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[v2-11] 2016-Jun-13

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AS3911 − Application Information

AM Modulation Depth Definition Using the ‘Calibrate

Modulation Depth’ Direct Command

Before sending the direct command Calibrate Modulation

Depth the AM Modulation Depth Control Register has to be

configured in the following way:

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

the command Calibrate Modulation Depth

• 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 ISO14443B modulation index is typically 10%

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

defined in the ISO15693 and 8% to 30% in the FeliCa™ and

NFCIP-1 212 kbit/s and 424 kbit/s.

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.

The table below depicts setting of the mod bits for some often

used modulation indexes.

Figure 125:

Setting of the mod Bits for Some Often Used Modulation Indexes

Modulation Index [%]

a/b [dec]

a/b [bin]

mod5 to mod0

1.1739

1.001011

001011

1.2222

1.001110

001110

1.3256

1.010100

010100

1.5000

1.100000

100000

1.8571

1.110111

110111

1.9843

1.111111

111111

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