Rainbow Electronics U3742BM - Manual (Page 7)

7
U3742BM
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
3
forms a series
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.
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 on page 6 to
determine the center frequency.
The receiver U3742BM - M3 employs an IF bandwidth of B
IF
= 600 kHz and can be used
together with the U2741B in FSK and ASK mode.
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
V
Th_red
. V
Th_red
is determined by the value of the external resistor R
Sense
. R
Sense
is con-
nected between pin SENS and GND or V
S
. The output of the comparator is fed into the
digital control logic. By this means it is possible to operate the receiver at a lower
sensitivity.
Pin RSSI The output voltage of the RSSI amplifier (V
RSSI
) is available at pin RSSI. Using the RSSI
output signal, the signal strength of different transmitters can be distinguished. The
usable input-power range P
Ref
is -100 dBm to -55 dBm. The temperature coefficient TC
of V
RSSI
is typically -2.2 mV/K. Due to TC and gain tolerance, it is not possible to find out
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
RSSI
.