ST25R3911B datasheet(10/116 Pages) STMICROELECTRONICS

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AGC, Squelch and RSSI

As mentioned above, the second and third gain stage gain and the digitizing stage window are included in the

AGC and Squelch loops. Eleven settings are available. The default state features minimum digitizer window and

maximum gain. The first four steps increase the digitizer window in 3 dB steps, the next six steps additionally

reduce the gain in the second and third gain stage, again in 3 dB steps. The initial setting with whom Squelch and

AGC start is defined in Receiver configuration register 4. The Gain reduction state register displays the actual

state of gain that results from Squelch, AGC and initial settings in Receiver configuration register 4. During bit

anticollision like Type A, the AGC should be disabled.

Squelch

This feature is designed for operation of the receiver in noisy conditions. The noise can come from tags (caused

by the processing of reader commands), or it can come from a noisy environment. This noise may be

misinterpreted as start of transponder response, resulting in decoding errors.

During execution of the Squelch procedure the output of the digitizing comparator is observed. In case there are

more than two transitions on this output in a 50 μs time period, the receiver gain is reduced by 3 dB, and the

output is observed during the next 50 μs. This procedure is repeated until the number of transitions in 50 μs is

lower or equal to two, or until the maximum gain reduction is reached. This gain reduction can be cleared sending

the direct command Reset Rx Gain.

There are two possibilities of performing squelch: automatic mode and using the direct command Squelch.

Automatic mode is enabled in case bit sqm_dyn in the Receiver configuration register 2 is set. It is activated

automatically 18.88 μs after end of Tx and is terminated when the Mask Receive timer expires. This mode is

primarily intended to suppress noise generated by tag processing during the time when a tag response is not

expected (covered by Mask Receive timer).

Command Squelch is accepted in case it is sent when signal rx_on is low. It can be used when the time

window in which noise is present is known by the controller.

AGC

AGC (Automatic Gain Control) is used to reduce gain to keep the receiver chain out of saturation. With gain

properly adjusted the demodulation process is also less influenced by system noise.

AGC action starts when signal rx_on is asserted high and is reset when it is reset to low. At the high to low

transitions of the rx_on signal the state of the receiver gain is stored in the Gain reduction state register. Reading

this register at a later stage gives information on the gain setting used during last reception.

When AGC is switched on the receiver gain is reduced so that the input to the digitizer stage is not saturated. The

AGC system comprises a comparator with a window 3.5 times larger than that of the digitizing window

comparator. When the AGC function is enabled the gain is reduced until there are no transitions on the output of

its window comparator. This procedure ensures that the input to the digitizing window comparator is less than 3.5

times larger than its threshold.

AGC operation is controlled by the control bits agc_en, agc_m and agc_fast in the Receiver configuration register

2. Bit agc_en enables the AGC operation, bit agc_m defines the AGC mode, and bit agc_alg defines the AGC

algorithm.

Two AGC modes are available. The AGC can operate during the complete Rx process (as long as signal rx_on is

high), or it can be enabled only during the first eight sub-carrier pulses.

Two AGC algorithms are available. The AGC can either start by presetting code 4h (max digitizer window, max

gain) or by resetting the code to 0h (min digitizer window, max gain).

The algorithm with preset code is faster, therefore it is recommended for protocols with short SOF (like

ISO14443A fc/128).

Default AGC settings are:

•

AGC is enabled

•

AGC operates during complete Rx process

•

algorithm with preset is used.

ST25R3911B

Application information

DS11793 - Rev 6

page 10/116

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