3D structure of cellulose, a beta-glucan polysaccharideAmylose is a linear polymer of glucose mainly linked with α(1→4) bonds. It can be made of several thousands of glucose units. It is one of the two components of starch, the other being amylopectin.
Polysaccharides are often quite heterogeneous, containing slight
modifications of the repeating unit. Depending on the structure, these macromolecules can have distinct properties from their monosaccharide building blocks. They may be amorphous or even insoluble in water.
When all the monosaccharides in a polysaccharide are the same type, the polysaccharide is called a homopolysaccharide or homoglycan, but when more than one type of monosaccharide is present, they are called heteropolysaccharides or heteroglycans.
Natural saccharides are generally composed of simple carbohydrates called monosaccharides with general formula (CH2O)n where n is three or more. Examples of monosaccharides are glucose, fructose, and glyceraldehyde. Polysaccharides, meanwhile, have a general formula of Cx(H2O)y where x and y are usually large numbers between 200 and 2500. When the repeating units in the polymer backbone are six-carbon monosaccharides, as is often the case, the general formula simplifies to (C6H10O5)n, where typically 40 ≤ n ≤ 3000.
As a rule of thumb, polysaccharides contain more than ten monosaccharide units, whereas oligosaccharides
contain three to ten monosaccharide units, but the precise cutoff
varies somewhat according to the convention. Polysaccharides are an
important class of biological polymers. Their function in living organisms is usually either structure- or storage-related. Starch (a polymer of glucose) is used as a storage polysaccharide in plants, being found in the form of both amylose and the branched amylopectin. In animals, the structurally similar glucose polymer is the more densely branched glycogen,
sometimes called "animal starch". Glycogen's properties allow it to be
metabolized more quickly, which suits the active lives of moving
animals. In bacteria, they play an important role in bacterial multicellularity.
Cellulose and chitin are examples of structural polysaccharides. Cellulose is used in the cell walls of plants and other organisms and is said to be the most abundant organic molecule on Earth.
It has many uses such as a significant role in the paper and textile
industries and is used as a feedstock for the production of rayon (via
the viscose process), cellulose acetate, celluloid, and nitrocellulose. Chitin has a similar structure but has nitrogen-containing side branches, increasing its strength. It is found in arthropodexoskeletons and in the cell walls of some fungi. It also has multiple uses, including surgical threads. Polysaccharides also include callose or laminarin, chrysolaminarin, xylan, arabinoxylan, mannan, fucoidan and galactomannan.
Function
Structure
Nutrition
polysaccharides are common sources of energy. Many organisms can easily
break down starches into glucose; however, most organisms cannot
metabolize cellulose or other polysaccharides like cellulose, chitin, and arabinoxylans. Some bacteria and protists can metabolize these carbohydrate types. Ruminants and termites, for example, use microorganisms to process cellulose.
Even though these complex polysaccharides are not very digestible, they provide important dietary elements for humans. Called dietary fiber, these carbohydrates enhance digestion. The main action of dietary fiber is to change the nature of the contents of the gastrointestinal tract and how other nutrients and chemicals are absorbed.Soluble fiber binds to bile acids in the small intestine, making them less likely to enter the body; this, in turn, lowers cholesterol levels in the blood.
Soluble fiber also attenuates the absorption of sugar, reduces sugar
response after eating, normalizes blood lipid levels and, once fermented
in the colon, produces short-chain fatty acids
as byproducts with wide-ranging physiological activities (discussion
below). Although insoluble fiber is associated with reduced diabetes
risk, the mechanism by which this occurs is unknown.
Not yet formally proposed as an essential macronutrient (as of
2005), dietary fiber is nevertheless regarded as important for the diet,
with regulatory authorities in many developed countries recommending
increases in fiber intake.
Starch is a glucose polymer in which glucopyranose units are bonded by alpha-linkages. It is made up of a mixture of amylose (15–20%) and amylopectin
(80–85%). Amylose consists of a linear chain of several hundred glucose
molecules, and Amylopectin is a branched molecule made of several
thousand glucose units (every chain of 24–30 glucose units is one unit
of Amylopectin). Starches are insoluble in water. They can be digested by breaking the alpha-linkages (glycosidic bonds). Both humans and other animals have amylases so that they can digest starches. Potato, rice, wheat, and maize are major sources of starch in the human diet. The formations of starches are the ways that plants store glucose.
Glycogen serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue. Glycogen is made primarily by the liver, and the muscles but can also be made by glycogenesis within the brain and stomach.
Glycogen is analogous to starch, a glucose polymer in plants, and is sometimes referred to as animal starch, having a similar structure to amylopectin
but more extensively branched and compact than starch. Glycogen is a
polymer of α(1→4) glycosidic bonds linked with α(1→6)-linked branches.
Glycogen is found in the form of granules in the cytosol/cytoplasm in many cell types and plays an important role in the glucose cycle. Glycogen forms an energy
reserve that can be quickly mobilized to meet a sudden need for
glucose, but one that is less compact and more immediately available as
an energy reserve than triglycerides (lipids).
In the liver hepatocytes, glycogen can compose up to 8 percent (100–120 grams in an adult) of the fresh weight soon after a meal. Only the glycogen stored in the liver can be made accessible to other organs. In the muscles, glycogen is found in a low concentration of one to two percent of the muscle mass. The amount of glycogen stored in the body—especially within the muscles, liver, and red blood cells—varies with physical activity, basal metabolic rate, and eating habits such as intermittent fasting. Small amounts of glycogen are found in the kidneys and even smaller amounts in certain glial cells in the brain and white blood cells. The uterus also stores glycogen during pregnancy to nourish the embryo.
Glycogen is composed of a branched chain of glucose residues. It is stored in liver and muscles.
It is an energy reserve for animals.
It is the chief form of carbohydrate stored in animal organisms.
It is insoluble in water. It turns brown-red when mixed with iodine.
Schematic 2-D cross-sectional view of glycogen. A core protein of glycogenin is surrounded by branches of glucose units. The entire globular granule may contain approximately 30,000 glucose units.
A view of the atomic structure of a single branched strand of glucose units in a glycogen molecule.
Galactogen
Galactogen is a polysaccharide of galactose that functions as energy storage in pulmonate snails and some Caenogastropoda.
This polysaccharide is exclusive of the reproduction and is only found
in the albumen gland from the female snail reproductive system and in
the perivitelline fluid of eggs. Furthermore, galactogen serves as an energy reserve for developing embryos and hatchlings, which is later replaced by glycogen in juveniles and adults.
Formed by crosslinking polysaccharide-based nanoparticles
and functional polymers, galactogens have applications within hydrogel
structures. These hydrogel structures can be designed to release
particular nanoparticle pharmaceuticals and/or encapsulated therapeutics
over time or in response to environmental stimuli.
