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I've seen many times that a low pass filter needs to be implemented before a switching power supply in DC power line communication transceiver boards.

Why is that? I assume that the switching power supply absorbs the modulated signal, right? Would it be the same if it was a linear power supply?

Also, do all the devices that are connected to the same DC bus need to have a low pass filter? I uploaded a schematic. Would DEVICE1 and DEVICE2 need a low pass filter as the transceivers PLC1 and PLC2 do?

PLC1 and PLC2 are two devices that contain power line communication transceivers and they have a low pass filter. Device1 and Device2 are devices that might exist on the bus, for example head lights on a car, the audio system etc...

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Yes (the downstream regulator and the upstream power source absorb signal power) and yes. For your last question, a schematic is needed because it is unclear. \$\endgroup\$ – Andy aka Sep 9 '19 at 11:51
  • \$\begingroup\$ @Andyaka Thank you! I uploaded a schematic about what I mean \$\endgroup\$ – MrBit Sep 9 '19 at 16:37
  • \$\begingroup\$ Can you share an example of where you saw such a set-up? I'd expect the PLC device to have two connections to the power bus. One with a LPF to power the device, and one with a HPF for sending and receiving data. \$\endgroup\$ – The Photon Sep 9 '19 at 16:55
  • \$\begingroup\$ I apologize my schematic is not very clear. A PLC device has two connection as you said. I put the inductors to mention the low pass filters cause my question was about low pass filters. In those two PLCx devices there are in the schematic there are 2 filters, a low pass filter before their power supply and a high pass filter for the data line. I'm just trying to show the connection diagram. \$\endgroup\$ – MrBit Sep 9 '19 at 16:59
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I would expect the PLC devices to connect to the power line through a diplexer. The data signal would be connected to the power line through a high pass filter (not a low pass filter). The PLC device might use a low pass filter to connect its power input to the power line, but it's possible this isn't needed.

You'd normally want to design your PLC system so that simple loads don't need to be modified when PLC devices are added to the power bus. If this is done right, then your DEVICE1 and DEVICE2 (which use the bus only for power) wouldn't need any special filter. If it isn't done right, or if you can't control how some of the power-only devices are designed, you might need to add a low pass filter to the connection for the power-only devices.

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  • \$\begingroup\$ Thank you! I edited my schematic. \$\endgroup\$ – MrBit Sep 9 '19 at 17:10

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