EVB7108
868/915MHz Transmitter
Evaluation Board Description
390120710801 2 of 8 EVB Description
Rev. 007 Aug/02
Theory of Operation
General
As depicted in Fig.1, the TH7108 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 P
o
can be adjusted in six
steps from P
o
= –15 dBm to +1 dBm either by
changing the value of resistor R
PS
or by varying the
voltage V
PS
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.
FSK Modulation
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 C
X1
and C
X2
allow the FSK deviation and
center frequency to be adjusted independently. At
DATA = LOW C
X2
is connected in parallel to C
X1
leading to the low-frequency component of the FSK
spectrum (f
min
); while at DATA = HIGH C
X2
is
deactivated and the XOSC is set to its high
frequency, leading to f
max
.
An external reference signal can be directly AC-
coupled to pin RO1. Then the TH7108 is used
without an XTAL. The reference signal has to
contain the FSK (or FM) and sets the carrier
frequency.
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.
ASK Modulation
The TH7108 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 1/8 of the reference crystal frequency.
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 approx. 2 V
pp
at C
load
= 5 pF.
ENCK ENTX Mode Description
0 0 all OFF whole circuit in standby
0 1 TX only TX functionality only, no
clock available
10clock
only
TX in standby and clock
available
1 1 all ON TX functional and clock
available
Table 1: Modes of operation
For more detailed information, please refer to the latest TH7108 data sheet revision.
