Single-Chip Low Power RF Transceiver for Narrowband Systems ...

12.5. RSSI
CC1021 has a built-in RSSI (Received
Signal Strength Indicator) giving a digital
value that can be read form the RSSI
register. The RSSI reading must be offset
and adjusted for VGA gain setting
(VGA_SETTING[4:0] in the VGA3
register).
The digital RSSI value is ranging from 0 to
106 (7 bits).
The RSSI reading is a logarithmic
measure of the average voltage amplitude
after the digital filter in the digital part of
the IF chain:
RSSI = 4 log2(signal amplitude)
The relative power is then given by RSSI x
1.5 dB in a logarithmic scale.
The number of samples used to calculate
the average signal amplitude is controlled
by AGC_AVG[1:0] in the VGA2 register.
The RSSI update rate is given by:
f
filter _ clock
RSSI = AGC _ AVG
2
f
[ 1:
0]
+ 1
where AGC_AVG[1:0] is set in the VGA2
register and filt = 2⋅
ChBW .
f er _ clock
Maximum VGA gain is programmed by the
VGA_SETTING[4:0] bits. The VGA gain is
programmed in approximately 3 dB/LSB.
The RSSI measurement can be referred to
the power (absolute value) at the RF_IN
pin
by using the following equation:
CC1021
P = 1.5·RSSI - 3·VGA_SETTING -
RSSI_Offset [dBm]
The RSSI_Offset depends on the channel
filter bandwidth used due to different VGA
settings. Figure 14 and Figure 15 show
typical plots of RSSI reading as a function
of input power for different channel filter
bandwidths. Refer to Application Note
AN030 CC1020/1021 RSSI for further
details.
The following method can be used to
calculate the power P in dBm from the
RSSI readout values in Figure 14 and
Figure 15:
P = 1.5·[RSSI – RSSI_ref] + P_ref
where P is the output power in dBm for the
current RSSI readout value. RSSI_ref is
the RSSI readout value taken from
Figure 14 or Figure 15 for an input power
level of P_ref. Note that the RSSI readings
in decimal value changes for different
channel filter bandwidths.
The analog filter has a finite dynamic
range and is the reason why the RSSI
reading is saturated at lower channel filter
bandwidths. Higher channel filter
bandwidths are typically used for high
frequency deviation and data rates. The
analog filter bandwidth is about 160 kHz
and is bypassed for high frequency
deviation and data rates and is the reason
why the RSSI reading is not saturated for
153.6 kHz and 307.2 kHz channel filter
bandwidths in Figure 14 and Figure 15.
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