Galactogens are polysaccharides with binding affinity for bioanalytes.
With this, by end-point attaching galactogens to other polysaccharides
constituting the surface of medical devices, galactogens have use as a
method of capturing bioanalytes (e.g., CTC's), a method for releasing
the captured bioanalytes and an analysis method.
Inulin is a naturally occurring polysaccharide complex carbohydrate composed of fructose, a plant-derived food that human digestive enzymes cannot completely break down. The inulins belong to a class of dietary fibers known as fructans. Inulin is used by some plants as a means of storing energy and is typically found in roots or rhizomes. Most plants that synthesize and store inulin do not store other forms of carbohydrates such as starch. In the United States in 2018, the Food and Drug Administration approved inulin as a dietary fiber ingredient used to improve the nutritional value of manufactured food products.
Structural polysaccharides
Some important natural structural polysaccharides
Arabinoxylans
Arabinoxylans are found in both the primary and secondary cell walls of plants and are the copolymers of two sugars: arabinose and xylose. They may also have beneficial effects on human health.
Cellulose
The structural components of plants are formed primarily from cellulose. Wood is largely cellulose and lignin, while paper and cotton are nearly pure cellulose. Cellulose is a polymer made with repeated glucose units bonded together by beta-linkages. Humans and many animals lack an enzyme to break the beta-linkages, so they do not digest cellulose. Certain animals, such as termites
can digest cellulose, because bacteria possessing the enzyme are
present in their gut. Cellulose is insoluble in water. It does not
change color when mixed with iodine. On hydrolysis, it yields glucose.
It is the most abundant carbohydrate in nature.
Chitin
Chitin is one of many naturally occurring polymers. It forms a structural component of many animals, such as exoskeletons. Over time it is bio-degradable in the natural environment. Its breakdown may be catalyzed by enzymes called chitinases, secreted by microorganisms such as bacteria and fungi and produced by some plants. Some of these microorganisms have receptors to simple sugars from the decomposition of chitin. If chitin is detected, they then produce enzymes to digest it by cleaving the glycosidic bonds in order to convert it to simple sugars and ammonia.
Chemically, chitin is closely related to chitosan (a more water-soluble derivative of chitin). It is also closely related to cellulose in that it is a long unbranched chain of glucose derivatives. Both materials contribute structure and strength, protecting the organism.
Pectins
Pectins are a family of complex polysaccharides that contain 1,4-linked α-D-galactosyl uronic acid residues. They are present in most primary cell walls and in the nonwoody parts of terrestrial plants.
Acidic polysaccharides
Acidic polysaccharides are polysaccharides that contain carboxyl groups, phosphate groups and/or sulfuric ester groups.
Polysaccharides containing sulfate groups can be isolated from algae or obtained by chemical modification.
Polysaccharides are major classes of biomolecules. They are long
chains of carbohydrate molecules, composed of several smaller
monosaccharides. These complex bio-macromolecules functions as an
important source of energy in animal cell and form a structural
component of a plant cell. It can be a homopolysaccharide or a
heteropolysaccharide depending upon the type of the monosaccharides.
Polysaccharides can be a straight chain of monosaccharides known
as linear polysaccharides, or it can be branched known as a branched
polysaccharide.
Bacterial polysaccharides
Pathogenic bacteria commonly produce a bacterial capsule, a thick, mucous-like, layer of polysaccharide. The capsule cloaks antigenicproteins
on the bacterial surface that would otherwise provoke an immune
response and thereby lead to the destruction of the bacteria. Capsular
polysaccharides are water-soluble, commonly acidic, and have molecular weights on the order of 100,000 to 2,000,000 daltons. They are linear and consist of regularly repeating subunits of one to six monosaccharides. There is enormous structural diversity; nearly two hundred different polysaccharides are produced by E. coli alone. Mixtures of capsular polysaccharides, either conjugated or native, are used as vaccines.
Bacteria and many other microbes, including fungi and algae,
often secrete polysaccharides to help them adhere to surfaces and to
prevent them from drying out. Humans have developed some of these
polysaccharides into useful products, including xanthan gum, dextran, welan gum, gellan gum, diutan gum and pullulan.
Most of these polysaccharides exhibit useful visco-elastic properties when dissolved in water at very low levels.
This makes various liquids used in everyday life, such as some foods,
lotions, cleaners, and paints, viscous when stationary, but much more
free-flowing when even slight shear is applied by stirring or shaking,
pouring, wiping, or brushing. This property is named pseudoplasticity or
shear thinning; the study of such matters is called rheology.
Aqueous solutions of the polysaccharide alone have a curious behavior
when stirred: after stirring ceases, the solution initially continues
to swirl due to momentum, then slows to a standstill due to viscosity
and reverses direction briefly before stopping. This recoil is due to
the elastic effect of the polysaccharide chains, previously stretched in
solution, returning to their relaxed state.
Cell-surface polysaccharides play diverse roles in bacterial ecology and physiology. They serve as a barrier between the cell wall and the environment, mediate host-pathogen interactions. Polysaccharides also play an important role in formation of biofilms and the structuring of complex life forms in bacteria like Myxococcus xanthus.
These polysaccharides are synthesized from nucleotide-activated precursors (called nucleotide sugars)
and, in most cases, all the enzymes necessary for biosynthesis,
assembly and transport of the completed polymer are encoded by genes
organized in dedicated clusters within the genome of the organism. Lipopolysaccharide
is one of the most important cell-surface polysaccharides, as it plays a
key structural role in outer membrane integrity, as well as being an
important mediator of host-pathogen interactions.
The enzymes that make the A-band (homopolymeric) and B-band (heteropolymeric) O-antigens have been identified and the metabolic pathways defined. The exopolysaccharide alginate is a linear copolymer of β-1,4-linked D-mannuronic acid and L-guluronic acid residues, and is responsible for the mucoid phenotype of late-stage cystic fibrosis disease. The pel and psl loci are two recently discovered gene clusters that also encode exopolysaccharides found to be important for biofilm formation. Rhamnolipid is a biosurfactant whose production is tightly regulated at the transcriptional level, but the precise role that it plays in disease is not well understood at present. Protein glycosylation, particularly of pilin and flagellin,
became a focus of research by several groups from about 2007, and has
been shown to be important for adhesion and invasion during bacterial
infection.
Chemical identification tests for polysaccharides
Periodic acid-Schiff stain (PAS)
Polysaccharides with unprotected vicinal diols or amino sugars (where some hydroxyl groups are replaced with amines) give a positive periodic acid-Schiff stain (PAS). The list of polysaccharides that stain with PAS is long. Although mucins
of epithelial origins stain with PAS, mucins of connective tissue
origin have so many acidic substitutions that they do not have enough
glycol or amino-alcohol groups left to react with PAS.
