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TH72001 Datasheet(PDF) 4 Page - Melexis Microelectronic Systems |
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TH72001 Datasheet(HTML) 4 Page - Melexis Microelectronic Systems |
4 / 20 page TH72001 315MHz FSK Transmitter 3901072001 Page 4 of 20 Data Sheet Rev. 007 June/07 f min fc f fmax eff CL eff CL R1 C1 C0 L1 XTAL CL CX1 CRO CX1+CRO (CX1+CX2) CRO CX1+CX2+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 FSK Modulation FSK modulation can be achieved by pulling the crystal oscillator frequency. A CMOS- compatible data stream applied at the pin FSKDTA digitally modulates the XOSC via an integrated NMOS switch. Two external pulling capacitors CX1 and CX2 allow the FSK devia- tion Δf and the center frequency f c to be ad- justed independently. At FSKDTA = 0, CX2 is connected in parallel to CX1 leading to the low- frequency component of the FSK spectrum (fmin); while at FSKDTA = 1, CX2 is deactivated and the XOSC is set to its high frequency fmax. An external reference signal can be directly AC- coupled to the reference oscillator input pin ROI. Then the transmitter is used without a crystal. Now the reference signal sets the car- rier frequency and may also contain the FSK (or FM) modulation. Fig. 2: Crystal pulling circuitry FSKDTA Description 0 fmin= fc - Δf (FSK switch is closed) 1 fmax= fc + Δf (FSK switch is open) 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 3 shows the oscillation frequency of a crystal as a function of the effective load ca- pacitance. This capacitance changes in accor- dance with the logic level of FSKDTA around the specified load capacitance. The figure illus- trates the relationship between the external pulling capacitors and the frequency deviation. It can also be seen that the pulling sensitivity increases with the reduction of CL. Therefore, applications with a high frequency deviation require a low load capacitance. For narrow band FSK applications, a higher load capaci- tance could be chosen in order to reduce the frequency drift caused by the tolerances of the chip and the external pulling capacitors. Fig. 3: Crystal pulling characteristic CX2 VCC XTAL CX1 ROI FSKSW VEE |
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