach to tuning Removal of the AM tuned

circuits eliminates the cost of expensive matched varactor

diodes and reduces the amount of labor required for align-

ment down from 6 adjustments to 2 Additional cost savings

are realized because up-conversion enables both the AM

and FM bands to be tuned using a single Voltage Controlled

Oscillator (VCO) operating between 98 and 120 MHz (The

2 to 1 LO tuning range found in conventional AM down con-

version radios is reduced to a 10% tuning range 994 MHz

to 1102 MHz)

Up-conversion AM tuning is accomplished by first dividing

the VCO signal down by a modulus 10 to obtain the LO

signal This LO in turn is mixed on chip with the RF signal to

obtain a first IF at the MIXER output pins This first IF after

crystal filtering is mixed (externally) with a reference fre-

quency provided by the PLL to obtain a 450 kHz second IF

frequency The DS8911 derives the 450 kHz second IF by

mixing an 1155 MHz first IF with a 1200 MHz reference

frequency

FM and WB (weather band) tuning is done using the con-

ventional down conversion approach Here the VCO signal

is buffered to produce the LO signal and then mixed on chip

with the RF signal to obtain an IF frequency at the MIXER

output pins This IF frequency is typically chosen to be 107

MHz although placement at 1150 MHz can further enhance

AM mode performance and minimize IF circuitry

The PLL provides phase comparator reference frequencies

of 10 125 25 and 100 kHz The tuning resolutions result-

ing from these reference frequencies are determined by di-

viding the reference by the premix modulus Table II shows

the tuning resolutions possible

The DS8911 contains the following logic elements a volt-

age controlled oscillator a reference oscillator a 14-bit pro-

grammable dual-modulus counter a reference frequency di-

vider chain a premix divider a mixer a phase comparator a

charge pump an operational amplifier and control circuitry

for latched serial data entry

The DS8913 includes all the above logic elements except

that it requires a 10 MHz reference frequency instead of

12 MHz

Features

Y

Direct synthesis of LW MW SW FM and WB

frequencies

Y

Serial data entry for simplified processor control

Y

10 125 25 and 100 kHz reference frequencies

Y

8 possible tuning resolutions (see Table II)

Y

An op amp with high impedance inputs for loop filtering

Y

Programmable mixer with high dynamic range

TRI-STATE

is a registered trademark of National Semiconductor Corporation

C1995 National Semiconductor Corporation

RRD-B30M105Printed in U S A