U3742BM Datasheet, PDF (5/32 Pages) ATMEL Corporation – UHF ASK/FSK RECEIVER

4735A–RKE–11/03
U3742BM
The passive loop filter connected to pin LF is designed for a loop bandwidth of
B_Loop = 100 kHz. This value for B_Loop exhibits the best possible noise performance of the
LO. Figure 4 on page 4 shows the appropriate loop filter components to achieve the
desired loop bandwidth. If the filter components are changed for any reason, please
note that the maximum capacitive load at pin LF is limited. If the capacitive load is
exceeded, a bit check may no longer be possible since f_LO cannot settle in time before
the bit check starts to evaluate the incoming data stream. Self-polling does therefore
also not work in that case.
f_LO is determined by the RF input frequency f_RF and the IF frequency f_IF using the follow-
ing formula:
f_LO = f_RF - f_IF
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 fre-
quencies, 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 formulas:
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
applications. For applications where f_RF = 315 MHz, MODE must be set to '0'. In the
case of f_RF = 433.92 MHz, 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
1 on page 6 summarizes 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 parame-
ters. The parasitic board inductances and capacitances also influence the input
matching. The RF receiver U3742BM exhibits its highest sensitivity at the best signal-to-
noise ratio in the LNA. Hence, noise matching is the best choice for designing the trans-
formation 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
DP_Ref = 40 dB can be achieved. There are SAWs available that exhibit a notch at
Df = 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 appli-
cations, a SAW is used.
Figure 5 on page 6 shows a typical input matching network for f_RF = 315 MHz and
f_RF = 433.92 MHz using a SAW. Figure 6 on page 6 illustrates input matching to 50 W
without a SAW. The input matching networks shown in Figure 6 on page 6 are the refer-
ence networks for the parameters given in the electrical characteristics.
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