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The classic use of a scope is to diagnose a failing piece of electronic equipment. In a radio, for example, one looks at the schematic and tries to locate the connections between stages (e.g. electronic mixers, electronic oscillators, amplifiers).
Then one puts the scope's ground on the circuit's ground, and the probe of the scope on a connection between two of the stages in the middle of the train of stages.
When the expected signal is absent, one knows that some preceding stage of the electronics has failed. Since most failures occur because of a single faulty component, each measurement can prove that half of the stages of a complex piece of equipment either work, or probably did not cause the fault.
Once the failing stage is found, further probing of the defective stage can usually tell a skilled technician exactly which component is broken. Once the technician replaces the component, the unit can be restored to service, or at least the next fault can be isolated.
Another use is to check newly designed circuitry. Very often a newly-designed circuit will misbehave because of bad voltage levels, electrical noise or design errors. Digital electronics usually operates from a clock, so a dual-trace scope is needed to check digital circuits. "Storage scopes" are helpful for "capturing" rare electronic events that cause defective operation.
Another use is for software engineers who must program electronics. Often a scope is the only way to see if the software is running the electronics properly.
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:
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 curveIn mathematics, a Lissajous curve Lissajous figure or Bowditch curve is the graph of the system of parametric equations : which describes complex harmonic motion. This family of curves was investigated by Nathaniel Bowditch in 1815, and later in more deta.
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.
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