An oscilloscope is a laboratory instrument commonly used to display and analyze

thewaveform of electronic signals. In effect, the device draws a graph of the instantaneous
signal voltage as a function of time.

A typical oscilloscope can display alternating current (AC) or pulsating direct current (DC)
waveforms having a frequency as low as approximately 1 hertz (Hz) or as high as several
megahertz (MHz). High-end oscilloscopes can display signals having frequencies up to several
hundred gigahertz (GHz). The display is broken up into so-called horizontal divisions (hor div)
and vertical divisions (vert div). Time is displayed from left to right on the horizontal scale.
Instantaneous voltage appears on the vertical scale, with positive values going upward and
negative values going downward.

The oldest form of oscilloscope, still used in some labs today, is known as the cathode-ray
oscilloscope. It produces an image by causing a focused electron beam to travel, or sweep, in
patterns across the face of a cathode ray tube (CRT). More modern oscilloscopes electronically
replicate the action of the CRT using a liquid crystal display (liquid crystal display) similar to
those found on notebook computers. The most sophisticated oscilloscopes employ computers
to process and display waveforms. These computers can use any type of display, including CRT,
LCD, and gas plasma.

In any oscilloscope, the horizontal sweep is measured in seconds per division (s/div),
milliseconds per division (ms/div), microseconds per division (s/div), or nanoseconds per
division (ns/div). The vertical deflection is measured in volts per division (V/div), millivolts per
division (mV/div), or microvolts per division (?V/div). Virtually all oscilloscopes have adjustable
horizontal sweep and vertical deflection settings.

The illustration shows two common waveforms as they might appear when displayed on an
oscilloscope screen. The signal on the top is a sine wave; the signal on the bottom is a ramp
wave. It is apparent from this display that both signals have the same, or nearly the
same,frequency. They also have approximately the same peak-to-peak amplitude. Suppose the
horizontal sweep rate in this instance is 1 µs/div. Then these waves both complete a full cycle
every 2 µs, so their frequencies are both approximately 0.5 MHz or 500 kilohertz (kHz). If the
vertical deflection is set for, say, 0.5 mV/div, then these waves both have peak-to-peak
amplitudes of approximately 2 mV.

These days, typical high-end oscilloscopes are digital devices. They connect to personal
computers and use their displays. Although these machines no longer employ scanning electron
beams to generate images of waveforms in the manner of the old cathode-ray "scope," the
basic principle is the same. Software controls the sweep rate, vertical deflection, and a host of
other features which can include:

 Storage of waveforms for future reference and comparison

An oscilloscope, previously called an oscillograph,[1][2] and informally known as a scope, CRO (for
cathode-ray oscilloscope), or DSO (for the more modern digital storage oscilloscope), is a type
of electronic test instrument that allows observation of constantly varying signal voltages, usually as
a two-dimensional plot of one or more signals as a function of time. Non-electrical signals (such as
sound or vibration) can be converted to voltages and displayed.

Oscilloscopes are used to observe the change of an electrical signal over time, such that voltage
and time describe a shape which is continuously graphed against a calibrated scale. The
observed waveform can be analyzed for such properties as amplitude, frequency, rise time, time
interval, distortion and others. Modern digital instruments may calculate and display these properties
directly. Originally, calculation of these values required manually measuring the waveform against
the scales built into the screen of the instrument.[3]
The oscilloscope can be adjusted so that repetitive signals can be observed as a continuous shape
on the screen. A storage oscilloscope allows single events to be captured by the instrument and
displayed for a relatively long time, allowing human observation of events too fast to be directly
perceptible.

Oscilloscopes are used in the sciences, medicine, engineering, and telecommunications industry.
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 as an electrocardiogram.

