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MAS9081A1UC06 Datasheet(PDF) 3 Page - Micro Analog systems |
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MAS9081A1UC06 Datasheet(HTML) 3 Page - Micro Analog systems |
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3 / 12 page  3 (12) DA9081.006 1 August, 2005 FREQUENCY SELECTION The frequency selection and power down control is accomplished via two digital control pins PDN1 and PDN2. The control logic is presented in table 1. Table 1 Frequency selection and power down control PDN1 PDN2 RFI2 Switch Selected Crystal Output Description High High Open - Power down High Low Open QO1 Frequency 1 Low High Closed QO2 Frequency 2, RFI2 capacitor connected in parallel with antenna Low Low Closed QO2 Frequency 2, RFI2 capacitor connected in parallel with antenna The internal antenna tuning capacitor switch (RFI2) and crystal filter output switches (QO1, QO2) are controlled according table 1. See switch in block diagram on page 1. If frequency 1 is selected the RFI2 switch is open and only crystal output QO1 is active. Antenna frequency is determined by antenna inductor LANT (see Typical Application on page 5), antenna capacitor CANT1 and parasitic capacitances related to antenna inputs RFI1, RFI2 and RFI3 (see Antenna Tuning Considerations below). Frequency 1 is the higher frequency of two selected frequencies. If frequency 2 is selected then RFI2 switch is closed to connect CANT2 in parallel with ferrite antenna and tune it to frequency 2. Then only crystal output QO2 is active. Frequency 2 is lower frequency of the two selected frequencies. It is recommended to switch the device to power down for 50ms before switching to another frequency. This guarantees fast startup in switching to another frequency. The 50ms power down period is used to discharge AGC capacitor and to initialize fast startup conditions. ANTENNA TUNING CONSIDERATIONS The ferrite bar antenna having inductance LANT and parasitic coil capacitance CCOIL is tuned to two reception frequencies f1 and f2 by parallel capacitors CANT1 and CANT2. The receiver input stage and internal antenna capacitor switch have capacitances (CRFI1, COFF2) which affect the resonance frequencies. COFF2 is switch capacitance when switch is open. When the switch is closed this capacitance is shorted by on resistance of the switch and is effectively eliminated. Following relationships can be written into two tuning frequencies. Frequency f1 (higher frequency): CTOT1=CCOIL+CANT1+CRFI1+COFF2=CCOIL+CANT1+6.5pF+37pF=CCOIL+CANT1+ 43.5pF, 1 1 2 1 TOT ANT C L f ⋅ = π Frequency f2 (lower frequency): CTOT2=CCOIL+CANT1+CANT2+CRFI1=CCOIL+CANT1+CANT2+ 6.5pF, 2 2 2 1 TOT ANT C L f ⋅ = π |
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