ST25R3911B datasheet(48/116 Pages) STMICROELECTRONICS

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Execution of direct command Calibrate Modulation Depth

The modulation level is adjusted by increasing the RFO1 and RFO2 driver output resistance. The RFO drivers are

composed of 8 binary weighted segments. Usually all these segments are turned on to define the normal,

non

‑modulated level, there is also a possibility to increase the output resistance of the non‑modulated state by

writing the RFO normal level definition register.

Before sending the direct command Calibrate Modulation Depth the oscillator and regulators have to be turned

on. When the direct command Calibrate Modulation Depth is sent the following procedure is executed:

The transmitter is turned on, non

‑modulated level is established.

The amplitude of the non

‑modulated carrier level established on the inputs RFI1 and RFI2 is measured by

the A/D converter and stored in the A/D converter output register.

Based on the measurement of the non

‑modulated level and the target modulated level defined by the bits

mod5 to mod0 the target modulated level is calculated.

The output driver strength is adjusted using a successive approximation algorithm until the field strength is

as close as possible to the calculated target modulated level.

The result of the output driver strength adjustment is copied in the AM modulation depth display register.

Content of this register is used to define the AM modulated level.

Note:

After the calibration procedure is finished, the content of the RFO normal level definition register should not be

changed. Modifications of the content of this register will change the non

‑modulated amplitude and therefore the

ratio between the modulated and non

‑modulated level.

In case the calibration of antenna resonant frequency in used, the command Calibrate Antenna has to be run

before AM modulation depth adjustment.

AM modulation depth definition using the RFO AM Modulated Level Definition register

When bit 7 (am_s) of the AM modulation depth control register is set to 1 the AM modulated level is controlled by

writing the RFO normal 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 RFO normal level

definition register and the direct command Measure Amplitude. This procedure has to be used when the target

modulation depth is deeper than 33%.

The procedure is the following:

Write the non

‑modulated level in the RFO normal level definition register (usually it is all 0 to have the lower

possible output resistance).

Switch on the transmitter.

Send the direct command Measure Amplitude. Read result from the A/D converter 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 RFO normal level definition register is modified, the command

Measure Amplitude executed and the result compared with 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 RFO normal level definition register that results in the target modulated level is

written in the RFO AM modulated level definition register while the RFO normal level definition register is

restored with the non

‑modulated definition value.

1.2.21

Antenna tuning

The ST25R3911B integrates the blocks needed to check and to adjust the antenna LC tank resonance frequency.

The phase and amplitude detector block is used for resonance frequency checking and adjustment.

In order to implement the antenna LC tank calibration tuning capacitors have to be connected between the two

coil terminals to the pins TRIM1_3 to TRIM1_0 and TRIM2_3 to TRIM2_0. In case single driver is used only the

pins TRIM1_3 to TRIM1_0 are used, pins TRIM2_3 to TRIM2_0 are left open. Figure 25 shows the connection of

the trim capacitors for both single (left side) and differential (right side) driving for the simple case where the

antenna LC tank is directly connected to RFO pins.

The TRIMx_y pins contain the HVNMOS switching transistors to VSS.

ST25R3911B

Application information

DS11793 - Rev 6

page 48/116

ST25R3911B Datasheet(PDF) 48 Page - STMicroelectronics

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