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Oscilloscope, Oscilloscope

Although an oscilloscope displays voltage on its vertical axis, any other quantity that can be converted to a voltage can be displayed as well.In most instances, oscilloscopes show events that repeat with either no change, or change slowly.The oscilloscope is one of the most versatile and widely-used electronic instruments.

In addition to the amplitude of the signal, an oscilloscope can show distortion and measure frequency, time between two events (such as pulse width or pulse rise time), and relative timing of two related signals. Some modern digital oscilloscopes can analyze and display the spectrum of a repetitive event. Special-purpose oscilloscopes, called spectrum analyzers, have sensitive inputs and can display spectra well into the GHz range. A few oscilloscopes that accept plug-ins can display spectra in the audio range.

General-purpose instruments are used for maintenance of electronic equipment and laboratory work. Special-purpose oscilloscopes may be used for such purposes as analyzing an automotive ignition system, or to display the waveform of the heartbeat.

Nowadays even a very capable laboratory instrument can be lifted by a single person, and hand-held digital oscilloscopes are made by several manufacturers.

Binding posts or banana plugs may be used for lower frequencies.If the signal source has its own coaxial connector, then a simple coaxial cable is used; otherwise, a specialised cable called a "scope probe", supplied with the oscilloscope, is used. In general, for routine use, an open wire test lead for connecting to the point being observed is not satisfactory, and a probe is generally necessary.General-purpose oscilloscopes have a standardised input resistance of 1 megohm in parallel with a capacitance of around 20 picofarads. This allows the use of standard oscilloscope probes. Scopes for use with very high frequencies may have 50-ohm inputs, which must be either connected directly to a 50-ohm signal source or used with Z 0 or active probes.

To display events with unchanging or slowly (visibly) changing waveforms, but occurring at times that may or may not be evenly spaced, modern oscilloscopes have triggered sweeps. Compared to simpler 'scopes with sweep oscillators that are always running, triggered-sweep 'scopes are markedly more versatile.

Early (tube) 'scopes and lowest-cost 'scopes have sweep oscillators that run continuously, and are uncalibrated. Such oscilloscopes are very simple, comparatively inexpensive, and were useful in radio servicing and some TV servicing. Measuring voltage or time is possible, but only with extra equipment, and is quite inconvenient. They are primarily qualitative instruments.

The current equivalent time sampling bandwidth record for sampling digital storage oscilloscopes, , is held by the LeCroy WaveExpert series with a 100 GHz bandwidth.

In such instances, the waveform of the applied RF could generally not be shown, because the frequency was much too high. In such monitors, the CRT's bandwidth, which is typically a few hundred MHz, permits the envelope of the high-frequency RF to be displayed. The display is not a trace, but a solid triangle of light. Some bench-top oscilloscopes brought out terminals for the deflection plates for such uses. (Edited; basically from D. S. Evans and G. R. Jessup (ed), VHF-UHF Manual (3rd Edition) , Radio Society of Great Britain, London, 1976 page 10.15) The response of any oscilloscope is much faster than that of mechanical measuring devices such as the multimeter, where the inertia of the pointer (and perhaps damping) slows down its response to the input.

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. Flood guns cover the entire screen, ideally uniformly. The electrons from the flood guns 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.

The need for this instrument grew out of the requirement of nuclear scientists at Harwell to capture the waveform of very fast repetitive pulses. The current state-of-the-art oscilloscopes -- with bandwidths of typically 20 MHz -- were not able to do this and the 300 MHz effective bandwidth of their analog sampling oscilloscope represented a considerable advance.

Oscilloscopes: How to Use Them, How They Work, 5th ed., Newness, 2001 p.88-91.

However, the oscilloscope is able to vary its timebase to precisely time its sample, thus building up the picture of the signal over the subsequent repeats of the signal. This requires that either a clock or repeating pattern be provided. This type of scope is frequently used for very high speed communication because it allows for a very high "sample rate" and low amplitude noise compared totraditional real-time scopes.

Today, a hand held oscilloscope is usually a digital sampling oscilloscope, using a liquid crystal display. Typically, a hand held oscilloscope has two analog input channels, but four-input-channel versions are also available. Some instruments combine the functions of a digital multimeter with the oscilloscope.

Mainstream oscilloscope vendors manufacture large-screen, PC-based oscilloscopes, with very fast (multi-GHz) input digitizers and highly-customized user interfaces.

Karl Ferdinand Braun invented the CRT oscilloscope as a physics curiosity in 1897, by applying an oscillating signal to electrically charged deflector plates in a phosphor-coated CRT.

If the period of the horizontal sweep did not match the period of the waveform to be obeserved, each subsequent trace would start at a different place in the waveform leading to a jumbled display or a moving image on the screen. The sweep could be synchronized with the period of the signal, but then the sweep speed was uncalibrated. Many oscilloscopes had a synchronization feature which fed a signal from the vertical deflection into the sweep generator circuit, but the equivalent of trigger level had at best a narrow range, and trigger polarity was not selectable.

The Dumont 248, a commercially available oscilloscope produced in 1945, had this feature.

LeCroy remains one of the three largest manufacturers of oscilloscopes in the world.

Unlike its analog predecessor, the digital storage oscilloscope can show pre-trigger events, opening another dimension to the recording of rare or intermittent events and troubleshooting of electronic glitches. As of 2006 most new oscilloscopes (aside from education and a few niche markets) are digital.

The 1963–65 U.S. TV show The Outer Limits famously used an image of fluctuating Lissajous figures on an oscilloscope as the background to its opening credits (" There is nothing wrong with your television set.... ").

Source: Wikipedia > Oscilloscope