ST25R3910 datasheet(32/65 Pages) STMICROELECTRONICS

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Functional overview

ST25R3910

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DocID029768 Rev 2

AM modulation depth definition using the Antenna Driver AM Modulated

Level Definition Register

When bit 7 (am_s) of the Modulation Depth Definition Register is set to 1 the AM modulated

level is controlled by writing the Antenna Driver Non-Modulated Level Definition Register. If

the setting of the modulated level is already known it is not necessary to run the calibration

procedure, the modulated level can be defined just by writing this register.

It is also possible to implement calibration procedure through an external controller using

the Antenna Driver Non-Modulated Level Definition Register and the direct command

Measure RF.

The procedure is the following:

Write the non-modulated level in the Antenna Driver Non-Modulated Level Definition

Switch on the transmitter.

After the setting time, send the direct command Measure RF. Read result from the A/D

Output Register.

Calculate the target modulated level from the target modulation index and result of the

previous point.

In the following iterations content of the Antenna Driver Non-Modulated Level Definition

the target modulated level as long as the result is not equal (or as close as possible) to

the target modulated level.

At the end the content of the Antenna Driver Non-Modulated Level Definition Register

that results in the target modulated level is written in the Antenna Driver AM Modulated

Level Definition Register while the Antenna Driver Non-Modulated Level Definition

1.2.15

Antenna tuning

The ST25R3910 integrates the blocks needed to check and to adjust the antenna LC tank

resonance frequency.

The key block in the resonance frequency checking and adjustment is the Phase detector,

which measures the phase shift between the Transmitter output signals (RFO1 and RFO2)

and the inputs RFI1 and RFI2 (proportional to the voltage on the antenna LC tank). In case

of perfect tuning there is a 90º phase shift between them.

Check Antenna Resonance

In case of the perfect 90º phase shift mentioned above, the Phase Detector output results in

VSP/2 output voltage. A phase shift of 1% of the carrier frequency period (3.6º) results in the

output voltage change of 2% of VSP (1% phase shift results in 60mV change at VSP = 3 V).

During execution of the direct command Check Antenna Resonance the Phase Detector

output is multiplexed on the input of A/ D converter which is set in relative mode. 1 LSB of

the A/D conversion output represents 0.13% of carrier frequency period (0.468º). The result

of A/D conversion is in case of the perfect tuning exactly in the middle of range (1000 0000

or 0111 1111).

Values higher than 1000 0000 mean that phase detector output voltage is higher than

VSP/2, which correspond to resonance frequency higher than target 13.56 MHz. In the

ST25R3910 Datasheet(PDF) 32 Page - STMicroelectronics

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