Derivatives
By
chemical modifications certain properties of polysaccharides can be
improved. Various ligands can be covalently attached to their hydroxyl
groups. Due to the covalent attachment of methyl-, hydroxyethyl- or
carboxymethyl- groups on cellulose, for instance, high swelling properties in aqueous media can be introduced. Another example are thiolated polysaccharides ( see thiomers). Thiol groups are covalently attached to polysaccharides such as hyaluronic acid or chitosan.
As thiolated polysaccharides can crosslink via disulfide bond
formation, they form stable three-dimensional networks. Furthermore,
they can bind to cysteine subunits of proteins via disulfide bonds.
Because of these bonds polysaccharides can be covalently attached to
endogenous proteins such as mucins or keratins.
A Tektronix model 475A portable analog oscilloscope, a typical instrument of the late 1970sOscilloscope cathode-ray tube, the left square-shaped end would be the blue screen in the upper device when built in.Typical display of an analog oscilloscope measuring a sine wave signal with 10 kHz.
From the grid inherent to the screen together with the user-set
parameters of the device shown at the upper display rim, the user may
calculate the frequency and the voltage of the measured signal. Modern
digital oscilloscopes set the measurement parameters and
calculate/display the signal values automatically.
An oscilloscope (informally scope or O-scope) is a type of electronic test instrument that graphically displays varying voltages
of one or more signals as a function of time. Their main purpose is
capturing information on electrical signals for debugging, analysis, or
characterization. The displayed waveform can then be analyzed for properties such as amplitude, frequency, rise time, time interval, distortion,
and others. Originally, calculation of these values required manually
measuring the waveform against the scales built into the screen of the
instrument. Modern digital instruments may calculate and display these properties directly.
Oscilloscopes are used in the sciences, engineering, biomedical,
automotive and the telecommunications industry. General-purpose
instruments are used for maintenance of electronic equipment and
laboratory work. Special-purpose oscilloscopes may be used to analyze an
automotive ignition system or to display the waveform of the heartbeat
as an electrocardiogram, for instance.
Early high-speed visualisations of electrical voltages were made with an electro-mechanical oscillograph.
These gave valuable insights into high speed voltage changes, but had a
very low frequency response, and were superseded by the oscilloscope
which used a cathode ray tube (CRT) as its display element.
The Braun tube, forerunner of the CRT, was known in 1897, and in 1899 Jonathan Zenneck equipped it with beam-forming plates and a magnetic field for deflecting the trace, and this formed the basis of the CRT.
Early cathode ray tubes had been applied experimentally to laboratory
measurements as early as the 1920s, but suffered from poor stability of
the vacuum and the cathode emitters. V. K. Zworykin
described a permanently sealed, high-vacuum cathode ray tube with a
thermionic emitter in 1931. This stable and reproducible component
allowed General Radio to manufacture an oscilloscope that was usable outside a laboratory setting.
After World War II surplus electronic parts became the basis for the revival of Heathkit Corporation, and a $50 oscilloscope kit made from such parts proved its premiere market success.
Features and uses
Oscilloscope showing a trace with standard inputs and controls
An analog oscilloscope is typically divided into four sections: the
display, vertical controls, horizontal controls and trigger controls.
The display is usually a CRT with horizontal and vertical reference lines called the graticule. CRT displays also have controls for focus, intensity, and beam finder.
The vertical section controls the amplitude of the displayed
signal. This section has 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 can be
configured to respond to an internal or external event. The principal
controls of this section are the source and coupling selector switches,
and an external trigger input (EXT Input) and level adjustment.
In addition to the basic instrument, most oscilloscopes are
supplied with a probe. The probe connects to any input on the instrument
and typically has a resistor of ten times the oscilloscope's input
impedance. This results in a 0.1 (‑10×) attenuation factor; this 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.
Size and portability
Most
modern oscilloscopes are lightweight, portable instruments compact
enough for a single person to carry. In addition to portable units, the
market offers a number of miniature battery-powered instruments for
field service applications. Laboratory grade oscilloscopes, especially
older units that use vacuum tubes, are generally bench-top devices or are mounted on dedicated carts. Special-purpose oscilloscopes may be rack-mounted or permanently mounted into a custom instrument housing.
Inputs
The signal to be measured is fed to one of the input connectors, which is usually a coaxial connector such as a BNC or UHF type. 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 specialized 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 usually present an input impedance of 1 megohm in parallel with a small but known capacitance such as 20 picofarads. This allows the use of standard oscilloscope probes.
Scopes for use with very high frequencies may have 50 Ω inputs. These
must be either connected directly to a 50 Ω signal source or used with Z0 or active probes.
Less-frequently-used inputs include one (or two) for triggering
the sweep, horizontal deflection for X‑Y mode displays, and trace
brightening/darkening, sometimes called z'‑axis inputs.
Open wire test leads (flying leads) are likely to pick up
interference, so they are not suitable for low level signals.
Furthermore, the leads have a high inductance, so they are not suitable
for high frequencies. Using a shielded cable (i.e., coaxial cable) is
better for low level signals. Coaxial cable also has lower inductance,
but it has higher capacitance: a typical 50 ohm cable has about 90 pF
per meter. Consequently, a one-meter direct (1×) coaxial probe loads a
circuit with a capacitance of about 110 pF and a resistance of 1 megohm.
To minimize loading, attenuator probes (e.g., 10× probes) are
used. A typical probe uses a 9 megohm series resistor shunted by a
low-value capacitor to make an RC compensated divider with the cable
capacitance and scope input. The RC time constants are adjusted to
match. For example, the 9 megohm series resistor is shunted by a 12.2 pF
capacitor for a time constant of 110 microseconds. The cable
capacitance of 90 pF in parallel with the scope input of 20 pF and
1 megohm (total capacitance 110 pF) also gives a time constant of 110
microseconds. In practice, there is an adjustment so the operator can
precisely match the low frequency time constant (called compensating the
probe). Matching the time constants makes the attenuation independent
of frequency. At low frequencies (where the resistance of R is much less than the reactance of C),
the circuit looks like a resistive divider; at high frequencies
(resistance much greater than reactance), the circuit looks like a
capacitive divider.
The result is a frequency compensated probe for modest
frequencies. It presents a load of about 10 megohms shunted by 12 pF.
Such a probe is an improvement, but does not work well when the time
scale shrinks to several cable transit times or less (transit time is
typically 5 ns).
In that time frame, the cable looks like its characteristic impedance,
and reflections from the transmission line mismatch at the scope input
and the probe causes ringing.
