AS3933 datasheet(33/64 Pages) AMSCO

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[v1-08] 2015-Sep-02

AS3933 − Detailed Description

Channel Amplifier and Frequency Detector

Each of the 3 channels consists of a variable gain amplifier (VGA)

with automatic gain control (AGC) and a frequency detector.

When the AS3933 is in listening mode (waiting for RF signal)

the gain of all channel amplifiers is set to maximum. The

frequency detector counts the zero crossing of the amplified RF

signal to detect the presence of the wanted 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. The RSSI (Received Signal

Strength Indicator) represents how strong the input signal is

and it is the inverse representation of the gain of the VGA. In

fact, if for example the input signal is very strong the AGC will

reduce the gain of the VGA. The gain reduction will correspond

to a big RSSI, as it is the inverse of the gain setting of the VGA

(small gain corresponds to a big RSSI and vice versa).

The AS3933 is a pretty wide LF wake-up receiver and can work

between 15 kHz and 150 kHz. Once the carrier frequency has

been chosen the user must set the amplifier working in the

appropriate frequency band using the bits R8<7:5>, as

described in the Figure 36.

It is possible to boost the gain of the amplifiers for +3dB with

an improvement of the sensitivity, as shown in the Figure 14

(R2<5>=1). The gain boost will increase the current

consumption of 100nA (typ) per channel. In case the lowest

frequency band is used (15kHz – 23 kHz) the gain boost is

automatically enabled from the logic.

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.

Frequency Detector / RSSI / Channel Selector

The frequency detection is based on a zero crossing counter

and uses the Clock Generator as time base. This counter counts

the zero crossing of the input signal within a time window

defined by the clock generator and if it matches to the expected

value it enabled the AGC (the RSSI measurement can get

started). The Clock Generator can be based either on the

internal RC-oscillator or on the Crystal oscillator or on the

external clock source. The details on the choice of the Clock

Generator are discussed in the Clock Generator. The Clock

Generator generates time windows equal to N times its period,

where N depends on the operating frequency band, as shown

in the Figure 36.

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