Transmitter Chain
Transmitter Theory of Operation
Introduction
The transmitter chain consists of a SAW coupled-resonator oscillator fol-
lowed by a modulated buffer amplifier. The SAW coupled resonator output
filter suppresses transmitter harmonics to the antenna.
RFM’s TX-series hybrid transmitters are specifically designed for short-
range wireless data communication applications. These transmitters pro-
vide robust operation, very small size, low power consumption and low im-
plementation cost. All critical RF functions are contained in the hybrid,
simplifying and speeding design-in. The transmitters can be readily con-
figured 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 operation supports two modulation formats, on-off keyed
(OOK) modulation, and amplitude-shift keyed (ASK) modulation. When
OOK modulation is chosen, the transmitter output turns completely off be-
tween “1” data pulses. When ASK modulation is chosen, a “1” pulse is rep-
resented 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 inter-
ference and allows the transmitted pulses to be shaped to control modula-
tion bandwidth.
Transmitter Block Diagram
Figure 1 is the general block diagram of the transmitter. Please refer to Fig-
ure 1 for the following discussions.
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 os-
cillator 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.
Antenna Port
The only external RF components needed for the transmitter are the anten-
na and its matching components. Antennas presenting an impedance 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 com-
ponents. For some impedances, two inductors and a capacitor will be re-
quired. A DC path from RFIO to ground is required for ESD protection.
Transmitter Block Diagram
Antenna
SAW
Coupled
Resonator
Ant
Tune
SAW
CR
TXA1
TXA2
Filter
Tune/ESD
Modulation
&BiasControl
RTXM
TX
IN
CN
TRL1
CN
TRL0
Figure 1
RF Monolithics, Inc.
RFM Europe
Phone: (972) 233-2903
Phone: 44 1963 251383
Fax: (972) 387-8148
Fax: 44 1963 251510
E-mail: info@rfm.com
http://www.rfm.com
TX6004-070805
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