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ATA5823-PLQW Datasheet(PDF) 10 Page - ATMEL Corporation |
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ATA5823-PLQW Datasheet(HTML) 10 Page - ATMEL Corporation |
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10 / 96 page  10 4829D–RKE–06/06 ATA5823/ATA5824 7. RF Transceiver in Half-duplex Mode According to Figure 2-2 on page 5, the RF transceiver consists of an LNA (Low-Noise Amplifier), PA (Power Amplifier), RX/TX switch, fractional-N frequency synthesizer and the signal process- ing part with mixer, IF filter, IF amplifier with analog RSSI, FSK/ASK demodulator, data filter and data slicer. In receive mode the LNA pre-amplifies the received signal which is converted down to 226 kHz intermediate frequency (IF), filtered and amplified before it is fed into an FSK/ASK demodulator, data filter and data slicer. The RSSI (Received Signal Strength Indicator) signal and the raw dig- ital output signal of the demodulator are available at the pins RSSI and on TEST3 (open drain output). The demodulated data signal Demod_Out is fed into the digital control logic where it is evaluated and buffered as described in section “Digital Control Logic” on page 35. In transmit mode the fractional-N frequency synthesizer generates the TX frequency which is fed into the PA. In ASK mode the PA is modulated by the signal PA_Enable. In FSK mode the PA is enabled and the signal TX_DATA (FSK) modulates the fractional-N frequency synthesizer. The frequency deviation is digitally controlled and internally fixed to about ±19.5 kHz (see Table 9-1 on page 30 for exact values). The transmit data can also be buffered as described in section “Digital Control Logic” on page 35. A lock detector within the synthesizer ensures that the trans- mission will only start if the synthesizer is locked. In half-duplex mode the RX/TX switch can be used to combine the LNA input and the PA output to a single antenna with a minimum of losses. In full-duplex mode more isolation between receive and transmit antenna is needed, therefore two antennas have to be used. Transparent modes without buffering of RX and TX data are also available to allow protocols and coding schemes other than the internal supported Manchester encoding, like PWM and pulse position coding. 7.1 Low-IF Receiver The receive path consists of a fully integrated low-IF receiver. It fulfills the sensitivity, blocking, selectivity, supply voltage and supply current specification needed to manufacture an automo- tive key fob for RKE and PEG systems without the use of a SAW blocking filter (see Figure 3-1 on page 6 and Figure 5-1 on page 8). The receiver can be connected to the roof antenna in the car when using an additional blocking SAW front-end filter as shown in Figure 4-1 on page 7. At 433.92 MHz the receiver has a typical system noise figure of 6.5 dB, a system I1dBCP of – 30 dBm and a system IIP3 of –20 dBm. The signal path is linear for disturbers up to the I1dBCP and there is hence no AGC or switching of the LNA needed to achieve a better blocking perfor- mance. This receiver uses an IF of about 226 kHz (see table “Electrical Characteristics” number 2.10 for exact values), the typical image rejection is 30 dB and the typical 3 dB system band- width is 220 kHz (f IF =2 2 6 k H z ± 1 1 0k H z , f l o_I F = 1 16 kHz an d f hi _I F = 3 36 kHz). The demodulator needs a signal to noise ratio of 8 dB for 20 Kbit/s Manchester with ±19.5 kHz fre- quency deviation in FSK mode, thus, the resulting sensitivity at 433.92 MHz is typically – 105.5 dBm. Due to the low phase noise and spurious of the synthesizer in receive mode (1) together with the eighth order integrated IF filter the receiver has a better selectivity and blocking performance than more complex double superhet receivers, without using external components and without numerous spurious receiving frequencies. Note: 1. –120 dBC/Hz at ±1 MHz and –72 dBC at ±fXTO at 433.92 MHz |
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