U3741BM Datasheet, PDF (4/25 Pages) ATMEL Corporation – UHF ASK RECEIVER IC

U3741BM
V_S
DVCC
C_L
XTO
LFGND
LF
R1 = 820 W
C9 = 4.7 nF
C10 = 1 nF
LFVCC ^VS
R1 _C10
C9
Figure 4. PLL peripherals
The passive loop filter connected to Pin LF is designed for
a loop bandwidth of BLoop = 100 kHz. This value for
BLoop exhibits the best possible noise performance of the
LO. Figure 4 shows the appropriate loop filter
components to achieve the desired loop bandwidth. If the
filter components are changed for any reason please
notify that the maximum capacitive load at Pin LF is
limited. If the capacitive load is exceeded, a bitcheck may
no longer be possible since f_LO cannot settle in time
before the bitcheck 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 following 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 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. This relation is dependent on the
logic level at pin mode. This is described by the following
formulas:
+ + MODE
0 (USA) f_IF
f_LO
314
+ + MODE
1 (Europe) f_IF
f_LO
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 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
parameters. The parasitic board inductances and
capacitances also influence the input matching. The RF
receiver U3741BM 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 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
applications, a SAW is used.
Figure 5 shows a typical input matching network, for
f_RF = 315 MHz and f_RF = 433.92 MHz using
Figure 6 illustrates an according input matching
a SAW.
to 50 W
without a SAW. The input matching networks shown in
figure 6 are the reference networks for the parameters
given in the electrical characteristics.
Table 1. Calculation of LO and IF frequency
Conditions
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{fRF = 315 MHz, MODE = 0}
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ f_RF = 433.92 MHz, MODE = 1
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{300 MHz < fRF < 365 MHz, MODE = 0}
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{365 MHz < fRF < 450 MHz, MODE = 1}
Local Oscillator Frequency
f_LO = 314 MHz
f_LO = 432.92 MHz
+ ₎ f_LO
f_RF
1
1
314
+ ₎ f_LO
f_RF
1
1
432.92
Intermediate Frequency
f_IF = 1 MHz
f_IF = 1 MHz
+ f_LO
f_RF
314
+ f_LO
f_RF
432.92
4 (25)
Rev. A1, 15-Oct-98
Preliminary Information