CC1021-RTR1 datasheet(33/92 Pages) TI

33 / 92 page

CC1021

SWRS045

Page 33 of 91

12.5. RSSI

CC1021 has a built-in RSSI (Received

Signal Strength Indicator) giving a digital

value that can be read form the RSSI

and adjusted for VGA gain setting

(VGA_SETTING[4:0]

the

VGA3

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:

[] 1

AVG

AGC

clock

filter

RSSI

where AGC_AVG[1:0] is set in the VGA2

ChBW

clock

filter

⋅

= 2

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:

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 ñ 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.

CC1021-RTR1 Datasheet(PDF) 33 Page - Texas Instruments

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