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[burakkucat]

and how's the graph or plot looks like with and without this common mode filter?
noise level etc

I have never performed such an experiment, as it would be very difficult for me to simulate legitimate RF signals, inductive load switching and various pulsed e/m-fields.

My earlier post is a theoretical description of the operation of a common-mode filter within a circuit troubled by external interference. Against the positive aspects, described above, there is also a negative aspect . . . by inserting the filter into the circuit, the wanted (differential-mode signal) is also slightly attenuated. In other words the common-mode filter is responsible for a small, but measurable, insertion loss. With an essentially perfect xDSL circuit, i.e. one that is free from any of the troubles that a common-mode filter could mitigate, fitting such a filter will attenuate the wanted differential-mode signal (by virtue of the intrinsic insertion loss) with no obvious gains. So fitting a common-mode filter is not necessarily a "win" for every xDSL circuit.

[Weaver]

Agrees with Burakucat. You have on the A wire, voltage: A+interference, and on the B wire: B+interference; then the receiving unit subtracts the one from the other, giving: (A+interference) - (B+interference) = A+interference - B-interference = A - B : which is just the signal alone, ideally

[neil]

It is a common-mode filter.

An xDSL service exists between two analogue transceiver units, the xTU-C and the xTU-R, connected together by a radio frequency transmission line consisting of a pair of twisted wires carried within one or more sequentially connected cables. The analogue transceivers operate in differential mode. Think of one moment in time (if you like, consider the circuit "to be frozen" in time) and we look at the state of the circuit. The circuit consists of two wires, one twisted pair (with wires labelled A & B), and at that "frozen" moment in time let's say that the the electrons in the wire labelled A are "pushing" from the xTU-C to the xTU-R whilst the electrons in the wire labelled B are "pushing" from the xTU-C to the xTU-R. That is differential mode operation. Now consider the cable carrying the twisted pair of wires linking the xTU-C to the xTU-R. In a real world situation that cable has significant length and can pass through regions where there are (legitimate) high strength RF signals, pulsed electromagnetic-fields, switched high-current inductive loads (to name three possible sources of interference to the xDSL service.) As the xDSL service is carried over a twisted pair, whatever interference that is coupled into the A-wire of the pair is also coupled into the B-wire of the pair. The interference, therefore, appears in common-mode on the pair. Hence the usage of that common-mode filter. It rejects any common-mode signal on the pair whilst allowing the differential-mode signal on the pair to pass unhindered.

thank you 🤗