ST25R3911B datasheet(46/116 Pages) STMICROELECTRONICS

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1.2.20

AM modulation depth: definition and calibration

The ST25R3911B transmitter supports OOK and AM modulation.

The choice between OOK and AM modulation is done by writing bit tr_am in the Auxiliary definition register. AM

modulation is preset by direct command Analog Preset in case the following protocols are configured:

•

ISO14443B

•

FeliCa™

•

NFCIP-1 212 and 424 kb/s

The AM modulation depth can be automatically adjusted by setting the AM modulation depth control register and

sending the direct command Calibrate Modulation Depth. 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

RFO AM modulated level definition register.

AM modulation depth definition using the direct command Calibrate Modulation Depth

Before sending the direct command Calibrate Modulation Depth the AM modulation depth control register has to

be configured in the following way:

•

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 as (a - b) / (a + b), where a and b are, respectively, the amplitude of the

non

‑modulated carrier and 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%, while range from 10 to 30% is defined in the ISO-15693, and 8 to

30% in the FeliCa™ and NFCIP-1 212 kb/s and 424 kb/s.

The bits mod5 to mod0 are used to calculate the amplitude of the modulated level. The

non

‑modulated level that was before measured by the A/D converter and stored in an 8 bit register is divided by a

binary number in the range from 1 to 1.98. Bits mod5 to mod0 define binary decimals of this number.

ST25R3911B

Application information

DS11793 - Rev 6

page 46/116

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