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ATA5724P3-TKQY Datasheet(PDF) 5 Page - ATMEL Corporation |
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ATA5724P3-TKQY Datasheet(HTML) 5 Page - ATMEL Corporation |
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5 / 46 page  5 9106E–RKE–07/08 ATA5723/ATA5724/ATA5728 3. RF Front-end The RF front-end of the receiver is a low-IF heterodyne configuration that converts the input sig- nal into about 1 MHz IF signal with a typical image rejection of 30 dB. According to Figure Figure 1-2 on page 3 the front-end consists of an LNA (Low Noise Amplifier), LO (Local Oscillator), I/Q mixer, polyphase low-pass filter and an IF amplifier. The PLL generates the drive frequency f LO for the mixer using a fully integrated synthesizer with integrated low noise LC-VCO (Voltage Controlled Oscillator) and PLL-loop filter. The XTO (crys- tal oscillator) generates the reference frequency f REF =fXTO/2 (868 MHz and 433 MHz versions) or f REF =fXTO/3 (315 MHz version). The integrated LC-VCO generates two or four times the mixer drive frequency f VCO. The I/Q signals for the mixer are generated with a divide by two or four circuit (f LO =fVCO/2 for 868 MHz version, fLO =fVCO/4 for 433 MHz and 315 MHz versions). f VCO is divided by a factor of 128 or 64 and feeds into a phase frequency detector and is com- pared with f REF. The output of the phase frequency detector is fed into an integrated loop filter and thereby generates the control voltage for the VCO. If f LO is determined, fXTO can be calcu- lated using the following formula: f REF =fLO/128 for 868 MHz band, fREF =fLO/64 for 433 MHz bands, fREF =fLO/64 for 315 MHz bands. The XTO is a two-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 pins XTAL1 and XTAL2. According to Figure 3-1, the crystal should be connected to GND with two capacitors C L1 and CL2 from XTAL1 and XTAL2 respectively. The value of these capacitors are recommended by the crystal supplier. Due to an inductive impedance at steady state oscillation and some PCB parasitics, a lower value of C L1 and CL2 is normally necessary. The value of C Lx should be optimized for the individual board layout to achieve the exact value of f XTO and hence of fLO. (The best way is to use a crystal with known load resonance frequency to find the right value for this capacitor.) When designing the system in terms of receiving band- width and local oscillator accuracy, the accuracy of the crystal and the XTO must be considered. Figure 3-1. XTO Peripherals The nominal frequency f LO is determined by the RF input frequency fRF and the IF frequency fIF using the following formula (low-side injection): f LO = fRF – fIF XTAL2 TEST2 TEST3 XTAL1 DVCC VS CL2 CL1 |
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