Before the advent of digital electronics, oscilloscopes used cathode ray tubes (CRTs) as their
display element (hence were commonly referred to as CROs) and linear amplifiers for signal
processing. Storage oscilloscopes used special storage CRTs to maintain a steady display of a
single brief signal. CROs were later largely superseded by digital storage oscilloscopes (DSOs)
with thin panel displays, fast analog-to-digital converters and digital signal processors. DSOs without
integrated displays (sometimes known as digitisers) are available at lower cost and use a general-
purpose digital computer to process and display waveforms.

http://www-lhs.beth.k12.pa.us/faculty/Hoffman_M/Introduction%20to%20Oscilloscopes.pdf

Description[edit]
The basic oscilloscope, as shown in the illustration, is typically divided into four sections: the display,
vertical controls, horizontal controls and trigger controls. The display is usually a CRT or LCD panel
which is laid out with both horizontal and vertical reference lines referred to as the graticule. In
addition to the screen, most display sections are equipped with three basic controls: a focus knob,
an intensity knob and a beam finder button.

The vertical section controls the amplitude of the displayed signal. This section carries a Volts-per-
Division (Volts/Div) selector knob, an AC/DC/Ground selector switch and the vertical (primary) input
for the instrument. Additionally, this section is typically equipped with the vertical beam position
knob.

The horizontal section controls the time base or "sweep" of the instrument. The primary control is the
Seconds-per-Division (Sec/Div) selector switch. Also included is a horizontal input for plotting dual X-
Y axis signals. The horizontal beam position knob is generally located in this section.

The trigger section controls the start event of the sweep. The trigger can be set to automatically
restart after each sweep or it can be configured to respond to an internal or external event. The
principal controls of this section will be the source and coupling selector switches. An external trigger
input (EXT Input) and level adjustment will also be included.

In addition to the basic instrument, most oscilloscopes are supplied with a probe as shown. The
probe will connect to any input on the instrument and typically has a resistor of ten times the
oscilloscope's input impedance. This results in a .1 (-10X) attenuation factor, but helps to isolate the
capacitive load presented by the probe cable from the signal being measured. Some probes have a
switch allowing the operator to bypass the resistor when appropriate.[3]

Basic types of sweep[edit]

Type 465 Tektronix oscilloscope. This was a popular analog oscilloscope, portable, and is a representative
example.

To display events with unchanging or slowly (visibly) changing waveforms, but occurring at times
that may not be evenly spaced, modern oscilloscopes have triggered sweeps. Compared to simpler
oscilloscopes with sweep oscillators that are always running, triggered-sweep oscilloscopes are
markedly more versatile.
A triggered sweep starts at a selected point on the signal, providing a stable display. In this way,
triggering allows the display of periodic signals such as sine waves and square waves, as well as
nonperiodic signals such as single pulses, or pulses that do not recur at a fixed rate.

With triggered sweeps, the scope will blank the beam and start to reset the sweep circuit each time
the beam reaches the extreme right side of the screen. For a period of time, called holdoff,
(extendable by a front-panel control on some better oscilloscopes), the sweep circuit resets
completely and ignores triggers. Once holdoff expires, the next trigger starts a sweep. The trigger
event is usually the input waveform reaching some user-specified threshold voltage (trigger level) in
the specified direction (going positive or going negative—trigger polarity).

In some cases, variable holdoff time can be really useful to make the sweep ignore interfering
triggers that occur before the events one wants to observe. In the case of repetitive, but quite-
complex waveforms, variable holdoff can create a stable display that cannot otherwise practically be
obtained.

Holdoff[edit]

Trigger holdoff defines a certain period following a trigger during which the scope will not trigger
again. This makes it easier to establish a stable view of a waveform with multiple edges which would
otherwise cause another trigger.[10]

Automatic sweep mode[edit]

Triggered sweeps can display a blank screen if there are no triggers. To avoid this, these sweeps
include a timing circuit that generates free-running triggers so a trace is always visible. Once triggers
arrive, the timer stops providing pseudo-triggers. Automatic sweep mode can be de-selected when
observing low repetition rates.

Recurrent sweeps[edit]

If the input signal is periodic, the sweep repetition rate can be adjusted to display a few cycles of the
waveform. Early (tube) oscilloscopes and lowest-cost oscilloscopes 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.

