TH7108

868/915MHz

FSK/FM/ASK Transmitter

3901007108

Page 4 of 14

Data Sheet

Rev. 008

May/03

2

Functional Description

2.1

FSK Modulation

A Colpitts crystal oscillator (XOSC) is used as the reference oscillator of a phase-locked loop (PLL) synthe-

sizer. FSK modulation is achieved by pulling the crystal (XTAL) through the data. So a CMOS-compatible

data stream applied at input DATA digitally modulates the XOSC. Two external pulling capacitors CX1 and

CX2 allow the FSK deviation and center frequency to be adjusted independently. At DATA = LOW CX2 is

An external reference signal can be directly AC-coupled to pin RO1. Then the TH7108 is used without an

XTAL. The reference signal has to contain the FSK (or FM) and sets the carrier frequency.

2.2

Frequency Modulation

For FM operation an external varactor is required. It simply acts as a pulling capacitor connected in series to

the crystal. Then the analog modulation signal, applied through a series resistor, directly modulates the

XOSC.

2.3

ASK Modulation

The TH7108 can be ASK-modulated by applying data directly at pin PS. This turns the PA on and off and

therefore leads to an ASK signal at the output.

2.4

Mode Control Logic

The mode control logic allows four different modes of operation as listed in the following table. The mode

control pins ENCK and ENTX are pulled-down internally. This guarantees that the whole circuit is shut down

if these pins are left floating.

The clock output CKOUT can be used to drive a µC. This output can be activated by the ENCK pin as re-

quired for any specific application. Clock frequency is 1/8 of the reference crystal frequency.

ENCK

ENTX

Mode

Description

0

0

all OFF

whole circuit in standby

0

1

TX only

TX active, no clock available

1

0

clock only

TX standby and clock available

1

1

all ON

TX active and clock available