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4899B–RKE–10/06
ATA3741
4.
Analog Signal Processing
4.1
IF Amplifier
The signals coming from the RF front end are filtered by the fully integrated 4th-order IF filter.
The IF center frequency is f
IF
= 1 MHz for applications where f
RF
= 315 MHz or f
RF
= 433.92 MHz
is used. For other RF input frequencies, refer to
to determine the center
frequency.
The ATA3741 is available with 2 different IF bandwidths. ATA3741-M2, the version with
B
IF
= 300 kHz, is well suited for ASK systems where Atmel’s PLL transmitter U2741B is used.
The receiver ATA3741-M3 employs an IF bandwidth of B
IF
= 600 kHz. This version can be used
together with the U2741B in FSK and ASK mode. If used in ASK applications, it allows higher
tolerances for the receiver and PLL transmitter crystals. SAW transmitters exhibit much higher
transmit frequency tolerances compared to PLL transmitters. Generally, it is necessary to use
B
IF
= 600 kHz together with such transmitters.
4.2
RSSI Amplifier
The subsequent RSSI amplifier enhances the output signal of the IF amplifier before it is fed into
the demodulator. The dynamic range of this amplifier is DR
RSSI
= 60 dB. If the RSSI amplifier is
operated within its linear range, the best signal-to-noise ratio (SNR) is maintained in ASK mode.
If the dynamic range is exceeded by the transmitter signal, the SNR is defined by the ratio of the
maximum RSSI output voltage and the RSSI output voltage due to a disturber. The dynamic
range of the RSSI amplifier is exceeded if the RF input signal is about 60 dB higher compared to
the RF input signal at full sensitivity.
In FSK mode, the SNR is not affected by the dynamic range of the RSSI amplifier.
The output voltage of the RSSI amplifier is internally compared to a threshold voltage V
Th_red
.
V
Th_red
is determined by the value of the external resistor R
Sense
. R
Sense
is connected between
pin SENS and GND or VS. The output of the comparator is fed into the digital control logic. This
makes it possible to operate the receiver at lower sensitivity.
If R
Sense
is connected to VS, the receiver operates at a lower sensitivity. The reduced sensitivity
is defined by the value of R
Sense
, the maximum sensitivity by the SNR of the LNA input. The
reduced sensitivity is dependent on the signal strength at the output of the RSSI amplifier.
Since different RF input networks may exhibit slightly different values for the LNA gain, the sen-
sitivity values given in the electrical characteristics refer to a specific input matching. This
matching is illustrated in
and exhibits the best possible sensitivity.
R
Sense
can be connected to VS or GND via a microcontroller or by the digital output port POUT of
the ATA3741 receiver IC. The receiver can be switched from full sensitivity to reduced sensitivity
or vice versa at any time. In polling mode, the receiver will not wake up if the RF input signal
does not exceed the selected sensitivity. If the receiver is already active, the data stream at pin
DATA will disappear when the input signal is lower than defined by the reduced sensitivity.
Instead of the data stream, the pattern shown in
is issued at pin DATA to indicate that
the receiver is still active.
Figure 4-1.
Steady L State Limited DATA Output Pattern
DATA
t
DATA_L_max
t
min2
