U3742BM Datasheet, PDF (8/32 Pages) ATMEL Corporation – UHF ASK/FSK RECEIVER

Figure 7. Temperature Compensation of V_RSSI
I ~ Ig(V_{LNA_IN})
RSSI
50k
I
V _RSSI
V _{RSSI_temp_comp.}
180k
B _min = 60
47k
U3742BM
Figure 8. RSSI Characteristic
1.6
1.5
1.4
1.3
max
1.2
1.1
1.0
-40°C
0.9
25°C
0.8
min
0.7
105°C
0.6
0.5
-110
-100
-90
-80
-70
-60
-50
P_Ref (dBm)
If R_Sense is connected to V_S, the receiver operates at a lower sensitivity. The reduced
sensitivity is defined by the value of R_Sense, the maximum sensitivity by the signal-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 6 on page 6 and exhibits the best possi-
ble sensitivity.
R_Sense can be connected to V_S or GND via a microcontroller. 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 according to Figure 9 on page 9 is issued at pin DATA to indi-
cate that the receiver is still active.
8 U3742BM
4735A–RKE–11/03