Experimental diagnostics of multi-frequency quasiperiodic oscillations

We suggest a new technique of fold Poincaré section, allowing one to visualize an invariant curve of a multi-frequency invariant torus in a physical experiment. Details of the technique are presented, along with examples of its application to various experimental studies. Examples of how an invariant curve is visualized in double-, triple- and four-fold Poincaré sections are shown.