The modern scope probe uses lossy low capacitance transmission lines
and sophisticated frequency shaping networks to make the 10× probe
perform well at several hundred megahertz. Consequently, there are other
adjustments for completing the compensation.
Probes with 10:1 attenuation are by far the most common; for
large signals (and slightly-less capacitive loading), 100:1 probes may
be used. There are also probes that contain switches to select 10:1 or
direct (1:1) ratios, but the latter setting has significant capacitance
(tens of pF) at the probe tip, because the whole cable's capacitance is
then directly connected.
Most oscilloscopes provide for probe attenuation factors,
displaying the effective sensitivity at the probe tip. Historically,
some auto-sensing circuitry used indicator lamps behind translucent
windows in the panel to illuminate different parts of the sensitivity
scale. To do so, the probe connectors (modified BNCs) had an extra
contact to define the probe's attenuation. (A certain value of resistor,
connected to ground, "encodes" the attenuation.) Because probes wear
out, and because the auto-sensing circuitry is not compatible between
different oscilloscope makes, auto-sensing probe scaling is not
foolproof. Likewise, manually setting the probe attenuation is prone to
user error. Setting the probe scaling incorrectly is a common error, and
throws the reading off by a factor of 10.
Special high voltage probes
form compensated attenuators with the oscilloscope input. These have a
large probe body, and some require partly filling a canister surrounding
the series resistor with volatile liquid fluorocarbon
to displace air. The oscilloscope end has a box with several
waveform-trimming adjustments. For safety, a barrier disc keeps the
user's fingers away from the point being examined. Maximum voltage is in
the low tens of kV. (Observing a high voltage ramp can create a
staircase waveform with steps at different points every repetition,
until the probe tip is in contact. Until then, a tiny arc charges the
probe tip, and its capacitance holds the voltage (open circuit). As the
voltage continues to climb, another tiny arc charges the tip further.)
There are also current probes, with cores that surround the
conductor carrying current to be examined. One type has a hole for the
conductor, and requires that the wire be passed through the hole for
semi-permanent or permanent mounting. However, other types, used for
temporary testing, have a two-part core that can be clamped around a
wire. Inside the probe, a coil wound around the core provides a current
into an appropriate load, and the voltage across that load is
proportional to current. This type of probe only senses AC.
A more-sophisticated probe includes a magnetic flux sensor (Hall effect
sensor) in the magnetic circuit. The probe connects to an amplifier,
which feeds (low frequency) current into the coil to cancel the sensed
field; the magnitude of the current provides the low-frequency part of
the current waveform, right down to DC. The coil still picks up high
frequencies. There is a combining network akin to a loudspeaker
crossover.
Front panel controls
Focus control
This
control adjusts CRT focus to obtain the sharpest, most-detailed trace.
In practice, focus must be adjusted slightly when observing very
different signals, so it must be an external control. The control varies
the voltage applied to a focusing anode within the CRT. Flat-panel
displays do not need this control.
Intensity control
This
adjusts trace brightness. Slow traces on CRT oscilloscopes need less,
and fast ones, especially if not often repeated, require more
brightness. On flat panels, however, trace brightness is essentially
independent of sweep speed, because the internal signal processing
effectively synthesizes the display from the digitized data.
Astigmatism
This
control may instead be called "shape" or "spot shape". It adjusts the
voltage on the last CRT anode (immediately next to the Y deflection
plates). For a circular spot, the final anode must be at the same
potential as both of the Y-plates (for a centred spot the Y-plate
voltages must be the same). If the anode is made more positive, the spot
becomes elliptical in the X-plane as the more negative Y-plates will
repel the beam. If the anode is made more negative, the spot becomes
elliptical in the Y-plane as the more positive Y-plates will attract the
beam. This control may be absent from simpler oscilloscope designs or
may even be an internal control. It is not necessary with flat panel
displays.
Beam finder
Modern
oscilloscopes have direct-coupled deflection amplifiers, which means
the trace could be deflected off-screen. They also might have their beam
blanked without the operator knowing it. To help in restoring a visible
display, the beam finder circuit overrides any blanking and limits the
beam deflection to the visible portion of the screen. Beam-finder
circuits often distort the trace while activated.
Graticule
The
graticule is a grid of lines that serve as reference marks for
measuring the displayed trace. These markings, whether located directly
on the screen or on a removable plastic filter, usually consist of a
1 cm grid with closer tick marks (often at 2 mm) on the centre vertical
and horizontal axis. One expects to see ten major divisions across the
screen; the number of vertical major divisions varies. Comparing the
grid markings with the waveform permits one to measure both voltage
(vertical axis) and time (horizontal axis). Frequency can also be
determined by measuring the waveform period and calculating its
reciprocal.
On old and lower-cost CRT oscilloscopes the graticule is a sheet
of plastic, often with light-diffusing markings and concealed lamps at
the edge of the graticule. The lamps had a brightness control.
Higher-cost instruments have the graticule marked on the inside face of
the CRT, to eliminate parallax errors;
better ones also had adjustable edge illumination with diffusing
markings. (Diffusing markings appear bright.) Digital oscilloscopes,
however, generate the graticule markings on the display in the same way
as the trace.
External graticules also protect the glass face of the CRT from
accidental impact. Some CRT oscilloscopes with internal graticules have
an unmarked tinted sheet plastic light filter to enhance trace contrast;
this also serves to protect the faceplate of the CRT.
Accuracy and resolution of measurements using a graticule is
relatively limited; better instruments sometimes have movable bright
markers on the trace. These permit internal circuits to make more
refined measurements.
Both calibrated vertical sensitivity and calibrated horizontal time are set in 1 – 2 – 5 – 10 steps. This leads, however, to some awkward interpretations of minor divisions.
Digital oscilloscopes generate the graticule digitally. The
scale, spacing, etc., of the graticule can therefore be varied, and
accuracy of readings may be improved.
Timebase controls
Computer model of the impact of increasing the timebase time/division
These select the horizontal speed of the CRT's spot as it creates the
trace; this process is commonly referred to as the sweep. In all but
the least-costly modern oscilloscopes, the sweep speed is selectable and
calibrated in units of time per major graticule division. Quite a wide
range of sweep speeds is generally provided, from seconds to as fast as
picoseconds (in the fastest) per division. Usually, a
continuously-variable control (often a knob in front of the calibrated
selector knob) offers uncalibrated speeds, typically slower than
calibrated. This control provides a range somewhat greater than the
calibrated steps, making any speed between the steps available.
