u3745bm ATMEL Corporation, u3745bm Datasheet - Page 6

• Manufacturer Part Number: u3745bm
• Description: Receiver
• Manufacturer: ATMEL Corporation

Table 1. Calculation of LO and IF Frequency
6
Conditions
f
f
300 MHz < f
365 MHz < f
RF
RF
= 315 MHz, MODE = 0
= 433.92 MHz, MODE = 1
U3745BM
RF
RF
< 365 MHz, MODE = 0
< 450 MHz, MODE = 1
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
not equal to 1 MHz. f
Table 1 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 U3745BM 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.
F i g u r e 3 s h o w s a t y p i c a l i n p u t m a t c h i n g n e t w o r k f o r f
f
SAW. The input matching networks shown in Figure 4 are the reference networks for the
parameters given in the section “Electrical Characteristics”.
MODE
MODE
RF
Ref
= 433.92 MHz using a SAW. Figure 4 illustrates an input matching to 50 W without a
Local Oscillator Frequency
f
f
f
f
LO
LO
LO
LO
= 40 dB can be achieved. There are SAWs available that exhibit a notch at
=
=
= 314 MHz
= 432.92 MHz
RF
0 (USA) f
0 (Europe) f
=
=
= 433.92 MHz, the MODE must be set to ‘1’. For other RF frequencies, f
-------------------
1
--------------------------- -
1
+
+
f
RF
LO
--------- -
314
----------------- -
432.92
f
RF
1
, the construction of the IF filter must be considered at this point. The
1
IF
=
IF
IF
--------- -
314
=
f
is then dependent on the logical level at Pin MODE and on f
LO
IF
IF
----------------- -
432.92
= 1 MHz. To achieve a good accuracy of the filter’s corner fre-
and f
f
LO
LO
that depends on the logic level at pin mode. This is
RF
= 315 MHz, the MODE must be set to ‘0’. In the
Intermediate Frequency
f
f
IF
IF
f
IF
f
= 1 MHz
= 1 MHz
IF
=
=
----------------- -
432.92
--------- -
314
f
f
LO
LO
XTO
. This means that there is a
R F
IF
= 3 1 5 M H z a n d
= 1 MHz for most
4663A–RKE–06/03
IF
RF
is
.