Rainbow Electronics ATA5746 - Manual (Page 10)

10
4596A–RKE–05/06
ATA5745/ATA5746 [Preliminary]
As can be seen in Figure 2-5 on page 9 , the supply voltage has almost no influence. The tem-
perature has an influence of about ±1.0 dB, and a frequency offset of ±160 kHz also influences
by about ±1 dB. All these influences, combined with the sensitivity of a typical IC (–105 dB), are
then within a range of –103.0 dBm and –107.0 dBm over temperature, supply voltage, and fre-
quency offset. The integrated IF filter has an additional production tolerance of ±10 kHz, hence,
a frequency offset between the receiver and the transmitter of ±160 kHz can be accepted for
XTAL and XTO tolerances.
Note: For the demodulator used in the ATA5745/ATA5746, the tolerable frequency offset does not
change with the data frequency. Hence, the value of ±160 kHz is valid for 1 Kbit/s to 10 Kbits/s.
This small sensitivity change over supply voltage, frequency offset, and temperature is very
unusual in such a receiver. It is achieved by an internal, very fast, and automatic frequency cor-
rection in the FSK demodulator after the IF filter, which leads to a higher system margin. This
frequency correction tracks the input frequency very quickly. If, however, the input frequency
makes a larger step (for example, if the system changes between different communication part-
ners), the receiver has to be restarted. This can be done by switching back to Standby mode
and then again to Active mode (pin RX 1
→
0
→
1) or by generating a positive pulse on pin
ASK_NFSK (0
→
1
→
0).
2.4 Frequency Accuracy of the Crystals in a Combined RKE and TPM System
In a tire pressure measurement system working at 315 MHz and using an ATA5756 as transmit-
ter and an ATA5746 is receiver, the higher frequency tolerances and the tolerance of the
frequency deviation of the transmitter has to be considered.
In the TPM transmitter, the crystal has a frequency error over temperature –40°C to 125°C,
aging, and tolerance of ±80 ppm (±25.2 kHz at 315 MHz). The tolerances of the XTO, the
capacitors used for FSK modulation, and the stray capacitances cause an additional frequency
error of ±30 ppm (±9.45 kHz at 315 MHz). The frequency deviation of such a transmitter varies
between ±16 kHz and ±24 kHz, since a higher frequency deviation is equivalent to a frequency
error this has to be considered as an additional ±24 kHz – ±19.5 kHz = ±4.5kHz frequency toler-
ance (19.5 kHz is constant). All tolerances added, these transmitters have a worst-case
frequency offset of ±39.15 kHz.
For the receiver in the car, a tolerance of ±160 kHz – ±39.15 kHz = ±120.85 kHz (±383.6 ppm)
remains. The needed frequency stability of the crystals over temperature and aging is
±383.6 ppm – ±5 ppm = ±378.6 ppm. The aging of such a crystal is ±10 ppm, leaving a reason-
able ±368.6 ppm for the temperature dependency of the crystal frequency in the car.
Since the receiver in the car is able to receive these TPM transmitter signals with high frequency
offsets, the component specification in the key can be largely relaxed.
This system calculation is based on worst-case tolerances of all the components; this leads in
practice to a system with margin.
For a 433.92 MHz TPM system using ATA5757 as transmitter and ATA5745 as receiver, the
same calculation must be done, but since the RF frequency is higher, every ppm of crystal toler-
ances results in higher frequency offset and either the system must have lower tolerances or a
lower margin at this frequency.