ST25R3918 datasheet(33/141 Pages) STMICROELECTRONICS

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DS13490 Rev 2

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ST25R3918

Application information

140

Relative mode is only used in phase measurement (phase detector output is proportional to

power supply). In all other cases absolute mode is used.

4.2.7

Phase and amplitude detector

This block is used to provide inputs to the A/D converter to perform measurements of

amplitude and phase, expected by direct commands Measure amplitude and Measure

phase.

Phase detector

The phase detector observes phase difference between the transmitter output signals

(RFO1 and RFO2) and the receiver input signals RFI1 and RFI2, proportional to the signal

on the antenna LC tank. These signals are first passed by digitizing comparators. Digitized

signals are processed by a phase detector with a strong low-pass filter characteristics to get

the average phase difference. The phase detector output is inversely proportional to the

phase difference between the two inputs. The 90° phase shift results in VDD_A/2 output

voltage, if both inputs are in phase the output voltage is VDD_A, if they are in opposite phase

the output voltage is 0 V. During execution of direct command Measure phase this output is

multiplexed to the A/D converter input (A/D converter is in relative mode during the

execution of this command). Since the A/D converter range is from 1/11 VDD_A to 10/11

VDD_A the actual phase detector range is from 17º to 163º. Figure 6 and Figure 7 show the

two inputs and output of phase detector in case of 90º and 135º phase shift, respectively.

Figure 6. Phase detector inputs and output in case of 90º phase shift

MS42426V1

VSP_A

VSP_A/2

Input 1

Input 2

Output

ST25R3918 Datasheet(PDF) 33 Page - STMicroelectronics