ST25R3910 datasheet(13/65 Pages) STMICROELECTRONICS

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ST25R3910

Functional overview

Gain reduction, AGC and Squelch

The total gain of receiver chain is 160. In certain conditions it is desirable to reduce this gain.

There are several features implemented in the Receiver to reduce this gain.

Automatic Gain Reduction (AGC)

The automatic gain control feature is useful in case the tag is close to the reader. In such

conditions the receiver chain is in saturation and demodulation can be influenced by system

noise and saturation of last gain stage. When AGC is switched on receiver gain is reduced

so that the input to digitizer stage is not saturated. The AGC system comprises a window

comparator with a window three times larger than the one of the digitalization window

comparator. When the AGC function is enabled the gain is reduced until there are no

transitions on its output. Such procedure ensures that the input to digitalization window

comparator is less than three times larger than its window.

AGC operation is controlled by the Receiver Configuration Register bits agc_en and agc_m.

Agc_en bit enables AGC operation, agc_m defines AGC operating mode. The AGC action

is started 20 μs after the rising edge of signal rx_on. In case agc_m bit is 0 it will operate

during a complete receive period, in case it is 1 it will operate on the first 8 subcarrier pulses.

The AGC is reducing gain by 21 dB in 7 steps of 3 dB. When signal rx_on is low AGC is in

reset.

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 tag 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 Clear Squelch.

During execution of the direct command Squelch the digital output of receiver (output of

window comparator mentioned above) is observed. In case there are more than two

transitions on this output in 50 μs time period, the gain is reduced by 3 dB and output is

observed during 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 (21 dB) is reached. This

setting is cleared with direct command Clear Squelch.

Setting gain reduction

By setting bits rg2 to rg0 in Receiver Configuration Register receiver gain can also be

reduced in seven steps of 3 dB.

Actual gain reduction is combination of all three gain reduction features mentioned above

(AGC, Squelch and setting gain reduction in Receiver Configuration Register). Actual gain

reduction state can also be observed by reading the Receiver State Display Register bits

gr_2 to gr_0.

ST25R3910 Datasheet(PDF) 13 Page - STMicroelectronics

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