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AS3911 − Application Information

It is also possible to switch off 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 without

making any hardware changes. The RFO Normal Level

Definition Register defines which segments are turned on to

define the normal transmission (non-modulated) level. Default

setting is that all segments are turned on.

Using the single driver mode the number and therefore the cost

of the antenna LC tank components is halved, but also the

output power is reduced. In single mode it possible to connect

two antenna LC tanks to the two RFO outputs and multiplex

between them by controlling the IO Configuration Register 1

bit rfo2.

In order to transmit the data the transmitter output level needs

to be modulated. The AM and OOK modulation are supported.

The type of modulation is defined by setting the bit tr_am in

the Auxiliary Definition Register. For the operation modes

supported by the AS3911 framing the setting of modulation

type is done automatically by sending direct command Analog

Preset.

During the OOK modulation (for example ISO14443A) the

Transmitter drivers stop driving the carrier frequency; drivers

are frozen in state before the modulation. As consequence the

amplitude of the antenna LC tank oscillation decays, the time

constant of the decay is defined with the LC tank Q factor. The

decay time in case of OOK modulation can be shortened by

asserting the Auxiliary Definition Register bit ook_hr. When this

bit is set to logic one the drivers are put in tristate during the

OOK modulation.

AM modulation (for example ISO14443B) is done by increasing

the output driver impedance during the modulation time. This

is done by reducing the number of driver segments which are

turned on. The AM modulated level can be automatically

adjusted to the target modulation depth by defining the target

modulation depth in the AM Modulation Depth Control

command. Please refer to AM Modulation Depth: Definition and

Calibration for further details.

Slow Transmitter Ramping

When transmitter is enabled it starts to drive the antenna LC

tank with full power, the ramping of field emitted by antenna

is defined by antenna LC tank Q factor.

However there are some reader systems where the reader field

has to transition with a longer transition time when it is

enabled. The STIF (Syndicat des transports d'Ile de France)

specification requires a transition time from 10% to 90% of field

longer than or equal to 10 μs.

The AS3911 supports that feature. It is realized by collapsing

VSP_RF regulated voltage when transmitter is disabled and

ramping it when transmitter is enabled. Typical transition time

is 15 μs at 3V supply and 20 μs at 5V supply.

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