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EVB72036-868-FSK-A Datasheet(PDF) 4 Page - Melexis Microelectronic Systems |
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EVB72036-868-FSK-A Datasheet(HTML) 4 Page - Melexis Microelectronic Systems |
4 / 12 page PR EL IM INA RY EVB72036 868/915MHz FSK/ASK Transmitter Evaluation Board Description 3901272036 01 Page 4 of 12 EVB Description Rev. 002 June/07 fmin fc f fmax eff CL eff CL R1 C1 C0 L1 XTAL CL CX1 CRO CX1+CRO (CX1+CX2) CRO CX1+CX2+CRO 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 fc 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 For ASK applications CX2 can be omitted. Then CX1 has to be adjusted for center frequency. CX2 VCC XTAL CX1 ROI FSKSW VEE |
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