One of the nice things of my Fritzbox DSL router is that it draws nice graphs of what is happening on my VDSL line. And one thing I find particularly puzzling in the top graph in the screenshot above is that the signal to noise ratio on the line starts at around 60 db and then quickly decreases down to only 20 db at around 2 MHz. After that it skyrockets again to 60 db and then expectedly degrades again slowly.
What I don’t understand is why there is such a significant dip at 2 MHz? And why is the SNR above 2.2 MHz higher than at the lower end of the spectrum? At first I thought that perhaps this is something particular to my line. But in the meantime I had a look at this graph on a few other routers connected to different lines, including to a line that is very short and thus should be almost ideal. In all cases that SNR dip at 2 MHz is present. Any idea why???