ASK Transmitter / LO Source
The TH71071 ASK 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 TH71071 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 = –12 dBm to +2 dBm either by
changing the value of resistor R1 or by varying the
voltage VPS at pin PS/DATA. 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.
The TH71071 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 TH71071 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
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