AS3911 datasheet(23/149 Pages) AMSCO

23 / 149 page

ams Datasheet, Confidential: 2013-Oct [2-03]

AS3911 – 23

Appl icatio n I n f o rm a t i o n

Gain.

There are two possibilities of performing squelch: automatic

mode and using direct command Squelch.

• Automatic mode is started 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

with Mask Receive timer expire. This mode is primarily

intended to suppress noise generated by tag processing

during the time when the 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 in case 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

receiver chain out of saturation. In case gain is 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 low to high transitions of the rx_on

the state of the receiver gain is stored in the Gain Reduction

State Register, therefore reading this register later gives the

information of the gain setting used during last reception.

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 which has its window 3.5 times

larger than window of digitalization window comparator. When

the AGC function is enabled gain is reduced until there are no

transitions on its output. Such procedure assures that the input

to digitalization window comparator is less than 3.5 times larger

than its window.

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 while bit agc_alg define the AGC algorithm.

Two AGC modes are available, AGC can operate during

complete Rx process (as long as signal rx_on is high) or it can

be enabled only during first eight sub-carrier pulses.

Two AGC algorithms are available; AGC can either start by

presetting of code 4h (max digitizer window, max gain) or by

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

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.

AS3911 Datasheet(PDF) 23 Page - ams AG

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