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T5761 Datasheet(PDF) 3 Page - ATMEL Corporation |
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T5761 Datasheet(HTML) 3 Page - ATMEL Corporation |
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3 / 32 page  T5761 T5760 / Preliminary Information Rev. A2, 19-Oct-00 3 (32) Block Diagram SENS CDEM AVCC AGND DGND LNAGND LNA_IN DATA POLLING/_ON DATA_CLK DVCC XTAL Polling circuit and control logic Rssi Limiter out LC–VCO f :256 XTO Standby logic FE CLK FSK/ASK– demodulator and data filter RSSI IF Amp. Poly–LPF fg=7MHz LNA 4. Order f0=950 kHz/ Dem_out Sensitivity– reduction LPF fg=2.2MHz IF Amp. IC_ACTIVE Data – interface LNAREF f :2 Loop– filter 1 MHz Figure 3. Block diagram RF Front End The RF front end of the receiver is a low-IF heterodyne configuration that converts the input signal into a 950-kHz/ 1-MHz IF signal with an image rejection of typ- ical 30dB. According to figure 3 the front end consists of an LNA (low noise amplifier), LO (local oscillator), I/Q mixer, polyphase lowpass filter and an IF amplifier. The PLL generates the carrier frequency for the mixer via a full integrated synthesizer with integrated low noise LC-VCO (voltage controlled oscillator ) and PLL-loop- filter. The XTO ( crystal oscillator ) generates the reference frequency fXTO. The integrated LC-VCO gen- erates two times the mixer drive frequency fVCO. The I/Q signals for the mixer are generated with a divide by two circuit ( fLO = fVCO/2 ). fVCO is divided by a factor of 256 and feed into a phase frequency detector and compared with fXTO. The output of the phase frequency detector is feed into an integrated loopfilter and thereby generates the control voltage for the VCO. If fLO is determined, fXTO can be calculated using the following formula: fXTO = fLO / 128 The XTO is a one-pin oscillator that operates at the series resonance of the quartz crystal with high current but low voltage signal, so that there is only a small voltage at the crystal oscillator frequency at Pin XTAL. According to figure 4, the crystal should be connected to GND with a series capacitor CL. The value of that capacitor is recom- mended by the crystal supplier. Due to a somewhat inductive impedance at steady state oscillation and some PCB parasitics a lower value of CL is normally necessary. |
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