Holdoff control
Some
higher-end analog oscilloscopes have a holdoff control. This sets a
time after a trigger during which the sweep circuit cannot be triggered
again. It helps provide a stable display of repetitive events in which
some triggers would create confusing displays. It is usually set to
minimum, because a longer time decreases the number of sweeps per
second, resulting in a dimmer trace. See Holdoff for a more detailed description.
Vertical sensitivity, coupling, and polarity controls
To
accommodate a wide range of input amplitudes, a switch selects
calibrated sensitivity of the vertical deflection. Another control,
often in front of the calibrated selector knob, offers a continuously
variable sensitivity over a limited range from calibrated to
less-sensitive settings.
Often the observed signal is offset by a steady component, and
only the changes are of interest. An input coupling switch in the "AC"
position connects a capacitor in series with the input that blocks
low-frequency signals and DC. However, when the signal has a fixed
offset of interest, or changes slowly, the user will usually prefer "DC"
coupling, which bypasses any such capacitor. Most oscilloscopes offer
the DC input option. For convenience, to see where zero volts input
currently shows on the screen, many oscilloscopes have a third switch
position (usually labeled "GND" for ground) that disconnects the input
and grounds it. Often, in this case, the user centers the trace with the
vertical position control.
Better oscilloscopes have a polarity selector. Normally, a
positive input moves the trace upward; the polarity selector offers an
"inverting" option, in which a positive-going signal deflects the trace
downward.
Vertical position control
Computer model of vertical position y offset varying in a sine wave
The vertical position control moves the whole displayed trace up and
down. It is used to set the no-input trace exactly on the center line of
the graticule, but also permits offsetting vertically by a limited
amount. With direct coupling, adjustment of this control can compensate
for a limited DC component of an input.
Horizontal sensitivity control
This
control is found only on more elaborate oscilloscopes; it offers
adjustable sensitivity for external horizontal inputs. It is only active
when the instrument is in X-Y mode, i.e. the internal horizontal sweep
is turned off.
Horizontal position control
Computer model of horizontal position control from x offset increasing
The horizontal position control moves the display sidewise. It usually
sets the left end of the trace at the left edge of the graticule, but it
can displace the whole trace when desired. This control also moves the
X-Y mode traces sidewise in some instruments, and can compensate for a
limited DC component as for vertical position.
Dual-trace controls green trace = y = 30 sin(0.1t) + 0.5teal trace = y = 30 sin(0.3t)
Each input channel usually has its own set of sensitivity, coupling,
and position controls, though some four-trace oscilloscopes have only
minimal controls for their third and fourth channels.
Dual-trace oscilloscopes have a mode switch to select either
channel alone, both channels, or (in some) an X‑Y display, which uses
the second channel for X deflection. When both channels are displayed,
the type of channel switching can be selected on some oscilloscopes; on
others, the type depends upon timebase setting. If manually selectable,
channel switching can be free-running (asynchronous), or between
consecutive sweeps. Some Philips dual-trace analog oscilloscopes had a
fast analog multiplier, and provided a display of the product of the
input channels.
Multiple-trace oscilloscopes have a switch for each channel to enable or disable display of the channel's trace.
These include controls for the delayed-sweep timebase, which is
calibrated, and often also variable. The slowest speed is several steps
faster than the slowest main sweep speed, though the fastest is
generally the same. A calibrated multiturn delay time control offers
wide range, high resolution delay settings; it spans the full duration
of the main sweep, and its reading corresponds to graticule divisions
(but with much finer precision). Its accuracy is also superior to that
of the display.
A switch selects display modes: Main sweep only, with a
brightened region showing when the delayed sweep is advancing, delayed
sweep only, or (on some) a combination mode.
Good CRT oscilloscopes include a delayed-sweep intensity control,
to allow for the dimmer trace of a much-faster delayed sweep which
nevertheless occurs only once per main sweep. Such oscilloscopes also
are likely to have a trace separation control for multiplexed display of
both the main and delayed sweeps together.
A switch selects the trigger source. It can be an external input, one
of the vertical channels of a dual or multiple-trace oscilloscope, or
the AC line (mains) frequency. Another switch enables or disables auto
trigger mode, or selects single sweep, if provided in the oscilloscope.
Either a spring-return switch position or a pushbutton arms single
sweeps.
A trigger level control varies the voltage required to generate a
trigger, and the slope switch selects positive-going or negative-going
polarity at the selected trigger level.
Basic types of sweep
Triggered sweep
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 older, simpler
oscilloscopes with continuously-running sweep oscillators,
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 blanks the beam and starts 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 useful to make the
sweep ignore interfering triggers that occur before the events to be
observed. In the case of repetitive, but complex waveforms, variable
holdoff can provide a stable display that could not otherwise be
achieved.
Holdoff
Trigger holdoff
defines a certain period following a trigger during which the sweep
cannot be triggered again. This makes it easier to establish a stable
view of a waveform with multiple edges, which would otherwise cause additional triggers.
Example
Imagine the following repeating waveform:
The green line is the waveform, the red vertical partial line represents
the location of the trigger, and the yellow line represents the trigger
level. If the scope was simply set to trigger on every rising edge,
this waveform would cause three triggers for each cycle:
Assuming the signal is fairly high frequency, the scope display would probably look something like this:
On an actual scope, each trigger would be the same channel, so all would be the same color.
It is desirable for the scope to trigger on only one edge per
cycle, so it is necessary to set the holdoff at slightly less than the
period of the waveform. This prevents triggering from occurring more
than once per cycle, but still lets it trigger on the first edge of the
next cycle.
Automatic sweep mode
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. This is referred to as "auto
sweep" or "automatic sweep" in the controls. Once triggers arrive, the
timer stops providing pseudo-triggers. The user will usually disable
automatic sweep when observing low repetition rates.
Recurrent sweeps
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
Some
oscilloscopes offer these. The user manually arms the sweep circuit
(typically by a pushbutton or equivalent). "Armed" means it's ready to
respond to a trigger. Once the sweep completes, it resets, and does not
sweep again 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, also known as TV 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, though 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
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 this.
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, though 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 can display waveforms this way, without offering a delayed timebase
as such.
Dual and multiple-trace oscilloscopes
Oscilloscopes with two vertical inputs, referred to as dual-trace oscilloscopes, are extremely useful and commonplace.
Using a single-beam CRT, they multiplex
the inputs, usually switching between them fast enough to display two
traces apparently at once. Less common are oscilloscopes with more
traces; four inputs are common among these, but a few (Kikusui, for one)
offered a display of the sweep trigger signal if desired. Some
multi-trace oscilloscopes use the external trigger input as an optional
vertical input, and some have third and fourth channels with only
minimal controls. In all cases, the inputs, when independently
displayed, are time-multiplexed, but dual-trace oscilloscopes often can
add their inputs to display a real-time analog sum. Inverting one
channel while adding them together results in a display of the
differences between them, provided neither channel is overloaded. This
difference mode can provide a moderate-performance differential input.)
