Most oscilloscopes have two types of input coupling to handle both alternating current and direct current signals; typically, a switch lets you select AC or DC to suit your measurement needs. When you set an input to DC coupling, the oscilloscope displays both AC and DC signals, although AC signals may pose a problem. By switching to AC coupling, the scope displays only AC signals; this simplifies measuring certain electronic circuits.
The DC coupling setting provides a direct electrical path into the scope; it accepts all types of signals, including unchanging DC voltages, time-varying DC voltages, AC, and combinations of AC and DC. In the last case, technicians call it an AC signal with a DC offset. Sometimes, DC offsets can be bothersome; the total signal voltage may push the signal past the top or bottom of the display, hiding the parts you want to see. However, under most other circumstances, DC coupling is all you need.
With AC coupling, the oscilloscope’s input has a capacitor in the signal path, removing DC offset from any mixed signal and letting you see the AC part more easily. For example, some transistor and vacuum-tube amplifiers have a significant DC offset; removing it with AC coupling helps you troubleshoot these circuits. Although it is most helpful with mixed signals, AC coupling also works with pure AC signals. Because it blocks DC, it is not suitable for DC signals.