AS3909 datasheet(60/77 Pages) AMSCO

60 / 77 page

AS3909/AS3910 – 60

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

AM Modulation Depth: Definition and

Calibration

The AM modulation of the transmitted carrier is used for

communication reader to transponder in two configuration

cases:

• The ISO-14443B mode is configured in the ISO Mode

Definition Register (#01)

• The Transparent mode with AM modulation [direct

command Transparent Mode, the bit 6(am) of the

Configuration Register 5 (#05) is set to 1]

In other cases the OOK modulation is used.

The AM modulation depth can be automatically adjusted by

setting the Modulation Depth Definition Register (#10) and

sending the direct command Calibrate Modulation Depth. This

procedure is patent pending. There is also an alternative

possibility where the command Calibrate Modulation Depth is

not used and the modulated level is defined by writing the

Antenna driver AM Modulated Level Definition Register (#12).

AM Modulation Depth Definition Using Direct

Command Calibrate Modulation Depth

Before sending the direct command Calibrate Modulation Depth

the Modulation Depth Definition Register (#10) has to be

configured in the following way:

•The bit 7 (am_s) has to be set to 0 to chose definition by

the command Calibrate Modulation Depth.

• The bits 6 to 1 (mod5 to mod0) define target AM

modulation depth.

Definition of Modulation Depth Using Bits mod5 to mod0

The RFID standard documents usually define the AM

modulation level in form of the modulation index. The

modulation index is defined by formula (a-b)/(a+b) where ‘a’ is

amplitude of the non-modulated carrier and ‘b’ is the amplitude

of the modulated carrier.

The modulation index specification is different for different

standards. The ISO-14443B modulation index is typically 10%

with allowed range from 8% to 14%, range from 10 to 30% is

defined in the ISO-15693 and 8% to 30% in the Felica™.

The bits mod5 to mod0 are used to calculate the amplitude of

the modulated level. The non-modulated level which was

before measured by the A/D converter and stored in an 8 bit

The bits mod5 to mod0 define binary decimals of this number.

Example: In case of the modulation index 10% the modulated

level amplitude is 1.2222 times lower than the non-modulated

level. 1.2222 converted to binary and truncated to 6 decimals

is 1.001110. So in order to define the modulation index 10% the

bits mod5 to mod0 have to be set to 001110.

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