Switching channels can be asynchronous, i.e. free-running, with
respect to the sweep frequency; or it can be done after each horizontal
sweep is complete. Asynchronous switching is usually designated
"Chopped", while sweep-synchronized is designated "Alt[ernate]". A given
channel is alternately connected and disconnected, leading to the term
"chopped". Multi-trace oscilloscopes also switch channels either in
chopped or alternate modes.
In general, chopped mode is better for slower sweeps. It is
possible for the internal chopping rate to be a multiple of the sweep
repetition rate, creating blanks in the traces, but in practice this is
rarely a problem. The gaps in one trace are overwritten by traces of the
following sweep. A few oscilloscopes had a modulated chopping rate to
avoid this occasional problem. Alternate mode, however, is better for
faster sweeps.
True dual-beam CRT oscilloscopes did exist, but were not common.
One type (Cossor, U.K.) had a beam-splitter plate in its CRT, and
single-ended deflection following the splitter. Others had two complete electron guns,
requiring tight control of axial (rotational) mechanical alignment in
manufacturing the CRT. Beam-splitter types had horizontal deflection
common to both vertical channels, but dual-gun oscilloscopes could have
separate time bases, or use one time base for both channels.
Multiple-gun CRTs (up to ten guns) were made in past decades. With ten
guns, the envelope (bulb) was cylindrical throughout its length. (Also
see "CRT Invention" in Oscilloscope history.)
The vertical amplifier
In
an analog oscilloscope, the vertical amplifier acquires the signal[s]
to be displayed and provides a signal large enough to deflect the CRT's
beam. In better oscilloscopes, it delays the signal by a fraction of a
microsecond. The maximum deflection is at least somewhat beyond the
edges of the graticule, and more typically some distance off-screen. The
amplifier has to have low distortion to display its input accurately
(it must be linear), and it has to recover quickly from overloads. As
well, its time-domain response has to represent transients
accurately—minimal overshoot, rounding, and tilt of a flat pulse top.
A vertical input goes to a frequency-compensated step attenuator
to reduce large signals to prevent overload. The attenuator feeds one or
more low-level stages, which in turn feed gain stages (and a delay-line
driver if there is a delay). Subsequent gain stages lead to the final
output stage, which develops a large signal swing (tens of volts,
sometimes over 100 volts) for CRT electrostatic deflection.
In dual and multiple-trace oscilloscopes, an internal electronic
switch selects the relatively low-level output of one channel's
early-stage amplifier and sends it to the following stages of the
vertical amplifier.
In free-running ("chopped") mode, the oscillator (which may be
simply a different operating mode of the switch driver) blanks the beam
before switching, and unblanks it only after the switching transients
have settled.
Part way through the amplifier is a feed to the sweep trigger
circuits, for internal triggering from the signal. This feed would be
from an individual channel's amplifier in a dual or multi-trace
oscilloscope, the channel depending upon the setting of the trigger
source selector.
This feed precedes the delay (if there is one), which allows the
sweep circuit to unblank the CRT and start the forward sweep, so the CRT
can show the triggering event. High-quality analog delays add a modest
cost to an oscilloscope, and are omitted in cost-sensitive
oscilloscopes.
The delay, itself, comes from a special cable with a pair of
conductors wound around a flexible, magnetically soft core. The coiling
provides distributed inductance, while a conductive layer close to the
wires provides distributed capacitance. The combination is a wideband
transmission line with considerable delay per unit length. Both ends of
the delay cable require matched impedances to avoid reflections.
X-Y mode
A 24-hour clock displayed on a CRT oscilloscope configured in X-Y mode as a vector monitor with dual R2R DACs to generate the analog voltages
Most modern oscilloscopes have several inputs for voltages, and thus
can be used to plot one varying voltage versus another. This is
especially useful for graphing I-V curves (current versus voltage characteristics) for components such as diodes, as well as Lissajous patterns. Lissajous figures are an example of how an oscilloscope can be used to track phase differences between multiple input signals. This is very frequently used in broadcast engineering to plot the left and right stereophonic channels, to ensure that the stereo generator is calibrated
properly. Historically, stable Lissajous figures were used to show that
two sine waves had a relatively simple frequency relationship, a
numerically-small ratio. They also indicated phase difference between
two sine waves of the same frequency.
The X-Y mode also lets the oscilloscope serve as a vector monitor to display images or user interfaces. Many early games, such as Tennis for Two, used an oscilloscope as an output device.
Complete loss of signal in an X-Y CRT display means that the beam
is stationary, striking a small spot. This risks burning the phosphor
if the brightness is too high. Such damage was more common in older
scopes as the phosphors previously used burned more easily. Some
dedicated X-Y displays reduce beam current greatly, or blank the display
entirely, if there are no inputs present.
Z input
Some
analogue oscilloscopes feature a Z input. This is generally an input
terminal that connects directly to the CRT grid (usually via a coupling
capacitor). This allows an external signal to either increase (if
positive) or decrease (if negative) the brightness of the trace, even
allowing it to be totally blanked. The voltage range to achieve cut-off
to a brightened display is of the order of 10–20 volts depending on the
CRT characteristics.
An example of a practical application is if a pair of sine waves
of known frequency are used to generate a circular Lissajous figure and a
higher unknown frequency is applied to the Z input. This turns the
continuous circle into a circle of dots. The number of dots multiplied
by the X-Y frequency gives the Z frequency. This technique only works if
the Z frequency is an integer ratio of the X-Y frequency and only if it
is not so large that the dots become so numerous that they are
difficult to count.
Bandwidth
As with all practical instruments, oscilloscopes do not respond equally to all possible input frequencies. The range of sinusoid frequencies an oscilloscope can usefully display is referred to as its bandwidth.
Bandwidth applies primarily to the Y-axis, though the X-axis sweeps
must be fast enough to show the highest-frequency waveforms.
The bandwidth is defined as the frequency at which the sensitivity is 0.707 of the sensitivity at DC or the lowest AC frequency
(a drop of 3 dB).
The oscilloscope's response drops off rapidly as the input frequency
rises above that point. Within the stated bandwidth the response is not
necessarily exactly uniform (or "flat"), but should always fall within a
+0 to −3 dB range. One source
says there is a noticeable effect on the accuracy of voltage
measurements at only 20 percent of the stated bandwidth. Some
oscilloscopes' specifications do include a narrower tolerance range
within the stated bandwidth.
Probes also have bandwidth limits and must be chosen and used to
handle the frequencies of interest properly. To achieve the flattest
response, most probes must be "compensated" (an adjustment performed
using a test signal from the oscilloscope) to allow for the reactance of the probe's cable.
