Oscilloscope

An oscilloscope or scope is an electronic measuring instrument that creates a visible two-dimensional graph of one or more electrical potential differences. The horizontal axis of the display normally represents time, making the instrument useful for displaying periodic signals. The vertical axis usually shows voltage. The display is caused by a "spot" that periodically "sweeps" the screen from left to right.

How it works

Cathode-ray oscilloscope (CRO)

The earliest and simplest type of oscilloscope consisted of a cathode ray tube, a vertical amplifier, a timebase, a horizontal amplifier and a power supply. These are now called 'analogue' scopes to distinguish them from the 'digital' scopes that became common in the 1990s and 2000s.

Related Topics:
Cathode ray tube - Amplifier - Power supply - 1990s - 2000s

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Before the introduction of the CRO in its current form, the cathode ray tube had already been in use as a measuring device.

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The cathode ray tube is an evacuated glass envelope, similar to that in a black-and-white television set, with its flat face covered in a phosphorescent material (the phosphor).

Related Topics:
Television - Phosphor

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The screen is typically less than 20 cm in diameter, much smaller than the one in a television set.

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In the neck of the tube is an electron gun, which is a heated metal plate with a wire mesh (the grid) in front of it. A small grid potential is used to block electrons from being accelerated when the electron beam needs to be turned off, as during sweep retrace or when no trigger events occur. A potential difference of at least several hundred volts is applied to make the heated plate (the cathode) negatively charged relative to the deflection plates. For higher bandwidth oscilloscopes where the trace may move more rapidly across the phosphor target, a positive post-deflection acceleration voltage of over 10,000 volts is often used, increasing the energy (speed) of the electrons that strike the phosphor. The kinetic energy of the electrons is converted by the phosphor into visible light at the point of impact.

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When switched on, a CRT normally displays a single bright dot in the center of the screen, but the dot can be moved about electrostatically or magnetically. The CRT in an oscilloscope uses electrostatic deflection.

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Between the electron gun and the screen are two opposed pairs of metal plates called the deflection plates. The vertical amplifier generates a potential difference across one pair of plates, giving rise to a vertical electric field through which the electron beam passes. When the plate potentials are the same, the beam is not deflected.

Related Topics:
Potential difference - Electric field

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When the top plate is positive with respect to the bottom plate, the beam is deflected upwards; when the field is reversed, the beam is deflected downwards. The horizontal amplifier does a similar job with the other pair of deflection plates, causing the beam to move left or right.

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This deflection system is called electrostatic deflection, and is different from the electromagnetic deflection system used in television tubes.

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In comparison to magnetic deflection, electrostatic deflection can more readily follow random changes in potential, but is limited to small deflection angles.

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The timebase is an electronic circuit that generates a ramp voltage.

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This is a voltage that changes continuously and linearly with time. When it reaches a predefined value the ramp is reset, with the voltage reestablishing its initial value. When a trigger event is recognized the reset is released, allowing the ramp to increase again.

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The timebase voltage usually drives the horizontal amplifier.

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Its effect is to sweep the electron beam at constant speed from left to right across the screen, then quickly return the beam to the left in time to begin the next sweep.

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The timebase can be adjusted to match the sweep time to the period of the signal.

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Meanwhile, the vertical amplifier is driven by an external voltage (the vertical input) that is taken from the circuit or experiment that is being measured. The amplifier has a very high input impedance, typically one megohm, so that it draws only a tiny current from the signal source.

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The amplifier drives with vertical deflection plates with a voltage that is proportional to the vertical input.

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The gain of the vertical amplifier can be adjusted to suit the amplitude of the input voltage. A positive input voltage bends the electron beam upwards, and a negative voltage bends it downwards, so that the vertical deflection of the dot shows the value of the input.

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The response of this system is much faster than that of mechanical measuring devices such as the multimeter, where the inertia of the pointer slows down its response to the input.

Related Topics:
Multimeter - Inertia

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When all these components work together, the result is a bright trace on the screen that represents a graph of voltage against time. Voltage is on the vertical axis, and time on the horizontal.

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Observing high speed signals, especially nonrepetitive signals, with a conventional CRO is difficult, often requiring the room to be darkened or a special viewing hood to be placed over the face of the display tube. To aid in viewing such signals, special oscilloscopes have borrowed from night vision technology, employing a microchannel plate in the tube face to amplify faint light signals.

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Although a CRO allows one to view a signal, in its basic form it has no means of recording that signal on paper for the purpose of documentation. Therefore, special oscilloscope cameras were developed to photograph the screen directly. Early cameras used roll or plate film, while in the 1970s Polaroid® instant cameras became popular.

Related Topics:
Cameras - 1970s - Polaroid

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Most multichannel scopes do not actually have multiple electron beams. Instead, they display only one dot at a time, but switch the dot between one channel and the other either on alternate sweeps (ALT mode) or many times per sweep (CHOP mode). A very few actual dual beam oscilloscopes were built; in these, the electron gun formed two electron beams and there were two sets of vertical deflection plates and one common set of horizontal deflection plates.

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The vertical amplifier and timebase controls are calibrated to show the vertical distance on the screen that corresponds to a given voltage difference, and the horizontal distance that corresponds to a given time interval.

