ST25R3918 datasheet(16/141 Pages) STMICROELECTRONICS

16 / 141 page

Description

ST25R3918

16/141

DS13490 Rev 2

2.2.2

Receiver

The receiver detects card modulation superimposed on the 13.56 MHz carrier signal. The

receiver consists of two receive chains that are built from a set of demodulators, followed by

two gain and filtering stages and a final digitizer stage. The demodulators can operate as

AM/PM demodulator or as I/Q demodulator. The filter characteristics can be adjusted to

match the selected RF mode and bit rate to optimize performance (subcarrier frequencies

from 212 to 848 kHz are supported). Apart from the filter stage the receiver incorporates

several other features (AGC, squelch) that enable reliable operation in noisy conditions.

The receiver is connected to the antenna via the pins RFI1 and RFI2. The output of the

receiver is connected to the framing block that decodes the demodulated and digitized

subcarrier signal.

2.2.3

Phase and amplitude detector

The phase detector measures the phase difference between the transmitter output signals

(RFO1 and RFO2) and the receiver input signals (RFI1 and RFI2).

The amplitude detector measures the amplitude of the differential RF carrier signal between

the receiver inputs RFI1 and RFI2. This differential amplitude signal is directly proportional

to the amplitude of the RF signal on the antenna LC tank.

The phase- and amplitude detectors are used for several purposes:



PM demodulation, by observing RFI1 and RFI2 phase variations (LF signal is fed to the

receiver)



Average phase difference between RFOx pins and RFIx pins, to check antenna tuning



Measure amplitude of signal present on pins RFI1 and RFI2, proportional to the

antenna voltage

2.2.4

A/D converter

The ST25R3918 features a built in A/D converter. Its input can be multiplexed from different

sources and it is used for the diagnostic functions and the low power card detection. The

result of the A/D conversion is stored in a register that can be read through the host

interface.

2.2.5

External field detector

This is a low power block used in the passive target mode to detect the presence of an

external RF field. It supports two different external field detection thresholds, namely Peer

detection and Collision avoidance threshold.

The Peer detection threshold is used in the passive peer to peer modes to detect when the

peer device turns on its RF field.

The Collision avoidance threshold is used to detect the presence of an external RF field

during the RF collision avoidance procedure.

2.2.6

Quartz crystal oscillator

The quartz crystal oscillator operates with 27.12 MHz crystals. At start-up the

transconductance of the oscillator is increased to achieve a fast start-up. Since the start-up

time varies with crystal type, temperature and other parameters, the oscillator amplitude is

observed and an interrupt is generated when stable oscillator operation is reached.

ST25R3918 Datasheet(PDF) 16 Page - STMicroelectronics

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