In “Dual” mode (=two-channel-mode) the signals in channel 1 and 2 seem to be displayed on the screen at the same time. The real oscilloscope however only has a single electronic beam. The beam draws smallest parts of both curves alternating between both channels on each sweep with a very high frequency. For the eye of he observer this looks as if both curves are drawn at the same time. As an alternative, “Chop” mode can be used to display two signals on the screen.
In “Chop” mode the oscilloscope either draws the signal of the first or the second channel alternating on each sweep of the electronic beam. In “Chop” mode, the channel does not alternate during a sweep but during the (invisible) reset of the beam. Chop-mode is on when both, the “Dual” button and the “Add” button are pressed.
The following screenshots show the different modes:
Dual or chop-mode? On short Time/Div. settings no difference is visible, because the toggle between drawing both curves is too fast for the eye to follow.
Long Time/Div. setting and Dual mode (without Chop-mode).