TH72032_12 Datasheet, PDF (4/20 Pages) Melexis Microelectronic Systems – ASK Transmitter

TH72032
868/915MHz
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
frequency 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
required oscillation frequency f₀ at a given load
f
capacitance CL and within the specified
calibration tolerance. The only way to pull the
oscillation frequency is to vary the effective load
capacitance CL_eff seen by the crystal.
Figure 2 shows the oscillation frequency of a
crystal as a function of the effective load
capacitance. This figure also illustrates the
relationship between the external pulling
capacitor and the center frequency.
f_c
It can be seen that the pulling sensitivity
increases with the reduction of CL. For high-
accuracy ASK applications, a higher load
capacitance 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
3901072032
Rev. 011
Page 4 of 20
Data Sheet
Jun/12