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ATA5812 Datasheet(PDF) 9 Page - ATMEL Corporation |
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ATA5812 Datasheet(HTML) 9 Page - ATMEL Corporation |
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9 / 90 page  9 ATA5811/ATA5812 [Preliminary] 4689B–RKE–04/04 RF Transceiver According to Figure 3 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 processing part with mixer, IF filter, IF amplifier, 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, 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 digital output signal of the demodulator are available at the pins RSSI and DEM_OUT. The demodulated data signal Demod_Out is fed to the digital control logic where it is evalu- ated and buffered as described in section “Digital Control Logic”. In transmit mode the fractional-N frequency synthesizer generates the TX frequency which is fed to 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 ±16 kHz (see Table 12 on page 24 for exact values). The transmit data can also be buffered as described in section “Digital Control Logic”. A lock detector within the synthesizer ensures that the transmission will only start if the synthesizer is locked. 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. Transparent modes without buffering of RX and TX data are also available to allow pro- tocols and coding schemes other than the internal supported Manchester encoding. 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 manu- facture an automotive key fob without the use of SAW blocking filter (see Figure 4 on page 6). 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 5 on page 7. At 433.92 MHz the receiver has a typical system noise figure of 7.0 dB, a system I1dBCP of -30 dBm and a system IIP3 of -20 dBm. There is no AGC or switching of the LNA needed, thus, a better blocking performance is achieved. This receiver uses an IF (Intermediate Frequency) of 226 kHz, the typical image rejection is 30 dB and the typical 3 dB IF filter bandwidth is 185 kHz (f IF = 226 kHz ±92.5 kHz, flo_IF = 133.5 kHz and f hi_IF = 318.5 kHz). The demodulator needs a signal to Gaussian noise ratio of 8 dB for 20 kBaud Manchester with ±16 kHz frequency deviation in FSK mode, thus, the result- ing sensitivity at 433.92 MHz is typically -106 dBm at 20 kBaud Manchester. 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 but without external compo- nents and without numerous spurious receiving frequencies. A low-IF architecture is also less sensitive to second-order intermodulation (IIP2) than direct conversion receivers where every pulse or AM-modulated signal (especially the signals from TDMA systems like GSM) demodulates to the receiving signal band at sec- ond-order non-linearities. Note: 1. -120 dBC/Hz at ±1 MHz and -75 dBC at ±FREF at 433.92 MHz |
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