A typical oscilloscope is a rectangular box with a small screen, numerous input connectors and control knobs and buttons on the front panel. To aid measurement, a grid called the graticule is drawn on the face of the screen. Each square in the graticule is known as a division. The signal to be measured is fed to one of the input connectors, which is usually a co-axial connector such as a BNC or N type. If the signal source has its own co-axial connector, then a simple co-axial cable is used; otherwise, a specialised cable called a scope probe, supplied with the oscilloscope, is used.
In its simplest mode, the oscilloscope repeatedly draws a horizontal line called the trace across the middle of the screen from left to right. One of the controls, the timebase control, sets the speed at which the line is drawn, and is calibrated in seconds per division. If the input voltage departs from zero, the trace is deflected either upwards or downwards. Another control, the vertical control, sets the scale of the vertical deflection, and is calibrated in volts per division. The resulting trace is a graph of voltage against time (the present plotted at a varying position, the most recent past to the left, the less recent past to the right).
If the input signal is periodic, then a nearly stable trace can be obtained just by setting the timebase to match the frequency of the input signal. For example, if the input signal is a 50 Hz sine wave, then its period is 20 ms, so the timebase should be adjusted so that the time between successive horizontal sweeps is 20 ms. This mode is called continual sweep. Unfortunately, an oscilloscope's timebase is not perfectly accurate, and the frequency of the input signal is not perfectly stable, so the trace will drift across the screen making measurements difficult.
To provide a more stable trace, an oscilloscope has a function called the trigger. This causes the scope to pause after reaching the right hand side of the screen, and wait for a specified event before returning to the left hand side of the screen and drawing the next trace.
The effect is to resynchronise the timebase to the input signal, preventing horizontal drift of the trace. Trigger circuits allow the display of nonperiodic signals such as single pulses, as well as periodic signals such as sine waves and square waves.
Types of trigger include:
- external trigger, a pulse from an external source connected to a dedicated input on the scope.
- edge trigger, an edge-detector that generates a pulse when the input signal crosses a specified threshold voltage in a specified direction.
- video trigger, a circuit that extracts synchronising pulses from video formats such as PAL and NTSC and triggers the timebase on every line, a specified line, every field, or every frame. This circuit is typically found in a waveform monitor device.
- delayed trigger, which waits a specified time after an edge trigger before starting the sweep. No trigger circuit acts instantaneously, so there is always a certain delay, but a trigger delay circuit extends this delay to a known and adjustable interval.
Most oscilloscopes also allow you to bypass the timebase and feed an external signal into the horizontal amplifier. This is called X-Y mode, and is useful for viewing the phase relationship between two signals, which is commonly done in radio and television engineering. When the two signals are sinusoids of varying frequency and phase, the resulting trace is called a Lissajous curve.
Some oscilloscopes have cursors, which are lines that can be moved about the screen to measure the time interval between two points, or the difference between two voltages.
Most oscilloscopes have two or more input channels, allowing them to display more than one input signal on the screen. Usually the oscilloscope has a separate set of vertical controls for each channel, but only one triggering system and timebase.
A dual-timebase oscilloscope has two triggering systems so that two signals can be viewed on different time axes. This is also known as a "magnification" mode. The user traps the desired, complex signal using a suitable trigger setting. Then he enables the "magnification", "zoom" or "dual timebase" feature, and can move a window to look at details of the complex signal.
Sometimes the event that the user wants to see may only happen occasionally. To catch these events, some oscilloscopes are "storage scopes" that preserve the most recent sweep on the screen.
Some digital oscilloscopes can sweep at speeds as slow as once per hour, emulating a strip chart recorder. That is, the signal scrolls across the screen from right to left. Most fancy oscilloscopes switch from a sweep to a strip-chart mode right around one sweep per ten seconds. This is because otherwise, the scope looks broken: it's collecting data, but the dot cannot be seen.