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Revision 1.2

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AS3932

The AGC can operate in two modes:

AGC down only (R1<5>=0)

AGC up and down (R1<5>=1)

As soon as the AGC starts to operate, the gain in the VGA is set to maximum. If the AGC down only mode is selected, the AGC can only

decrease the gain. Since the RSSI is directly derived from the VGA gain, the system holds the RSSI peak.

When the AGC up and down mode is selected, the RSSI can follow the input signal strength variation in both directions.

Regardless which AGC operation mode is used, the AGC needs maximum 35 carrier periods to settle.

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>)

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

following the internal wake-up. Then the AGC (RSSI) is frozen till the wake-up or RSSI reset occurs.

The RSSI is reset either with the direct command 'clear_wakeup' or 'reset_RSSI'. The 'reset_RSSI' command resets only the AGC setting but

does not terminate wake-up condition. This means that if the signal is still present the new AGC setting (RSSI) will appear not later than 300µs

(35 LF carrier periods) after the command was received. The AGC setting is reset 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 Table 14. In this way it is possible to reduce the false frequency detection.

8.4.2 Antenna Damper

The antenna damper allows the chip to deal with higher field strength, it is enabled by register R1<4>. It consists of shunt resistors which

degrade the quality factor of the resonator by reducing the signal at the input of the amplifier. In this way the resonator sees a smaller parallel

resistance (in the band of interest) which degrades its quality factor in order to increase the linear range of the channel amplifier (the amplifier

doesn't saturate in presence of bigger signals). Table 15 shows the bit setup.

Table 14. Bit Setting of Gain Reduction

R4<3>

R4<2>

R4<1>

R4<0>

Gain reduction

0

0

0

0

no gain reduction

00

0

1

n.a.

0

0

1

0 or 1

n.a.

0

1

0

0 or 1

-4dB

0

1

1

0 or 1

-8dB

1

0

0

0 or 1

-12dB

1

0

1

0 or 1

-16dB

1

1

0

0 or 1

-20dB

1

1

1

0 or 1

-24dB

Table 15. Antenna Damper Bit Setup

R4<5>

R4<4>

Shunt resistor (parallel to the resonator at 125 kHz)

00

1 k

Ω

01

3 k

Ω

10

9 k

Ω

11

27 k

Ω