They have a few (widely spaced) frequency ranges, and relatively wide-range continuous frequency
control within a given range. In use, the sweep frequency is set to slightly lower than some
submultiple of the input frequency, to display typically at least two cycles of the input signal (so all
details are visible). A very simple control feeds an adjustable amount of the vertical signal (or
possibly, a related external signal) to the sweep oscillator. The signal triggers beam blanking and a
sweep retrace sooner than it would occur free-running, and the display becomes stable.

Single sweeps[edit]

Some oscilloscopes offer these—the sweep circuit is manually armed (typically by a pushbutton or
equivalent) "Armed" means it's ready to respond to a trigger. Once the sweep is complete, it resets,
and will not sweep until re-armed. This mode, combined with an oscilloscope camera, captures
single-shot events.

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. These are the most-common types of triggers; the
level control sets the threshold voltage, and the slope control selects the direction (negative or
positive-going). (The first sentence of the description also applies to the inputs to some digital
logic circuits; those inputs have fixed threshold and polarity response.)
 video trigger, a circuit that extracts synchronizing 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, although some better
oscilloscopes include this function.
 delayed trigger, which waits a specified time after an edge trigger before starting the sweep. As
described under delayed sweeps, a trigger delay circuit (typically the main sweep) extends this
delay to a known and adjustable interval. In this way, the operator can examine a particular
pulse in a long train of pulses.

Some recent designs of oscilloscopes include more sophisticated triggering schemes; these are
described toward the end of this article.

Delayed sweeps[edit]

More sophisticated analog oscilloscopes contain a second timebase for a delayed sweep. A delayed
sweep provides a very detailed look at some small selected portion of the main timebase. The main
timebase serves as a controllable delay, after which the delayed timebase starts. This can start
when the delay expires, or can be triggered (only) after the delay expires. Ordinarily, the delayed
timebase is set for a faster sweep, sometimes much faster, such as 1000:1. At extreme ratios, jitter
in the delays on consecutive main sweeps degrades the display, but delayed-sweep triggers can
overcome that.

The display shows the vertical signal in one of several modes: the main timebase, or the delayed
timebase only, or a combination thereof. When the delayed sweep is active, the main sweep trace
brightens while the delayed sweep is advancing. In one combination mode, provided only on some
oscilloscopes, the trace changes from the main sweep to the delayed sweep once the delayed
sweep starts, although less of the delayed fast sweep is visible for longer delays. Another
combination mode multiplexes (alternates) the main and delayed sweeps so that both appear at
once; a trace separation control displaces them.

DSOs allow waveforms to be displayed in this way, without offering a delayed timebase as such.

Examples of use[edit]

Lissajous figures on an oscilloscope, with 90 degrees phase difference between x and y inputs.

One of the most frequent uses of scopes is troubleshooting malfunctioning electronic equipment.
One of the advantages of a scope is that it can graphically show signals: where a voltmeter may
show a totally unexpected voltage, a scope may reveal that the circuit is oscillating. In other cases
the precise shape or timing of a pulse is important.

In a piece of electronic equipment, for example, the connections between stages (e.g. electronic
mixers, electronic oscillators, amplifiers) may be 'probed' for the expected signal, using the scope as
a simple signal tracer. If the expected signal is absent or incorrect, some preceding stage of the
electronics is not operating correctly. 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 faulty stage is found, further probing can usually tell a skilled technician exactly which
component has failed. Once the component is replaced, the unit can be restored to service, or at
least the next fault can be isolated. This sort of troubleshooting is typical of radio and TV receivers,
as well as audio amplifiers, but can apply to quite-different devices such as electronic motor drives.

Another use is to check newly designed circuitry. Very often a newly designed circuit will misbehave
because of design errors, bad voltage levels, electrical noise etc. Digital electronics usually operate
from a clock, so a dual-trace scope which shows both the clock signal and a test signal dependent
upon the clock is useful. Storage scopes are helpful for "capturing" rare electronic events that
cause defective operation.