ST25R3916 datasheet(28/157 Pages) STMICROELECTRONICS

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Application information

ST25R3916/7

28/157

DS12484 Rev 4

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).

The gain in the second and third stage is 23 dB and can be reduced in six 3 dB steps. Gain

of these two stages is included in AGC and Squelch loops or can be manually set in

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

7 MHz

Others

Not used

Table 6. First and third zero setting

rec1<3>z600k rec1<2>h200

rec1<1>h80

rec1<0>z12k First stage zero Third stage zero

60 kHz

400 kHz

200 kHz

40 kHz

80 kHz

12 kHz

200 kHz

80 kHz

200 kHz

600 kHz

400 kHz

200 kHz

Others

Not used

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