MELEXIS TH71081 Datasheet
Features
! Fully integrated, PLL-stabilized VCO
! Flexible frequency range from 800 MHz to 930 MHz
! ASK achieved by on/off keying of internal
power amplifier
! FM possible with external varactor
! Wide power supply range from 2.2 V to 5.5 V
! High over-all frequency accuracy
! Very low standby current
TH71081
868/915MHz
ASK Transmitter / LO Source
! Adjustable output power range from
-15 dBm to +1 dBm
! Adjustable current consumption from
6.2 mA to 12.5 mA
! Differential output well-suited for loop
antenna
! Conforms to EN 300 220 and similar
standard
Ordering Information
Part No. Temperature Range Package
TH71081 -40 C° to 85 °C SOIC8
Application Examples Pin Description
! Keyless car and central locking
! Low-power telemetry
! Alarm and security systems
! General digital data transmission
! General analog audio signal transmission
! Local oscillator signal gener ation
LF
RO
ENTX
PS/DATA
1
2
3
4
TH71081
8
7
6
5
VEE
OUT1
OUT2
VCC
3901071081 Page 1 of 10 Data Sheet
Rev. 004 Dec. 2001
General Description
TH71081
868/915MHz
ASK Transmitter / LO Source
The TH71081 ASK transmitter IC is designed for
applications in the European 868MHz industrialscientific-m edical (ISM) band, according to the EN
300 220 telecommunications standard. It can also
be used for any other system with carrier
frequencies ranging from 800 MHz to 930 MHz
(e.g. for applications in the US 915MHz ISM band).
Theory of Operation
General
As depicted in Fig.1, the TH71081 transmitter
consists of a fully integrated voltage-controlled
oscillator (VCO), a divide-by-32 divider (div32), a
phase-frequency detector (PFD) and a charge
pump. An external loop filter at pin LF determines
the dynamic behavio ur of the PLL and suppres ses
reference spurious signals.
The VCO’s output signal fee ds the power amplifier
(PA). RF signal power P
steps from P
changing the valu e of resistor R
voltage V
differential output (OUT1, OUT2) can be used to
either directly drive a loop antenna or to be
converted to a single-ended impedance by means
of a balanced-to-unbalanced (balun) transformer.
For maximum available output power, the
differential output should be matched to a load of
approx. 1 kΩ.
Bandgap biasing ensures stab le operation of the IC
at a power supply range of 2.2 V to 5.5 V.
ASK Modulation
The TH71081 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.
= –15 dBm to +1 dBm either by
o
at pin PS/DATA. The open-collector
PS
can be adjusted in six
o
or by varying the
1
The transmitter's c arrier frequency f
by the frequency of the reference crystal f
used. The integrated P LL synthesizer ensur es that
each RF value, ranging fr om 800 MHz to 9 30 MHz,
can be achieved by using a cr ystal with reference
frequency accordin g to: f
the PLL feedback divider ratio.
LO Source
Many applications require a stable RF source. For
this purpose, the TH71081 can be used without
modulation as an easy-to-use, PLL-stabilized,
continuous wave (CW) generator.
Frequency Modulation (FM)
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.
Mode Control Logic
The mode control logic allows two different modes
of operation as listed in the following table. The
mode control pin ENTX is pulled-down internally.
This guarantees that th e whole circuit is shut down
if this pin is left floating.
ENTX Mode Description
0 TX disabled whole circuit in standby
1 TX enable TX active
Table 1: Modes of operation
= fc/N, where N = 32 is
ref
is determined
c
that is
ref
3901071081 Page 2 of 10 Data Sheet
Rev. 004 Dec. 2001
Block Diagram
TH71081
868/915MHz
ASK Transmitter / LO Source
VCC
7
PA
div32
6
OUT2
PFD
XOSC
OUT1
PS/DATA 4VCC 5
VCO
ENTX
3
mode
control
charge
pump
8 VEE
1
LF
CF1
CF2
RF1
2
RO
XTAL
CX1
Fig. 1: TH71081 block diagram with external components
3901071081 Page 3 of 10 Data Sheet
Rev. 004 Dec. 2001
Loading...