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The power supply is an important component of the scope. It provides low voltages to power the cathode heater in the tube, and the vertical and horizontal amplifiers. High voltages are needed to drive the electrostatic deflection plates. These voltages must be very stable. Any variations will cause errors in the position and brightness of the trace.

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Later analogue oscilloscopes added digital processing to the standard design. The same basic architecture - cathode ray tube, vertical and horizontal amplifiers - was retained, but the electron beam was controlled by digital circuitry that could display graphics and text mixed with the analogue waveforms. The extra features that this system provides include:

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• on-screen display of amplifier and timebase settings;
• voltage cursors - adjustable horizontal lines with voltage display;
• time cursors - adjustable vertical lines with time display;
• on-screen menus for trigger settings and other functions.

Analogue storage oscilloscope

An extra feature available on some analogue scopes is called 'storage'.

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This feature allows the trace pattern that normally decays in a fraction of a second to remain on the screen for several minutes or longer. An electrical circuit can then be deliberately activated to store and erase the trace on the screen.

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The storage is accomplished using the principle of secondary emission. When the ordinary writing electron beam passes a point on the phosphor surface, not only does it momentarily cause the phosphor to illuminate, but the kinetic energy of the electron beam knocks other electrons loose from the phosphor surface. This can leave a net positive charge. Storage oscilloscopes then provide one or more secondary electron guns (called the "flood guns") that provide a steady flood of low-energy electrons traveling towards the phosphor screen. The electrons from the flood gun are more strongly drawn to the areas of the phosphor screen where the writing gun has left a net positive charge; in this way, the electrons from the flood guns re-illuminate the phosphor in these positively-charged areas of the phosphor screen.

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If the energy of the flood gun electrons is properly balanced, each impinging flood gun electron knocks out one secondary electron from the phosphor screen, thus preserving the net positive charge in the illuminated areas of the phosphor screen. In this way, the image originally written by the writing gun can be maintained for a long time. Eventually, small imbalances in the secondary emission ratio cause the entire screen to "fade positive" (light up) or cause the originally-written trace to "fade negative" (extinguish). It is these imbalances that limit the ultimate storage time possible.

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Some oscilloscopes used a strictly binary (on/off) form of storage known as "bistable storage". Others permitted a constant series of short, incomplete erasure cycles which created the impression of a phosphor with "variable persistence". Certain oscilloscopes also allowed the partial or complete shutdown of the flood guns, allowing the preservation (albeit invisibly) of the latent stored image for later viewing. (Fading positive or fading negative only occurs when the flood guns are "on"; with the flood guns off, only leakage of the charges on the phosphor screen degrades the stored image).

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Digital storage oscilloscope

The digital storage oscilloscope, or DSO for short, is now the preferred type for most industrial applications, although simple analogue CROs are still used by hobbyists. It replaces the unreliable storage method used in analogue storage scopes with digital memory, which can store data as long as required without degradation. It also allows complex processing of the signal by high-speed digital signal processing circuits.

Related Topics:
Memory - Digital signal processing

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The vertical input, instead of driving the vertical amplifier, is digitised by an analog to digital converter to create a data set that is stored in the memory of a microprocessor.

Related Topics:
Analog to digital converter - Microprocessor

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The data set is processed and then sent to the display, which in early DSOs was a cathode ray tube, but is now more likely to be an LCD flat panel. DSOs with colour LCD displays are common. The data set can be sent over a LAN or a WAN for processing or archiving. The screen image can be directly recorded on paper by means of an attached printer or plotter, without the need for an oscilloscope camera. The scope's own signal analysis software can extract many useful time-domain features (e.g. rise time, pulse width, amplitude), frequency spectra, histograms and statistics, persistence maps, and a large number of parameters meaningful to engineers in specialized fields such as telecommunications, disk drive analysis and power electronics.

Related Topics:
LCD - LAN - WAN

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PC-based oscilloscope

Although most people think of an oscilloscope as a self-contained instrument in a box, a new type of "oscilloscope" is emerging that consists of an external analogue-to-digital converter (sometimes with its own memory and perhaps even some data-processing ability) connected to a PC that provides the display, control interface, disc storage, networking and often the electrical power. The viability of these so-called PC-based oscilloscopes depends on the current widespread use and low cost of standardised PCs. This makes the instruments particularly suitable for the educational market, where PCs are commonplace but equipment budgets are often low.

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The advantages of PC-based oscilloscopes include:

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• Lower cost (assuming the user already owns a PC).
• Easy exporting of data to standard PC software such as spreadsheets and word processors.
• Ability to control the instrument by running a custom program on the PC.
• Use of the PC's networking and disc storage functions, which cost extra when added to a self-contained oscilloscope.
• Easier portability when used with a laptop PC.

There are also some disadvantages, which include:

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• Need for the owner to install oscilloscope software on the PC.
• Time taken for the PC to boot, compared with the almost instant start-up of a self-contained oscilloscope (although, as some modern oscilloscopes are actually PCs or similar machines in disguise, this distinction is narrowing).
• Reduced portability when used with a desktop PC.
• Inconvenience of using part of the PC's screen for the oscilloscope display.