TH72012 Datasheet, PDF (4/20 Pages) Melexis Microelectronic Systems – 433MHz ASK Transmitter

TH72012
433MHz
ASK Transmitter
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.
2.2 ASK Modulation
The PLL transmitter can be ASK-modulated by
applying a data stream directly at the pin
ASKDTA. This turns the internal current
sources of the power amplifier on and off and
therefore leads to an ASK signal at the output.
ASKDTA
0
1
Description
Power amplifier is turned off
Power amplifier is turned on (according
to the selected output power step)
2.3 Crystal Pulling
A crystal is tuned by the manufacturer to the
f
required oscillation frequency f₀ at a given load
capacitance CL and within the specified calibra-
tion tolerance. The only way to pull the oscilla-
tion frequency is to vary the effective load ca-
pacitance CL_eff seen by the crystal.
Figure 2 shows the oscillation frequency of a
crystal as a function of the effective load ca-
pacitance. This figure also illustrates the rela-
tionship between the external pulling capacitor _fc
and the center frequency.
It can be seen that the pulling sensitivity in-
creases with the reduction of CL. For high-
accuracy ASK applications, a higher load ca-
pacitance should be chosen in order to reduce
the frequency drift caused by the tolerances of
the chip and the external pulling capacitor.
XTAL
L1
C1
C0
R1
CL _eff
CX1 CRO
CL= _CX1+CRO
CL _eff
Fig. 2: Crystal pulling characteristic
39010 72012
Rev. 009
Page 4 of 20
Data Sheet
July/06