AN2744 datasheet(27/56 Pages) STMICROELECTRONICS

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AN2744

Board description

27/56

5.1.4

Dual channel Rx active filter

An active filtering is suitable for receiving a highly attenuated signal. Without the gain of an

active filter, it could be impossible to detect a signal lower than the ST7538Q receiving

sensitivity even filtering the noise around it. Therefore, the choice of the Rx filter depends

mostly on the attenuation introduced by the network and then on the point of insertion of the

power line communication node.

It is possible to choose the received signal path on the board by configuring the three

jumpers shown in Figure 9 (JP1, JP2 and JP3) in different ways. The Rx path can include

only the passive filter, only the active filter, both of them or even none (no filtering).

Figure 18

and Figure 19 show the measured transfer function of the Rx active filter for both

channels. The curves show a 10 dB gain at center frequency and a -3 dB bandwidth of

about 20 kHz.

Figure 17.

Simulated frequency response of the Rx passive filter for 86 kHz channel with

tolerance effect

1E5

1E4

1E6

-17

-16

-15

-14

-13

-12

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

-18

1 dB

1E5

1E4

1E6

-17

-16

-15

-14

-13

-12

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

-18

1 dB

Figure 18.

Measured frequency response of the Rx active filter for 72 kHz channel (typical)

-8

-6

-4

-2

1.0E+04

1.0E+05

1.0E+06

Freq (Hz)

-8

-6

-4

-2

1.0E+04

1.0E+05

1.0E+06

Freq (Hz)

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