Another related specification is rise time.
This is the time taken between 10% and 90% of the maximum amplitude
response at the leading edge of a pulse. It is related to the bandwidth
approximately by:
Bandwidth in Hz × rise time in seconds = 0.35.
For example, an oscilloscope with a rise time of 1 nanosecond would have a bandwidth of 350 MHz.
In analog instruments, the bandwidth of the oscilloscope is
limited by the vertical amplifiers and the CRT or other display
subsystem. In digital instruments, the sampling rate of the analog-to-digital converter
(ADC) is a factor, but the stated analog bandwidth (and therefore the
overall bandwidth of the instrument) is usually less than the ADC's Nyquist frequency. This is due to limitations in the analog signal amplifier, deliberate design of the anti-aliasing filter that precedes the ADC, or both.
For a digital oscilloscope, a rule of thumb is that the
continuous sampling rate should be ten times the highest frequency
desired to resolve; for example a 20 megasample/second rate would be
applicable for measuring signals up to about 2 MHz. This lets the
anti-aliasing filter be designed with a 3 dB down point of 2 MHz and an
effective cutoff at 10 MHz (the Nyquist frequency), avoiding the
artifacts of a very steep ("brick-wall") filter.
A sampling oscilloscope
can display signals of considerably higher frequency than the sampling
rate if the signals are exactly, or nearly, repetitive. It does this by
taking one sample from each successive repetition of the input waveform,
each sample being at an increased time interval from the trigger event.
The waveform is then displayed from these collected samples. This
mechanism is referred to as "equivalent-time sampling". Some oscilloscopes can operate in either this mode or in the more traditional "real-time" mode at the operator's choice.
Other features
A computer model of the sweep of the oscilloscope
Some oscilloscopes have cursors. These are lines that can be
moved about the screen to measure the time interval between two points,
or the difference between two voltages. A few older oscilloscopes simply
brightened the trace at movable locations. These cursors are more
accurate than visual estimates referring to graticule lines.
Better quality general purpose oscilloscopes include a
calibration signal for setting up the compensation of test probes; this
is (often) a 1 kHz square-wave signal of a definite peak-to-peak voltage
available at a test terminal on the front panel. Some better
oscilloscopes also have a squared-off loop for checking and adjusting
current probes.
Sometimes a user wants to see an event that happens only occasionally. To catch these events, some oscilloscopes—called storage scopes—preserve the most recent sweep on the screen. This was originally achieved with a special CRT, a storage tube, which retained the image of even a very brief event for a long time.
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
oscilloscopes with this facility switch from a sweep to a strip-chart
mode at about one sweep per ten seconds. This is because otherwise, the
scope looks broken: it's collecting data, but the dot cannot be seen.
All but the simplest models of current oscilloscopes more often
use digital signal sampling. Samples feed fast analog-to-digital
converters, following which all signal processing (and storage) is
digital.
Many oscilloscopes accommodate plug-in modules for different
purposes, e.g., high-sensitivity amplifiers of relatively narrow
bandwidth, differential amplifiers, amplifiers with four or more
channels, sampling plugins for repetitive signals of very high
frequency, and special-purpose plugins, including audio/ultrasonic
spectrum analyzers, and stable-offset-voltage direct-coupled channels
with relatively high gain.
Examples of use
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. For example, 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 show that some 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. Often, a newly
designed circuit misbehaves because of design errors, bad voltage
levels, electrical noise etc. Digital electronics usually operate from a
clock, so a dual-trace scope showing 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.
Oscilloscopes are often used during real-time software development to check, among other things, missed deadlines and worst-case latencies.
Automotive use
First
appearing in the 1970s for ignition system analysis, automotive
oscilloscopes are becoming an important workshop tool for testing
sensors and output signals on electronic engine management systems, braking and stability systems. Some oscilloscopes can trigger and decode serial bus messages, such as the CAN bus commonly used in automotive applications.
Selection
For work at high frequencies and with fast digital signals, the bandwidth
of the vertical amplifiers and sampling rate must be high enough. For
general-purpose use, a bandwidth of at least 100 MHz is usually
satisfactory. A much lower bandwidth is sufficient for audio-frequency
applications only.
A useful sweep range is from one second to 100 nanoseconds, with
appropriate triggering and (for analog instruments) sweep delay. A
well-designed, stable trigger circuit is required for a steady display.
The chief benefit of a quality oscilloscope is the quality of the
trigger circuit.
Key selection criteria of a DSO (apart from input bandwidth) are
the sample memory depth and sample rate. Early DSOs in the mid- to late
1990s only had a few KB of sample memory per channel. This is adequate
for basic waveform display, but does not allow detailed examination of
the waveform or inspection of long data packets for example. Even
entry-level (<$500) modern DSOs now have 1 MB or more of sample
memory per channel, and this has become the expected minimum in any
modern DSO.
Often this sample memory is shared between channels, and can sometimes
only be fully available at lower sample rates. At the highest sample
rates, the memory may be limited to a few tens of KB.
Any modern "real-time" sample rate DSO typically has 5–10 times the
input bandwidth in sample rate. So a 100 MHz bandwidth DSO would have
500 Ms/s – 1 Gs/s sample rate. The theoretical minimum sample rate
required, using SinX/x interpolation, is 2.5 times the bandwidth.
Analog oscilloscopes have been almost totally displaced by
digital storage scopes except for use exclusively at lower frequencies.
Greatly increased sample rates have largely eliminated the display of
incorrect signals, known as "aliasing", which was sometimes present in
the first generation of digital scopes. The problem can still occur
when, for example, viewing a short section of a repetitive waveform that
repeats at intervals thousands of times longer than the section viewed
(for example a short synchronization pulse at the beginning of a
particular television line), with an oscilloscope that cannot store the
extremely large number of samples between one instance of the short
section and the next.
The used test equipment market, particularly on-line auction
venues, typically has a wide selection of older analog scopes available.
However it is becoming more difficult to obtain replacement parts for
these instruments, and repair services are generally unavailable from
the original manufacturer. Used instruments are usually out of
calibration, and recalibration by companies with the necessary equipment
and expertise usually costs more than the second-hand value of the
instrument.
As of 2007, a 350 MHz bandwidth (BW), 2.5 gigasamples per second (GS/s), dual-channel digital storage scope costs about US$7000 new.
On the lowest end, an inexpensive hobby-grade single-channel DSO
could be purchased for under $90 as of June 2011. These often have
limited bandwidth and other facilities, but fulfill the basic functions
of an oscilloscope.
