ST25R3911B datasheet(20/129 Pages) STMICROELECTRONICS

20 / 129 page

Functional overview

ST25R3911B

20/129

DocID029656 Rev 2

settings, 3 dB steps, adjustment range from ±33 mV to ±120 mV). Adjustment of the

digitizing window is included in the AGC and Squelch loops. In addition, the digitizing

window can also be set manually in the Receiver Configuration Register 4.

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

ST25R3911B Datasheet(PDF) 20 Page - STMicroelectronics

ST25R3911B Datasheet(HTML) 20 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL