Long story short. Some problems that I had with my ADSL connection comes from my old faulty ADSL filter, removing it leads to weird random ADSL drops, so I went to the nearest store a bought a cheap filter.
I know that the filter is a quite inexpensive product and buying different brands does not yeld more network performance. Just for fun I would like to build my own filter. The filter "model" that I need to follow is the Italian version like this one. Is there a good guide online on how to build your own ADSL filter?
ADSL filters are used to remove the ADSL signal from a line when old-fashioned analog service from the phone company is also carried on the same wires. DSL signals will sound like noise in an analog phone.
DSL modems contain their own filtering and are unaffected by analog signals also on the line. The DSL modem should ideally be connected directly to the phone company with a straight run of good modern twisted-pair wire such as cat5e. Do not use old wiring in the house for DSL. Do not place a filter or anything else in series in the line between the phone company and the modem.
If you don't have analog service riding on the phone company wires, your installation is now done. VOIP can be distributed on old phone wires as an entirely separate network.
If you do have analog service arriving direct from the phone company then you need a DSL filter to use your phones without noise. Have one filter between the main line and all of the phones. It should be connected directly without teeing off unfiltered signal into a long wire or old wires. After the filter it is OK to connect to old wires.
I guess that's different form a country to another. Here we use a "splitter", where you plug the twisted pair at one side, and in the other side you have that split to phone and modem.
These are distributed here along with ISP modem. According to the page below, they actually help filtering analogue signal off the DSL data. Though, when I have issues with the service, sometimes ISP ask me to bypass the filter while they are testing.
The usual splitter is a filter. It has to at least remove DSL noise from phone branch (and probably also remove analogue noise from DSL branch).
Anyway, I don't know about your background, but if you intend to make a filter (i.e. electronic circuit to remove analogue noise off DSL data) then that's sounds like re-inventing the wheel. These things are mass produced and I don't see the point of trying to make one. I think the best option would be to speak to your ISP (especially if you have an ISP modem) and see if they recommend a particular filter.
Yes I know the splitter is the filter. Btw I know the theory behind it... Sort of... I studied basic electronics (I reached filters, modulation, signals theory, etc...) at high school, I have to admit that I am more like a programmer (high school was more programming oriented, later got a bachelor degree in IT and in few moths I will get my master degree), but I do not want to forget about hardware stuffs :3
I would like to build a filter as exercise just to challenge my self a bit
Get yourself a basic simulator like LTSpice.
Read and understand the theory behind Low/High/Bandpass filters using RLC circuits (cutoff freq calculations, real world slopes).
Read and understand the frequency ranges used in telephonic voice and xDSL.
Design and test in LTSpice to satisfy your curiosity.
Go and buy the hardware and follow the advice above of consulting a dedicated electronics forum.