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TH72012KDC Datasheet(PDF) 4 Page - Melexis Microelectronic Systems |
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TH72012KDC Datasheet(HTML) 4 Page - Melexis Microelectronic Systems |
4 / 20 page TH72012 433MHz ASK Transmitter 39010 72012 Page 4 of 20 Data Sheet Rev. 009 July/06 fc f eff CL eff CL R1 C1 C0 L1 XTAL CL= CX1 CRO CX1+CRO 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 Description 0 Power amplifier is turned off 1 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 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 CLeff 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 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. Fig. 2: Crystal pulling characteristic |
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