u3742bm ATMEL Corporation, u3742bm Datasheet - Page 5

• Manufacturer Part Number: u3742bm
• Description: Uhf Ask/fsk Receiver
• Manufacturer: ATMEL Corporation

4735A–RKE–11/03
The passive loop filter connected to pin LF is designed for a loop bandwidth of
B
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
the bit check starts to evaluate the incoming data stream. Self-polling does therefore
also not work in that case.
f
ing formula:
f
To determine f
nominal IF frequency is f
quencies, the filter is tuned by the crystal frequency f
fixed relation between f
described by the following formulas:
The relation is designed to achieve the nominal IF frequency of f
applications. For applications where f
case of f
equal to 1 MHz. f
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
D P
D 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 appli-
cations, a SAW is used.
Figure 5 on page 6 shows a typical input matching network for f
f
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.
MODE
MODE
LO
LO
RF
Loop
Ref
is determined by the RF input frequency f
= f
= 433.92 MHz using a SAW. Figure 6 on page 6 illustrates input matching to 50 W
= 100 kHz. This value for B
RF
= 40 dB can be achieved. There are SAWs available that exhibit a notch at
=
=
- f
RF
0 (USA) f
1 (Europe) f
IF
= 433.92 MHz, MODE must be set to '1'. For other RF frequencies, f
LO
, the construction of the IF filter must be considered at this point. The
IF
IF
is then dependent on the logical level at pin MODE and on f
IF
=
--------- -
314
=
f
LO
IF
IF
----------------- -
432.92
and f
= 1 MHz. To achieve a good accuracy of the filter's corner fre-
f
LO
LO
Loop
, that depends on the logic level at pin MODE. This is
exhibits the best possible noise performance of the
RF
= 315 MHz, MODE must be set to '0'. In the
RF
and the IF frequency f
XTO
LO
. This means that there is a
cannot settle in time before
IF
RF
U3742BM
IF
= 1 MHz for most
= 315 MHz and
using the follow-
RF
IF
. Table
is not
5