ST25R3916B-AQWT datasheet(27/165 Pages) STMICROELECTRONICS

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DS13541 Rev 7

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ST25R3916B ST25R3917B

Application information

164

Demodulation stage

The first stage performs demodulation of the tag subcarrier response signal, superimposed

on the HF field carrier. Two different blocks are implemented for the AM demodulation:

•

peak detector

•

AM/I or PM/Q demodulator mixer.

The choice of the used demodulator is made by the Receiver configuration register 2 bit

amd_sel.

The peak detector performs AM demodulation using a peak follower. Both the positive and

negative peaks are tracked to suppress any common mode signals. Its demodulation gain is

G = 0.7 and the input is taken from RFI1 demodulator input only.

The AM demodulator mixer uses synchronous rectification of both receiver inputs (RFI1 and

RFI2). Its gain is G = 0.55. The PM demodulation is also done by a mixer. The PM

demodulator mixer has differential outputs with 60 mV differential signal for 1% phase

change (16.67 mV / °).

The I/Q demodulation is composed of two mixer circuits, driven with a 90° shifted local

oscillator (LO) signals derived from the crystal oscillator. The outputs of the two mixers are

connected to two equal base band reception chains and to the decoding logic.

Filtering and gain stages

The receiver chain has band pass filtering characteristics. The filtering is optimized to pass

subcarrier frequencies while rejecting carrier frequency, low frequency noise and DC

component. Filtering and gain is implemented in three stages, the first and the last stage

have first order high pass characteristics while the mid stage has second order low-pass

characteristic.

The gain and filtering characteristics can be optimized depending on the application by

writing the Receiver configuration register 1 (filtering), Receiver configuration register 3

(primarily gain in first stage) and Receiver configuration register 4 (gain in second and third

stage).

The gain of first stage is around 20 dB and can be reduced in six 2.5 dB steps. There is also

a special boost mode available, which increases the max gain by additional 5.5 dB. The first

stage gain can only be modified by writing Receiver configuration register 3. The default

setting of this register is the minimum gain. Default first stage zero is located at 60 kHz, it

can also be lowered to 40 or 12 kHz by writing option bits in the Receiver configuration

option bit z600k. The control of the first and third stage zeros is done with common control

bits (see Table 6).

Table 5. Low-pass control

rec1<5>lp2

rec1<4>lp1

rec1<3>lp0

-1 dB point

1200 kHz

600 kHz

300 kHz

2 MHz

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