CC1020-RTR1 datasheet(38/91 Pages) TI

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CC1020

SWRS046

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frequency hopping. This means that bit

synchronization can be maintained from

hop to hop.

VGA_WAIT determines the time to hold

the present bit synchronization and RSSI

levels after changing VGA gain. This

feature is useful to avoid AGC operation

during the settling of transients after a

VGA gain change. Some transients are

expected due to DC offsets in the VGA.

At the sensitivity limit, the VGA gain is set

by VGA_SETTING. In order to optimize

selectivity, this gain should not be set

higher than necessary. The SmartRF®

Studio software gives the settings for

VGA1 – VGA4 registers. For reference,

the following method can be used to find

the AGC settings:

1. Disable AGC and use maximum LNA2 gain by

writing BFh to the VGA2 register. Set minimum

VGA gain by writing to the VGA3 register with

VGA_SETTING = 0.

2. Apply no RF input signal, and measure ADC noise

floor by reading the RSSI register.

3. Apply no RF input signal, and write VGA3 register

with increasing VGA_SETTING value until the

RSSI register value is approximately 4 larger than

the value read in step 2. This places the front-end

noise floor around 6 dB above the ADC noise floor.

4. Apply an RF signal with strength equal the desired

carrier sense threshold. The RF signal should

preferably be modulated with correct Baud rate and

deviation. Read the RSSI register value, subtract 8,

and write to CS_LEVEL in the VGA4 register. Vary

the RF signal level slightly and check that carrier

sense indication (bit 3 in STATUS register)

switches at the desired input level.

5. If desired, adjust the VGA_UP and VGA_DOWN

settings according to the explanation in Figure 18.

6. Enable AGC and select LNA2 gain change level.

Write 55h to VGA2 register if the resulting

VGA_SETTING>10. Otherwise, write 45h to VGA2.

Modify AGC_AVG in the above VGA2 value if

faster carrier sense and AGC settling is desired.

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.

Figure 18. Relationship between RSSI, carrier sense level, and AGC settings CS_LEVEL,

VGA_UP and VGA_DOWN

12.9. 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

the

VGA1

B) The AGC waits 16-48 FILTER_CLK

periods, depending on the VGA_WAIT

setting in the VGA1 register, for settling in

the analog parts and the digital channel

filter.

C) The AGC calculates the RSSI value as

the average magnitude over the next 2-16

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