ST25R3918 datasheet(28/141 Pages) STMICROELECTRONICS

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Application information

ST25R3918

28/141

DS13490 Rev 2

AGC

The AGC (automatic gain control) can reduce the gain to keep the receiver chain and input

to the digitizing stage out of saturation. The demodulation process is also less influenced by

system noise when the gain is properly adjusted.

The AGC logic starts operating when the signal rx_on is asserted to high and is reset when

it is reset to low. The state of the receiver gain is stored in the Gain reduction state register

during a high to low transition of bit rx_on. Reading this register later on gives information of

the gain setting used during the last reception.

The AGC system comprises a window comparator and an AGC ratio that can be set to 3 or

to 6. As an example, when the AGC ratio is set to 6 the window is six times larger than the

data digitalization window comparator. When the AGC function is enabled the gain is

reduced until there are no transitions on its output. Such procedure assures that the input to

digitalization window comparator is up to 6 times larger than its window.

If the AGC ratio is set to three, the input to the digitalization window comparator is set to be

up to 3 times larger than its window.

The AGC operation is controlled by the control bits agc_en, agc_m, agc_alg, and agc6_3 in

Receiver configuration register 2.

The bit agc_m defines the AGC mode when two AGC modes are available. The AGC can

operate during the complete RX process as long as the signal rx_on is high and it can be

enabled only during first eight subcarrier pulses.

There are two AGC algorithms to choose from bit agc_alg. The AGC can start either by

pre-setting (maximum digitizer window and maximum gain) or by resetting (minimum

digitizer window and maximum gain) it. The algorithm with preset is faster and therefore

recommended for protocols with short SOF (like ISO14443A at 106 kbps).

Correlator

The correlators correlate the incoming filtered subcarrier with 848 kHz. The aim of the

correlation is to maximize the system sensitivity at 848 kHz, while rejecting other

frequencies. There are two correlators in the system for AM (or I) channel and PM (or Q)

channel.

Correlator settings are defined in Correlator configuration register 1 and Correlator

configuration register 2.

RSSI

The receiver also performs the RSSI (received signal strength indicator) measurement for

both channels. The RSSI measurement is started after the rising edge of rx_on. It stays

active while the signal rx_on is high and frozen while rx_on is low. It is a peak hold system

where the value can only increase from initial 0 value. Every time the AGC reduces the gain

the RSSI measurement is reset and starts from 0. The result of RSSI measurements is a

4-bit value that can be observed by reading the RSSI display register. The LSB step is

2.8 dB, the maximum value is Dh (13d).

Since the RSSI measurement is of peak hold type, the result does not follow any variations

in the signal strength (the highest value will be kept). To follow RSSI variation it is possible

to reset RSSI bits and restart the measurement by sending direct command Clear RSSI.

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