AS3909 datasheet(15/77 Pages) AMSCO

15 / 77 page

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

AS3909/AS3910 – 15

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

certain segments when driving the non modulated level to

reduce the amplitude of signal on the antenna and/or to reduce

the antenna Q factor.

The driver impedance in increased in case of AM modulation

(ISO-14443B), in case of OOK modulation (ISO-14443A) both

drivers are blocked to low state. In the single driver mode (bit

sing of configuration register 2 set to 1) only RFO1 output is

driven, RFO2 output is disabled.

AM modulation and operation of the driver segments is

controlled by writing AM modulation depth and antenna driver

registers (see “AM Modulation Depth and Antenna Driver

Registers” on page 46). Register #13 defines which segments

will be used to define normal, non-modulated level. The default

setting is that all segments are used. Registers #10 to #12 are

used to define how the AM modulated level is set-up. It can be

set-up automatically by definition of modulation depth and the

direct command Calibrate Modulation Depth or by a direct

definition of segments which are turned off during AM

modulation.

Receiver

The receiver performs demodulation of the transponder

subcarrier modulation which is superimposed on the 13.56 MHz

carrier frequency. It performs AM or PM demodulation,

band-pass filtering and digitalization of subcarrier signals, 848,

424 and 212 kHz subcarrier frequencies are supported.

Additionally, it performs RSSI measurement, automatic gain

control (AGC) and Squelch function.

The receiver is switched on when Operation Control Register bit

rx_en is set. The operation of the receiver is additionally

controlled by the signal rx_on which is set high when

modulated signal is expected on the receiver input. This is

automatically done after every Transmit command. Signal

rx_on can be also forced high by sending direct command

Unmask Receive Data. Signal rx_on is used to control features

like RSSI and AGC.

AM demodulation is performed using a peak follower. Both the

positive and negative peaks are tracked to suppress common

mode signal. In case external demodulation is carried out the

peak follower stage can be bypassed by setting bit envi in

Configuration Register 2. In case of PM demodulation signal

coming from the phase detector is replacing the output of peak

follower.

Next stage in signal processing is the buffer amplifier followed

by second order low pass filter with adjustable corner

frequency. Final stage is a first order high pass filter with

adjustable corner frequency. The digital signal representing

transponder subcarrier modulation is produced by a window

comparator.

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