ST ST7538Q, Application Note, Page: 27 / 56

AN2744 Board description
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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
0
1 dB
1E5 1E4 1E6
-17
-16
-15
-14
-13
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
-18
0
1 dB
Figure 18. Measured frequency response of the Rx active filter for 72 kHz channel (typical)
-8
-6
-4
-2
0
2
4
6
8
10
12
1.0E+04 1.0E+05 1.0E+06
Freq (Hz)
Gain
(
d
B)
-8
-6
-4
-2
0
2
4
6
8
10
12
1.0E+04 1.0E+05 1.0E+06
Freq (Hz)
Gain
(
d
B)