how is the magnetic field measured by ST25R200 Eval GUI?

[JW1]

Hi,

I am evaluating the ST25R200 using the ST25R200 Eval GUI. In Reader Mode, there is a Field Measurement tab that displays a numerical value together with a circular indicator. However, the displayed value does not appear to have any unit.

Could you please clarify:

1. What exactly is being measured in this tab? Does the value correspond to a physical quantity (e.g., magnetic field strength, ADC count, RSSI, amplitude, etc.)?
2. Is there a conversion from the displayed value to a standard unit (A/m, µT, V, etc.)?
3. Which ST25R200 registers or measurement circuitry are used to obtain this value?
4. Is this functionality available through the ST25R200 firmware/API so that it can be implemented in a custom application?

Any explanation of the measurement principle and interpretation of the displayed value would be greatly appreciated.

Thank you.

[Grégoire Poulain]

Dear JW1,

The Field Measurement Tab on ST25R200 Eval GUI shows two distinct functionalities:

• Detecting the presence of an external RF field
  This is done by API rfalIsExtFieldOn(), which executes the direct command Sense RF
   
• Inferring the intensity of its own RF field
  This is achieved by measuring the I/Q channels of the antenna input, when ST25R200 RF Carrier is enabled.
  The API used is st25r200MeasureCombinedIQ(), which executs the direct command Calibrate WU measurement

In fact the GUI itself provides information on the APIs used (see Logging section) and additional context on the Help section.
Details of the commands and registers involved are described on device’s Datasheet.
 

Both commands make use of the I/Q Measurement also used for the Wake-up system.
These signals are processed (filtered/amplified) according to the system configuration.

While Sense RF is meant to be used as presence indicator (boolean) of an external field, the Calibrate WU does provide a value for each I/Q component, which is then combined into a squared sum, and depicted on the GUI as a value within [0,128].

Specifically to you questions:

1. The I and Q components of the antenna RFI signal.
   Amplitude is represented as the squared sum of the I and Q channel
    
2. Technically yes, but it depends of a few external parameters so that the API / GUI is not converting into units.
   The external parameters include VDD, VDD_A, the configured gain (afe_gain_td), and (so some extend) temperature.
    
3. Described above and on ST25R200 device Datasheet.
    
4. Sure, all APIs the EVAL GUI uses are available on the protocol interface, and they also map directly to RFAL interfaces/functions(st25r200MeasureCombinedIQ()/st25r200IsExtFieldOn()), which will then execute the device commands (Calibrate WU measurement / Sense RF)

Hope it helps
GP