ATA3745_07 Datasheet, PDF (5/30 Pages) ATMEL Corporation – UHF ASK/FSK Receiver

4901B–RKE–11/07
ATA3745
To determine f_LO, the construction of the IF filter must be considered at this point. The nominal
IF frequency is f_IF = 1 MHz. To achieve a good accuracy of the filter’s corner frequencies, the
filter is tuned by the crystal frequency f_XTO. This means that there is a fixed relation between f_IF
and f_LO that depends on the logic level at pin MODE. This is described by the following formu-
las:
MODE
=
0 (USA) f_IF
=
-^f--^L---^O---
314
MODE
=
1 (Europe) f_IF
=
------^f--^L--^O-------
432.92
The relation is designed to achieve the nominal IF frequency of f_IF = 1 MHz for most applica-
tions. For applications where f_RF = 315 MHz, the MODE must be set to “0”. In the case of
f_RF = 433.92 MHz, the MODE must be set to “1”. For other RF frequencies, f_IF is not equal to
1 MHz. f_IF is then dependent on the logical level at pin MODE and on f_RF. Table 3-1 summa-
rizes the different conditions.
The RF input either from an antenna or from a generator must be transformed to the RF input
pin LNA_IN. The input impedance of that pin is provided in the electrical parameters. The par-
asitic board inductances and capacitances also influence the input matching. The RF receiver
ATA3745 exhibits its highest sensitivity at the best signal-to-noise ratio (SNR) in the LNA.
Hence, noise matching is the best choice for designing the transformation network.
A good practice when designing the network is to start with power matching. From that starting
point, the values of the components can be varied to some extent to achieve the best
sensitivity.
If a SAW is implemented into the input network, a mirror frequency suppression of
∆P_Ref = 40 dB can be achieved. There are SAWs available that exhibit a notch at ∆f = 2 MHz.
These SAWs work best for an intermediate frequency of IF = 1 MHz. The selectivity of the
receiver is also improved by using a SAW. In typical automotive applications, a SAW is used.
Figure 3-2 on page 6 shows a typical input matching network for f_RF = 315 MHz and
f_RF = 433.92 MHz using a SAW. Figure 3-3 on page 6 illustrates an input matching to 50Ω
without a SAW. The input matching networks shown in Figure 3-3 on page 6 are the reference
networks for the parameters given in the section “Electrical Characteristics” on page 23.
Table 3-1. Calculation of LO and IF Frequency
Conditions
Local Oscillator
Frequency
f_RF = 315 MHz, MODE = 0
f_RF = 433.92 MHz, MODE = 1
f_LO = 314 MHz
f_LO = 432.92 MHz
300 MHz < f_RF < 365 MHz, MODE =
0
f_LO
=
------^f--^R----^F-------
1 + ----¹-----
314
365 MHz < f_RF < 450 MHz, MODE =
1
f_LO
=
-----------^f-^R----^F-----------
1 + --------¹---------
432.92
Intermediate Frequency
f_IF = 1 MHz
f_IF = 1 MHz
f_IF
=
-^f--^L---^O---
314
f_IF
=
------^f--^L--^O-------
432.92
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