CC1021-RTR1 datasheet(35/91 Pages) TI1

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RSSI Level

Zero level depends on front-end settings and VGA_SETTING

value.

CS_LEVEL+8

VGA_UP

VGA_DOWN+3

(signal strength, 1.5dB/step)

Carrier sense is turned on here.

AGC increases gain if below this

level (unless at maximum).

AGC decreases gain if above

this level (unless at minimum).

To set CS_LEVEL, subtract 8 from RSSI readout with RF

input signal at desired carrier sense level.

Note that the AGC works with "raw" filter output signal

strength, while the RSSI readout value is compensated for

VGA gain changes by the AGC.

The AGC keeps the signal strength in this range. Minimize

VGA_DOWN for best selectivity, but leave some margin to

avoid frequent VGA gain changes during reception.

The AGC keeps the signal strength above carrier sense level

+ VGA_UP. Minimize VGA_UP for best selectivity, but

increase if first VGA gain reduction occurs too close to the

noise floor.

Not Recommended for New Designs NRND

CC1021

www.ti.com

SWRS045D – JANUARY 2006 – REVISED JANUARY 2015

Figure 5-17. Relationship Between RSSI, Carrier Sense Level,

and AGC Settings CS_LEVEL, VGA_UP and VGA_DOWN

5.9.9

AGC Settling

After turning on the RX chain, the following occurs:

A. The AGC waits 16-128 ADC_CLK (1.2288 MHz) periods, depending on the VGA_FREEZE setting in

the VGA1 register, for settling in the analog parts.

B. The AGC waits 16-48 FILTER_CLK periods, depending on the VGA_WAIT setting in the VGA1

C. The AGC calculates the RSSI value as the average magnitude over the next 2-16 FILTER_CLK

periods, depending of the AGC_AVG setting in the VGA2 register.

D. If the RSSI value is higher than CS_LEVEL+8, then the carrier sense indicator is set (if CS_SET = 0).

If the RSSI value is too high according to the CS_LEVEL, VGA_UP and VGA_DOWN settings, and the

VGA gain is not already at minimum, then the VGA gain is reduced and the AGC continues from B).

E. If the RSSI value is too low according to the CS_LEVEL and VGA_UP settings, and the VGA gain is

not already at maximum (given by VGA_SETTING), then the VGA gain is increased and the AGC

continues from B).

Two to three VGA gain changes should be expected before the AGC has settled. Increasing AGC_AVG

increases the settling time, but may be worthwhile if there is the time in the protocol, and for reducing false

wake-up events when setting the carrier sense close to the noise floor.

The AGC settling time depends on the FILTER_CLK (= 2 × ChBW). Thus, there is a trade off between

AGC settling time and receiver sensitivity because the AGC settling time can be reduced for data rates

lower than 76.8 kBaud by using a wider receiver channel filter bandwidth (that is, larger ChBW).

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