Rainbow Electronics U3741BM - Manual (Page 7)

7
U3741BM
4662B–RKE–10/04
Analog Signal Processing
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 Table 1 to determine
the center frequency.
The U3741BM is available with 2 different IF bandwidths. U3741BM-M2, the version
with B
IF
= 300 kHz, is well suited for ASK systems where Atmel’s PLL transmitter
U2741B is used. The receiver U3741BM-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 crys-
tals. 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.
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 S/N ratio is maintained in ASK
mode. If the dynamic range is exceeded by the transmitter signal, the S/N ratio 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 S/N ratio 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
VTh_red. VTh_red is determined by the value of the external resistor R
Sense
. R
Sense
is
connected between pin Sense and GND or VS. The output of the comparator is fed into
the digital control logic. By this means it is 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 sig-
nal-to-noise ratio 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 sensitivity values given in the electrical characteristics refer to a specific input
matching. This matching is illustrated in Figure 4 on page 6 and exhibits the best possi-
ble sensitivity.
R
Sense
can be connected to VS or GND via a microcontroller or by the digital output port
POUT of the U3741BM 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 accord-
ing to Figure 5 is issued at pin DATA to indicate that the receiver is still active.
Figure 5. Steady L State Limited DATA Output Pattern
DATA
t
DATA_L_max
t
min2