EVB72015
433MHz FSK/ASK Transmitter
Evaluation Board Description
1 Theory of Operation
1.1
General
As depicted in Fig.1, the TH72015 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 (CP). An inter-
nal loop filter determines the dynamic behavior of the PLL and suppresses reference spurious signals. A
Colpitts crystal oscillator (XOSC) is used as the reference oscillator of a phase-locked loop (PLL) synthe-
sizer. The VCO’s output signal feeds the power amplifier (PA). The RF signal power P
out
can be adjusted in
four steps from P
out
= –12 dBm to +10 dBm, either by changing the value of resistor RPS or by varying the
voltage V
PS
at pin PSEL. The open-collector output (OUT) can be used either to directly drive a loop antenna
or to be matched to a 50Ohm load. Bandgap biasing ensures stable operation of the IC at a power supply
range of 1.95 V to 5.5 V.
1.2
Block Diagram
RPS
VCC
PSEL
ASKDTA
10
ENTX
6
1
8
5
PLL
mode
control
32
PA
OUT
antenna
matching
network
ROI
4
XTAL
FSKSW
CX2
CX1
3
2
FSKDTA
PFD
XOSC
XBUF
CP
VCO
low
voltage
detector
7
VEE
9
VEE
Fig. 1:
Block diagram with external components
2 Functional Description
2.1
Crystal Oscillator
A Colpitts crystal oscillator with integrated functional capacitors is used as the reference oscillator for the PLL
synthesizer. The equivalent input capacitance CRO offered by the crystal oscillator input pin ROI is about
18pF. The crystal oscillator is provided with an amplitude control loop in order to have a very stable fre-
quency over the specified supply voltage and temperature range in combination with a short start-up time.
39012 72015 01
Rev. 007
Page 3 of 14
EVB Description
June/07
MELEXIS [ Melexis Microelectronic Systems ]
