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An oscilloscope is an electronic tool used to graphically depict sound waves and environmental frequencies. This sort of tool is helpful in a number of different applications. Some of the most common include music, particularly radio frequencies and digital music remastering, but it can also be helpful in certain circuitry and engineering scenarios and in doing things like measuring seismic activity and certain other sounds in nature. Most devices are calibrated to not just depict the sound waves happening in one instant, but also to track them over time, noting changes and significant shifts. They’re usually fairly easy to control and manipulate, and users can calibrate them to achieve a number of different goals. Additionally there are many different models to choose from, often with a range of specifications. Some are basic and easy to use, whereas others are much more complicated and often require supporting software and other equipment. People looking to buy one of these tools are usually wise to research the available options and carefully consider their needs before making an investment.
A typical oscilloscope is a rectangular box with a small screen, numerous input connectors and control knobs, and buttons on the front panel. A grid called the graticule on the face of the screen helps with measurement. 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 coaxial connector that uses an electrical cord or other cabling. If the signal source has its own coaxial connector, then a simple coaxial cable may be all that’s required; otherwise, a specialized cable called a “scope probe” may be required, though in these cases the probe usually comes with the device.
In its simplest and most basic mode, the device draws a horizontal line called the trace across the middle of the screen from left to right that relates to the sounds heard and absorbed. One of the controls, the timebase control, sets the speed at which the line is drawn. It’s usually 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, with the most recent past to the left, the less recent past to the right.
When the input signal is what’s known as “periodic,” it’s usually possible to get a simple trace 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. The flaw with this is that the tool's primary timebase is not usually perfectly accurate, and the frequency of the input signal is not usually perfectly stable; as a result, the trace may drift across the screen, which can make measurements difficult.
These devices typically have a function called the “trigger” that helps provide a more stable trace. In essence, the trigger causes the scope to pause after reaching the right hand side of the screen, where it waits for a specified event before returning to the left hand side of the screen and drawing the next trace. The effect is a resynchronization of the timebase to the input signal, which prevents horizontal drift. 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; and
- delayed trigger, which waits a specified time after an edge trigger before starting the sweep.
External Signals and Input Channels
Most devices also allow users 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, as might be 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 devices also have two or more input channels, which allows them to display more than one input signal on the screen at a given time. Usually they has a separate set of vertical controls for each channel, but only one triggering system and timebase.
A dual-timebase device 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 first traps the desired signal using a suitable trigger setting. Then he or she 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 models 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.