An oscilloscope (often abbreviated to scope) is a type of electronic testing equipment used for measuring and visualising electrical signals. An oscilloscope uses a display to present of a graph of changes in voltage over a period of time, making them very useful for measuring the rapidly changing signals present in computer systems.

[wiki="File:Oscilloscope_Trace.jpg"]200px|thumb|right|Example trace on an oscilloscope display. Extra information regarding the oscilloscopes current settings is also displayed.[/wiki]
The display of an oscilloscope is generally split into divisions my means of a grid known as the graticule. A representation of the signal being measured is displayed on the screen my means of a line called the trace. When the signal is at ground potential (i.e. zero volts) the trace is a simple straight line. A negative voltage causes the trace to descend towards the bottom of the screen; conversely, a positive voltage deflects the trace upwards. The resulting pattern is referred to as the waveform of the signal.
Inputs, Channels and Probes

Different oscilloscopes are capable of measuring different numbers of signals simultaneously, known as the number of channels. A single-channel 'scope is adequate for many diagnostic tasks but two or more channels can be helpful when diagnosing more complex problems as signals can be compared to each other in real time. If measurment of more than three or four channels is necessary, consider whether a [wiki]logic analyser[/wiki] is more suitable.
Each channel has its own input to which a probe is connected. The probe is a specialised type of test lead, usually with a high [wiki]impedence[/wiki] and low [wiki]capacitance[/wiki] in order to prevent distrubing the circuit being tested.

The bandwidth of a 'scope determines the range of frequencies it is able to display. Note however that a scope with 100MHz bandwidth cannot measure a 100MHz digital signal with adequate definition. As a rule of thumb, a 'scope should have a bandwidth of roughly five times the highest frequency signal to be measured. For most vintage applications, a bandwidth of around 100 to 150 MHz is therefore adequate, allowing reliable measurement of signals up to around 20 or 30 MHz.