Rainbow Electronics ATA5746 - Manual (Page 5)

5
4596A–RKE–05/06
ATA5745/ATA5746 [Preliminary]
2. RF Receiver
As seen in Figure 1-3 on page 4 , the RF receiver consists of a low-noise amplifier (LNA), a local
oscillator, and the signal processing 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 a
440-kHz intermediate frequency (IF), then filtered and amplified before it is fed into an FSK/ASK
demodulator, data filter, and data slicer. The received signal strength indicator (RSSI) signal is
available at the pin RSSI.
2.1 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 design an automotive inte-
grated receiver for RKE and TPM systems. A benefit of the integrated receive filter is that no
external components needed.
At 315 MHz, the ATA5745 receiver (433.92 MHz for the ATA5746 receiver) has a typical system
noise figure of 6.0 dB (7.0 dB), a system I1dBCP of –31 dBm (–30 dBm), and a system IIP3 of
–24 dBm (–23 dBm). The signal path is linear for out-of-band disturbers up to the I1dBCP and
hence there is no AGC or switching of the LNA needed, and a better blocking performance is
achieved. This receiver uses an IF (intermediate frequency) of 440 kHz, the typical image rejec-
tion is 30 dB and the typical 3-dB IF filter bandwidth is 420 kHz (f
IF
= 440 kHz ± 210 kHz,
f
lo_IF
= 230 kHz and f
hi_IF
= 650 kHz). The demodulator needs a signal-to-noise ratio of 8.5 dB for
10 Kbits/s Manchester with ±38 kHz frequency deviation in FSK mode, thus, the resulting sensi-
tivity at 315 MHz (433.92 MHz) is typically –105 dBm (–104 dBm).
Due to the low phase noise and spurs of the synthesizer together with the 8th-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 receiv-
ing frequencies.
A low-IF architecture is also less sensitive to second-order intermodulation (IIP2) than direct
conversion receivers where every pulse or amplitude modulated signal (especially the signals
from TDMA systems like GSM) demodulates to the receiving signal band at second-order
non-linearities.