u3742bm ATMEL Corporation, u3742bm Datasheet - Page 7

• Manufacturer Part Number: u3742bm
• Description: Uhf Ask/fsk Receiver
• Manufacturer: ATMEL Corporation

Analog Signal
Processing
IF Amplifier
RSSI Amplifier
Pin RSSI
4735A–RKE–11/03
Please note that for all coupling conditions (see Figure 5 on page 6 and Figure 6 on
page 6), the bond wire inductivity of the LNA ground is compensated. C
resonance circuit together with the bond wire. L = 25 nH is a feed inductor to establish a
DC path. Its value is not critical but must be large enough not to detune the series reso-
nance circuit. For cost reduction, this inductor can be easily printed on the PCB. This
configuration improves the sensitivity of the receiver by about 1 dB to 2 dB.
The signals coming from the RF front end are filtered by the fully integrated 4th-order IF
filter. The IF center frequency is f
f
determine the center frequency.
The receiver U3742BM - M3 employs an IF bandwidth of B
together with the U2741B in FSK and ASK mode.
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 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
V
nected between pin SENS and GND or V
digital control logic. By this means it is possible to operate the receiver at a lower
sensitivity.
The output voltage of the RSSI amplifier (V
output signal, the signal strength of different transmitters can be distinguished. The
usable input-power range P
of V
the absolute level of each transmitter, but the level differences can be used to distin-
guish several transmitters. As illustrated in Figure 8 on page 8, the RSSI output voltage
is not constant over the temperature range. Figure 7 on page 8 illustrates an application
that realizes a temperature compensation of V
RF
Th_red
= 433.92 MHz is used. For other RF input frequencies, refer to Table 1 on page 6 to
RSSI
. V
is typically -2.2 mV/K. Due to TC and gain tolerance, it is not possible to find out
Th_red
is determined by the value of the external resistor R
Ref
is -100 dBm to -55 dBm. The temperature coefficient TC
IF
= 1 MHz for applications where f
S
. The output of the comparator is fed into the
RSSI
RSSI
) is available at pin RSSI. Using the RSSI
.
IF
= 600 kHz and can be used
Sense
RSSI
U3742BM
RF
3
. R
= 60 dB. If the
forms a series
= 315 MHz or
Sense
is con-
7