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AS3932

Datasheet - Detailed Descr i p ti on

8.3 Channel Amplifier and Frequency Detector

Each of the 3 channels consists of a variable gain amplifier, an automatic gain control and a frequency detector. The latter detects the presence

of a carrier. As soon as the carrier is detected the AGC is enabled, the gain of the VGA is reduced and set to the right value and the RSSI can be

displayed.

It is possible to enable/disable individual channels, in case not all three channels are needed. This enables to reduce the current consumption by

1.5 µA (typ.) per channel.

8.3.1 Frequency Detector / AGC

The frequency detection uses the RTC as time base. In case the internal RC oscillator is used as RTC, it must be calibrated, but the calibration

is guaranteed for a 32.768 kHz crystal oscillator only. The frequency detection criteria can be tighter or more relaxed according to the setup

described in R2<1:0> (see Table 14).

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 15. In this way it is possible to reduce the false frequency detection.

Table 14. Tolerance Settings for Wake-up

R2<1>

R2<0>

Tolerance

00

relaxed

0

1

tighter (medium)

1

0

stringent

1

1

reserved

Table 15. 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