AS3911B datasheet(23/155 Pages) AMSCO

23 / 155 page

ams Datasheet: 2014-Jun-12 [v1-08]

AS3911B – 23

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

AGC, Squelch and RSSI

As mentioned above second and third gain stage gain and the

Digitizing stage digitizing window are included in AGC and

Squelch loops. Eleven settings are available, default state

features minimum digitizer window and maximum gain, first

four steps increase the digitizer window in 3 dB steps, next six

steps additionally reduce the gain in 2nd and 3rd gain stage also

in 3 dB steps. The initial setting with which Squelch and AGC

start is defined in Receiver Configuration Register 4. The Gain

Reduction State Register displays the actual state of gain which

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 receiver in noisy

environment. The noise can come from tags in which

processing of data sent by the reader is going on and an answer

is being prepared. Noise can also be generated by noisy

environment. This noise may be misinterpreted as start of

transponder response which results in decoding error.

During execution of the Squelch procedure the output of

Digitizing comparator is observed. In case there are more than

two transitions on this output in 50 μs time period, gain is

reduced for 3 dB and output is observed during next 50 μs. This

procedure is repeated until number of transitions in 50 μs is

lower or equal to 2 or until maximum gain reduction is reached.

This setting is cleared by sending direct command Reset Rx

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.

AS3911B Datasheet(PDF) 23 Page - ams AG

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