Much like the line graph traced on a heart monitor, an oscilloscope displays changes in voltage in an electronic circuit over time. As the dot on a heart monitor moves up and down corresponding to a beating heart, the dot on an oscilloscope shows the way the voltage changes in a circuit.
Using the information provided by repeating the trace hundreds of times each second on the oscilloscope, a trained technician or engineer can visualize changes in voltages, which in turn provides information regarding what adjustments s/he needs to make as well as pinpoints trouble areas in the circuit.
Since the trace on the screen shows precisely what the circuit is doing — rather than how the designer intended the circuit to function — an oscilloscope will allow you to analyze what is happening in a circuit at a given moment.
The signal on the input of the digital oscilloscope is applied to the screen with an AC or DC coupling switch; each position of the switch is used to display different aspects of the signal. The third position of the coupling switch, the ground, is for setting the display to zero volts before you take any measurements.
Similar to a very fast voltmeter, the voltage on the input of the oscilloscope is traced on the display with DC coupling, enabling you to observe large transitions and, when the voltage remains low or high, the trace correspondingly remains low or high. The display shows exactly what is happening, whether the voltage is a constant zero volts or goes positive or negative.
Monitoring small changes in the voltage will be necessary on occasion. AC coupling will block the steady voltage, while allowing variations to be displayed. When, for example, the voltage is a steady hundred volts, it is difficult to see a change of three volts up or down. By blocking the hundred volts, AC coupling causes the display to become amplified, and you can then see small variations.
DC or AC coupling on an oscilloscope lets the technician or engineer to pick the portion of the signal s/he wants to observe. DC couples the entire signal to the screen, including constant positive or negative voltages. AC coupling will block the steady voltage, allowing you to observe small variations.