The TH7107 FSK/ASK/FM transmitter IC is
designed for applications in the European 433MHz
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 310 MHz to 450 MHz
(e.g. for applications in the US 315MHz 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 310 MHz to 450 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 TH7107 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 behavior 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 = –12 dBm to +2 dBm either by
changing the value of resistor RPS or by varying the
voltage VPS at pin PS. The open-collector 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 stable operation of the IC
at a power supply range of 2.2 V to 5.5 V.
A Colpitts crystal oscillator (XOSC) is used as the
reference oscillator of a phase-locked loop (PLL)
synthesizer. 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 connected in parallel to
CX1 leading to the low-frequency component of the
FSK spectrum (fmin); while at DATA = HIGH CX2 is
deactivated and the XOSC is set to its high
frequency, leading to fmax.
An external reference signal can be directly AC-
coupled to pin RO1. Then the TH7107 is used
without an XTAL. The reference signal has to
contain the FSK (or FM) and sets the carrier
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.
The TH7107 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.
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
required for any specific application. Clock
frequency is 1/4 of the reference crystal frequency.
ENCK ENTX Mode
0 0 all OFF whole circuit in standby
0 1 TX only TX functionality only, no
1 0 clock TX in standby and clock
1 1 all ON TX functional and clock
Table 1: Modes of operation
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