5.9.9 AGC Settling

After turning on the RX chain, the following occurs:

1. 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.
2. 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.
3. 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.
4. 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).
5. 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).