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T5744-TGQ Datasheet(PDF) 3 Page - ATMEL Corporation |
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T5744-TGQ Datasheet(HTML) 3 Page - ATMEL Corporation |
3 / 19 page 3 T5744 4521B–RKE–01/03 Figure 3. Block Diagram RF Front End The RF front end of the receiver is a heterodyne configuration that converts the input signal into a 1-MHz IF signal. According to Figure 3, the front end consists of an LNA (Low-Noise Amplifier), LO (Local Oscillator), a mixer and RF amplifier. The LO generates the carrier frequency for the mixer via a PLL synthesizer. The XTO (crystal oscillator) generates the reference frequency f XTO. The VCO (Voltage-Controlled Oscillator) generates the drive voltage frequency f LO for the mixer. fLO is dependent on the voltage at Pin LF. f LO is divided by factor 64. The divided frequency is compared to f XTO by the phase frequency detector. The current output of the phase frequency detec- tor is connected to a passive loop filter and thereby generates the control voltage VLF for the VCO. By means of that configuration, VLF is controlled in a way that f LO/64 is equal to f XTO. If fLO is determined, fXTO can be calculated using the following formula: f XTO = fLO/64 The XTO is a one-pin oscillator that operates at the series resonance of the quartz crys- tal. According to Figure 4, the crystal should be connected to GND via a capacitor CL. The value of that capacitor is recommended by the crystal supplier. The value of CL should be optimized for the individual board layout to achieve the exact value of f XTO and hereby of f LO. When designing the system in terms of receiving bandwidth, the accuracy of the crystal and the XTO must be considered. ASK- Demodulator and data filter RSSI IF Amp IF Amp 4. Order LPF 3 MHz LPF 3 MHz Dem_out RSSI Standby logic VCO XTO 64 f CDEM AVCC RSSI AGND DGND MIXVCC LNAGND LNA_IN DATA ENABLE TEST MODE LFGND LFVCC XTO LF DVCC LNA Data interface Test BR_0 BR_1 |
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