3

Transmitter Theory of Operation

Introduction

RFM’s TX-series hybrid transmitters are specifically designed for

short-range wireless data communication applications. These trans-

mitters provide robust operation, very small size, low power con-

sumption and low implementation cost. All critical RF functions are

contained in the hybrid, simplifying and speeding design-in. The

transmitters can be readily configured to support a wide range of

data rates and protocol requirements. TX-series transmitters feature

excellent suppression of output harmonics and virtually no other RF

emissions, making them easy to certify to short- range (unlicensed)

radio regulations.

Transmitter Block Diagram

Figure 1 is the general block diagram of the transmitter. Please refer

to Figure 1 for the following discussions.

Antenna Port

The only external RF components needed for the transmitter are the

antenna and its matching components. Antennas presenting an im-

pedance in the range of 35 to 72 ohms resistive can be satisfactorily

matched to the RFIO pin with a series matching coil and a shunt

matching/ESD protection coil. Other antenna impedances can be

matched using two or three components. For some impedances,

two inductors and a capacitor will be required. A DC path from RFIO

to ground is required for ESD protection.

Transmitter Chain

The transmitter chain consists of a SAW coupled-resonator oscilla-

tor followed by a modulated buffer amplifier. The SAW coupled res-

onator output filter suppresses transmitter harmonics to the

antenna.

Transmitter operation supports two modulation formats, on-off

keyed (OOK) modulation, and amplitude-shift keyed (ASK) modula-

tion. When OOK modulation is chosen, the transmitter output turns

completely off between “1” data pulses. When ASK modulation is

chosen, a “1” pulse is represented by a higher transmitted power

level, and a “0” is represented by a lower transmitted power level.

OOK modulation provides compatibility with first-generation ASH

technology, and provides for power conservation. ASK modulation

must be used for high data rates (data pulses less than 200 µs).

ASK modulation also reduces the effects of some types of interfer-

ence and allows the transmitted pulses to be shaped to control mod-

ulation bandwidth.

The modulation format is chosen by the state of the CNTRL0 and

the CNTRL1 mode control pins, as discussed below. In the OOK

mode, the oscillator amplifier TXA1 and buffer amplifier TXA2 are

turned off when the voltage to the TXMOD input falls below 220 mV.

In the OOK mode, the data rate is limited by the 20/15 µs turn-on

and turn-off time of the oscillator. In the ASK mode TXA1 is biased

ON continuously, and the output of TXA2 is modulated by the

TXMOD input current. Minimum output power occurs in the ASK

mode when the modulation driver sinks about 10 µA of current from

the TXMOD pin.

T r an sm i t t e r B l o ck D i ag r a m

T XA1

T XA2

A n t enna

SAW

Co u p l e d

R e s onat o r

SAW

CR

F ilt e r

M odul at i o n

& Bi a s C o n t r o l

TX

IN

CN

TR L 1

CN

TR L 0

T une/ E S D

An t

T une

Figure 1