AS3909 datasheet(16/77 Pages) AMSCO

16 / 77 page

AS3909/AS3910 – 16

ams Datasheet, Confidential: 2013-Oct [3-02]

A p pl i c at io n I n f o r m at i o n

Filter setting is done automatically when ISO mode and data

rate are chosen by writing ISO Mode Definition Register. Setting

is displayed in the Receiver Configuration Register (#06) and can

be changed by rewriting this register. In transparent mode ISO

mode register is not used and Filter selection has to be done by

writing Receiver Configuration register (#06).

By setting the Operation Control Register bit rx_lp receiver

operates in low power mode. In this mode, power consumption

is lower but as consequence also receiver sensitivity is reduced

(see “Electrical Characteristics” on page 7).

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)

AGC (automatic gain control) feature is useful in case the

transponder is close to the reader. In such conditions 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 which has its window three 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 three times

larger than its window.

AGC operation is controlled by the Receiver Configuration

operation, agc_m defines AGC operating mode. The AGC action

is started 20μs after 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 first 8 subcarrier pulses. The AGC is

reducing gain to -21dB in 7 steps of 3dB. When signal rx_on is

low AGC is in reset.

Squelch

This feature is designed to avoid demodulation problems of

transponders which produce a lot of noise during data

processing which takes place when the data sent by the reader

is being processed and an answer prepared. It can also be used

in noisy environment. Transponder processing noise (or

environment noise) may be misinterpreted as start of

transponder response which results in reader decoding error.

These problems are avoided by reducing receiver gain so that

there are no transitions of output when noise is present. This is

done by sending direct command 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

AS3909 数据表(PDF) 16 Page - ams AG

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