filter _ clock
RSSI
AGC _ AVG[1:0] 1
f
f
2
+
=
SWRS046H – NOVEMBER 2006 – REVISED MARCH 2015
5.9.5
RSSI
CC1020 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 as shown in
.
RSSI = 4 log
2
(signal amplitude)
(18)
The relative power is then given by RSSI × 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
.
(19)
Where:
AGC_AVG[1:0] is set in the VGA2 register.
f
filter_clock
= 2 × ChBW.
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
P = 1.5 × RSSI – 3 × VGA_SETTING – RSSI_Offset [dBm]
(20)
The RSSI_Offset depends on the channel filter bandwidth used due to different VGA settings.
and
show typical plots of RSSI reading as a function of input power for different channel
spacings. See
for a list of channel filter bandwidths corresponding to the various channel
spacings. Refer to
AN030 CC1020/1021 Received Signal Strength Indicator
(
) for further
details.
The method shown in
can be used to calculate the power (P) in dBm from the RSSI readout
values in
and
P = 1.5 × [RSSI – RSSI_ref] + P_ref
(21)
Where:
P is the output power in dBm for the current RSSI readout value.
RSSI_ref is the RSSI readout value taken from
or
for an input power level of
P_ref.
NOTE
The RSSI reading 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 spacings. Higher channel spacing is 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 200 kHz and 500 kHz channel spacing in
and
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Detailed Description
31
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