AS3933 datasheet(35/64 Pages) AMSCO

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[v1-08] 2015-Sep-02

AS3933 − Detailed Description

The AGC starts working after the frequency detection. At the

beginning the gain in the VGA is set to maximum and the AGC

reduce it according to the received signal input level. The AGC

needs maximum 35 carrier periods to settle, getting a stable

RSSI.

The AGC can operate in two modes:

• AGC down only (R1<5>=0)

• AGC up and down (R1<5>=1)

If the AGC down only mode is selected, the AGC can only

decrease the gain for the whole duration of the data reception;

in this mode the system holds the RSSI peak.

When the AGC up and down mode is selected, the RSSI can

dynamically follow the input signal strength variation in both

directions.

The RSSI is available for all 3 channels at the same time and it

is stored in 3 registers (R10<4:0>, R11<4:0>, R12<4:0>). Once

the RSSI gets stable (maximum after 35 carrier periods after

frequency detection) the channel selector checks which

channel receives the strongest signal. The channel selector

compares the RSSI on the active channels and freezes the AGC

on the channels which have the smaller RSSI. From this time on

the AGC is active only on the selected channel. It is possible to

set things back having the AGC active on all channels just

sending a clear_wake (sets the chip back to listening mode) or

reset_RSSI (resets the ACG) direct command.

Both AGC modes (only down or down and up) can also operate

with time limitation. This option allows AGC operation only in

time slot of 256μs after the frequency detection (during carrier

burst), then the RSSI is frozen till the wake-up or RSSI reset

occurs (clear_wakeup or reset_RSSI).

The RSSI is reset either with the direct command 'clear_wakeup'

or 'reset_RSSI'. The 'reset_RSSI' command resets only the VGA

setting but does not terminate wake-up frequency detection

condition. This means that if the signal is still present the new

AGC setting (RSSI) will appear not later than 35 LF carrier

periods after the command was received. The AGC setting is

reset during data receiving if for duration of 3 Manchester half

symbols no carrier is detected. If the wake-up IRQ is cleared the

chip will go back to listening mode.

In case the maximum amplification at the beginning is a

drawback (e.g. in noisy environment) it is possible to set a

smaller starting gain on the amplifier, according to the

Figure 39. In this way it is possible to reduce the false frequency

detection.

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