ASK Transmitter / LO Source
The TH71082 ASK transmitter IC is designed for
applications in the European 868MHz industrial-
scientific-medical (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).
The transmitter's carrier frequency fc is determined
by the frequency of the reference crystal fref that is
used. The integrated PLL synthesizer ensures that
each RF value, ranging from 800 MHz to 930 MHz,
can be achieved by using a crystal with reference
frequency according to: fref = fc/N, where N = 32 is
the PLL feedback divider ratio.
Theory of Operation
As depicted in Fig.1, the TH71082 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 behaviour of the PLL and suppresses
reference spurious signals.
The VCO’s output signal feeds the power amplifier
(PA). RF signal power Po can be adjusted in six
steps from Po = –15 dBm to -1 dBm, either by
changing the value of resistor R1 or by varying the
voltage VPS at pin PS/DATA. The open-collector
output (OUT) can be used to either drive a loop
antenna or to feed a single-ended load impedance.
This could be, for example λ /4 monopol antenna
or a 50Ω output port. In any case, an impedance
matching network should be added in order to
achieve maximum available RF power.
Bandgap biasing ensures stable operation of the IC
at a power supply range of 2.2 V to 5.5 V.
The TH71082 feature a clock output (CKOUT) that
can be used to drive a µC. The frequency at
CKOUT is fref/8. The clock output is slew-rate limited
in order to keep spurious signal emission as low as
possible. The voltage swing at CKOUT depends on
the capacitive loading at this pin.
It is approximate 2 Vpp at Cload = 5 pF
The TH71082 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.
Many applications require a stable RF source. For
this purpose, the TH71082 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 the whole circuit is shut down
if this pin is left floating.
0 TX disabled whole circuit in standby
1 TX enable TX active
Table 1: Modes of operation
TH71082 Data Sheet
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