An oscilloscope is an instrument that provides a graphical display of electrical signals. It presents a considerable amount of information about the operation of a circuit almost instantly, and the visual nature of the display provides insights that tables of numbers do not offer. Oscilloscopes are extremely useful for monitoring and diagnosing electrical circuits or devices. Though it can plot an electrical signal versus another signal, the most common oscilloscope display mode shows the behavior of an electrical signal as a function of time. The signal amplitude, or voltage, is displayed on the vertical axis of the screen, while the horizontal axis represents the time sweep. Electrical phenomena often happen faster than can been seen with the eye. At the same time they are generally oscillatory, or cyclical, and can be displayed as a motionless trace on a fast graphing device such as an oscilloscope.
Unlike other types of electrical meters, oscilloscopes show changes in voltage and circuit behavior instantly, and when power to a circuit is removed, the oscilloscope display response is immediate. Thus, an engineer or electrician diagnosing a system can observe such rapid, and potentially damaging phenomena as transients, or voltage surges. This is a significant benefit of the oscilloscope.
Oscilloscopes use a cathode ray tube, or an electron gun, to display data. Electrons are produced by the filament of the tube and focused into a tight beam. Two pairs of electrostatic deflection plates, vertically oriented and horizontally oriented, control the beam and direct it toward the phosphor coated screen. When the electron beam impacts a section of the phosphor screen, it causes the material to glow, thus emitting light and conveying information. If the vertical pair of deflection plates are connected to an amplified voltage and the horizontal pair are connected to a "clock," the beam deflection will map the voltage as a function of time. The electron beam is swept across the screen, creating the circuit trace.
The oscilloscope can be adjusted for maximum usefulness of display. In the time-sweep mode, the instrument can be adjusted to show multiple cycles on the screen, or just one. The vertical scale can be adjusted to match the amplitude of the signal being studied. A scope can be set up to "trigger" or begin to display a signal under certain conditions, for example, a rising signal or a falling signal. It can also be connected to two circuits at once, displaying both traces. Some oscilloscopes have memory, and can continue to display a given signal for a given time. Sampling oscilloscopes are useful for very high frequency applications.
See also Electric circuit.