ST25R3911B datasheet(7/116 Pages) STMICROELECTRONICS

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Enable transmitter, its output will ramp slowly.

Before sending any command set the bit slow_up back to 0.

1.2.3

Receiver

The receiver performs demodulation of the transponder sub-carrier modulation that is superimposed on the 13.56

MHz carrier frequency. It performs AM and/or PM demodulation, amplification, band-pass filtering and

digitalization of sub-carrier signals. Additionally it performs RSSI measurement, automatic gain control (AGC) and

Squelch.

In typical applications the receiver inputs RFI1 and RFI2 are outputs of capacitor dividers connected directly to the

terminals of the antenna coil. This concept ensures that the two input signals are in phase with the voltage on the

antenna coil. The design of the capacitive divider must ensure that the RFI1 and RFI2 input signal peak values do

not exceed the VSP_A supply voltage level.

The receiver comprises two complete receive channels, one for the AM demodulation and another one for the PM

demodulation. In case both channels are active the selection of the channel used for reception framing is done

automatically by the receive framing logic. The receiver is switched on when Operation control register bit rx_en is

set to 1. Additionally the Operation control register contains bits rx_chn and rx_man; rx_chn defines whether both,

AM and PM, demodulation channels will be active or only one of them, while bit rx_man defines the channel

selection mode in case both channels are active (automatic or manual). Operation of the receiver is controlled by

four receiver configuration registers.

The operation of the receiver is additionally controlled by the signal rx_on that is set high when a modulated

signal is expected on the receiver input. This signal is used to control RSSI and AGC and also enables

processing of the receiver output by the framing logic. Signal rx_on is automatically set to high after the Mask

Receive Timer expires. Signal rx_on can also be directly controlled by the controller by sending direct commands

Mask Receive Data and Unmask Receive Data. Figure 4 details the receiver block diagram.

Figure 4. Receiver block diagram

Peak detector

AGC

Squelch

RSSI

rec4<7:4>

AGC

Squelch

RSSI

rec4<3:0>

rec3<7:5>

rec3<4:2>

rec3<2:0>

rec1<5:3>

Digital sub-carrier

AC coupling

+ 1st gain stage

Low-pass

+ 2nd gain stage

High-pass

+ 3rd gain stage

Digitizing

stage

Demodulation

stage

Demodulator

Mixer

rec1<7:6>

rec2<6:5>

RF_IN1

RF_IN2

RSSI_AM<3:0>

RSSI_PM<3:0>

RX_on

sg_on

Demodulator

Mixer

Digital sub-carrier

Demodulation stage

The first stage performs demodulation of the transponder sub-carrier signal, superimposed on the HF field carrier.

Two different blocks are implemented for AM demodulation:

•

Peak detector

•

AM demodulator mixer.

The choice of the used demodulator is made by the Receiver configuration register 1 bit amd_sel.

ST25R3911B

Application information

DS11793 - Rev 6

page 7/116

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