Atmel ATA3741 Series, Operating

Page: 5 / 31

5
4899B–RKE–10/06
ATA3741
f
LO
is determined by the RF input frequency f
RF
and the IF frequency f
IF
using the following for-
mula:
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 formulas:
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, 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 3-1 summarizes the dif-
ferent 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 LNA_IN is specified in
“Electrical Characteristics” on page
23 . The parasitic board inductances and capacitances also influence the input matching. The RF
receiver ATA3741 exhibits its highest sensitivity at the best signal-to-noise ratio 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
R F
= 315 MHz and
f
RF
= 433.92 MHz using a SAW. Figure 3-3 on page 6 illustrates an input matching to 50
Ω
with-
out a SAW. The input matching networks shown in Figure 3-3 are the reference networks for the
parameters given in the “Electrical Characteristics” on page 23 .
f
LO
f
RF
f
IF
– =
MODE 0 (USA) f
IF
f
LO
314
---------- = =
MODE 1 (Europe) f
IF
f
LO
432.92
------------------ = =
Table 3-1. Calculation of LO and IF Frequency
Conditions Local Oscillator Frequency Intermediate Frequency
f
RF
= 315 MHz, MODE = 0 f
LO
= 314 MHz f
IF
= 1 MHz
f
RF
= 433.92 MHz, MODE = 1 f
LO
= 432.92 MHz f
IF
= 1 MHz
300 MHz < f
RF
< 365 MHz, MODE = 0
365 MHz < f
RF
< 450 MHz, MODE = 1
f
LO
f
RF
1
1
314
---------- +
------------------- =
f
IF
f
LO
314
---------- =
f
LO
f
RF
1
1
432.92
------------------ +
---------------------------- =
f
IF
f
LO
432.92
------------------ =