Software
Many oscilloscopes today provide one or more external interfaces to allow remote instrument control by external software. These interfaces (or buses) include GPIB, Ethernet, serial port, USB and Wi-Fi.
Example
of an analog oscilloscope Lissajous figure, showing a harmonic
relationship of 1 horizontal oscillation cycle to 3 vertical oscillation
cyclesFor analog television, an analog oscilloscope can be used as a vectorscope to analyze complex signal properties, such as this display of SMPTE color bars.
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 "analog" scopes to distinguish them from the "digital" scopes that became common in the 1990s and later.
Analog scopes do not necessarily include a calibrated reference
grid for size measurement of waves, and they may not display waves in
the traditional sense of a line segment sweeping from left to right.
Instead, they could be used for signal analysis by feeding a reference
signal into one axis and the signal to measure into the other axis. For
an oscillating reference and measurement signal, this results in a
complex looping pattern referred to as a Lissajous curve.
The shape of the curve can be interpreted to identify properties of the
measurement signal in relation to the reference signal, and is useful
across a wide range of oscillation frequencies.
Dual-beam oscilloscope
The dual-beam analog oscilloscope can display two signals simultaneously. A special dual-beam CRT
generates and deflects two separate beams. Multi-trace analog
oscilloscopes can simulate a dual-beam display with chop and alternate
sweeps—but those features do not provide simultaneous displays. (Real
time digital oscilloscopes offer the same benefits of a dual-beam
oscilloscope, but they do not require a dual-beam display.) The
disadvantages of the dual trace oscilloscope are that it cannot switch
quickly between traces, and cannot capture two fast transient events. A
dual beam oscilloscope avoids those problems.
Trace storage is an extra feature available on some analog scopes;
they used direct-view storage CRTs. Storage allows a trace pattern that
normally would decay 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.
While analog devices use continually varying voltages, digital
devices use numbers that correspond to samples of the voltage. In the
case of digital oscilloscopes, an analog-to-digital converter (ADC)
changes the measured voltages into digital information.
The digital storage oscilloscope, or DSO for short, is the
standard type of oscilloscope today for the majority of industrial
applications, and thanks to the low costs of entry-level oscilloscopes
even for hobbyists. It replaces the electrostatic storage method in
analog storage scopes with digital memory,
which stores sample data as long as required without degradation and
displays it without the brightness issues of storage-type CRTs. It also
allows complex processing of the signal by high-speed digital signal processing circuits.
A standard DSO is limited to capturing signals with a bandwidth of less than half the sampling rate of the ADC (called the Nyquist limit). There is a variation of the DSO called the digital sampling oscilloscope
which can exceed this limit for certain types of signal, such as
high-speed communications signals, where the waveform consists of
repeating pulses. This type of DSO deliberately samples at a much lower
frequency than the Nyquist limit and then uses signal processing to
reconstruct a composite view of a typical pulse.
Mixed-signal oscilloscopes
A
mixed-signal oscilloscope (or MSO) has two kinds of inputs: a small
number of analog channels (typically two or four), and a larger number
of digital channels (typically sixteen). It provides the ability to
accurately time-correlate analog and digital channels, thus offering a
distinct advantage over a separate oscilloscope and logic analyser.
Typically, digital channels may be grouped and displayed as a bus with
each bus value displayed at the bottom of the display in hexadecimal or
binary. On most MSOs, the trigger can be set across both analog and
digital channels.
Mixed-domain oscilloscopes
A
mixed-domain oscilloscope (MDO) is an oscilloscope that comes with an
additional RF input which is solely used for dedicated FFT-based spectrum analyzer
functionality. Often, this RF input offers a higher bandwidth than the
conventional analog input channels. This is in contrast to the FFT
functionality of conventional digital oscilloscopes, which use the
normal analog inputs.
Some MDOs allow time-correlation of events in the time domain (like a
specific serial data package) with events happening in the frequency
domain (like RF transmissions).
Handheld oscilloscopes are useful for many test and field service applications. Today, a handheld oscilloscope is usually a digital sampling oscilloscope, using a liquid crystal display.
Many handheld and bench oscilloscopes have the ground reference
voltage common to all input channels. If more than one measurement
channel is used at the same time, all the input signals must have the
same voltage reference, and the shared default reference is the "earth".
If there is no differential preamplifier or external signal isolator,
this traditional desktop oscilloscope is not suitable for floating
measurements. (Occasionally an oscilloscope user breaks the ground pin
in the power supply cord of a bench-top oscilloscope in an attempt to
isolate the signal common from the earth ground. This practice is
unreliable since the entire stray capacitance of the instrument cabinet
connects into the circuit. It is also a hazard to break a safety ground
connection, and instruction manuals strongly advise against it.)
Some models of oscilloscope have isolated inputs, where the
signal reference level terminals are not connected together. Each input
channel can be used to make a "floating" measurement with an independent
signal reference level. Measurements can be made without tying one side
of the oscilloscope input to the circuit signal common or ground
reference.
The isolation available is categorized as shown below:
Overvoltage category
Operating voltage (effective value of AC/DC to ground)
Peak instantaneous voltage (repeated 20 times)
Test resistor
CAT I
600 V
2500 V
30 Ω
CAT I
1000 V
4000 V
30 Ω
CAT II
600 V
4000 V
12 Ω
CAT II
1000 V
6000 V
12 Ω
CAT III
600 V
6000 V
2 Ω
PC-based oscilloscopes
PicoScope 6000 digital PC-based oscilloscope using a laptop computer for display and processing
Some digital oscilloscope rely on a PC platform for display and
control of the instrument. This can be in the form of a standalone
oscilloscope with internal PC platform (PC mainboard), or as external
oscilloscope which connects through USB or LAN to a separate PC or laptop.
Related instruments
A
large number of instruments used in a variety of technical fields are
really oscilloscopes with inputs, calibration, controls, display
calibration, etc., specialized and optimized for a particular
application. Examples of such oscilloscope-based instruments include waveform monitors for analyzing video levels in television productions
and medical devices such as vital function monitors and
electrocardiogram and electroencephalogram instruments. In automobile
repair, an ignition analyzer is used to show the spark waveforms for
each cylinder. All of these are essentially oscilloscopes, performing
the basic task of showing the changes in one or more input signals over
time in an X‑Y display.
Other instruments convert the results of their measurements to a
repetitive electrical signal, and incorporate an oscilloscope as a
display element. Such complex measurement systems include spectrum analyzers, transistor analyzers, and time domain reflectometers (TDRs). Unlike an oscilloscope, these instruments automatically generate stimulus or sweep a measurement parameter.
![Schematic 2-D cross-sectional view of glycogen. A core protein of glycogenin is surrounded by branches of glucose units. The entire globular granule may contain approximately 30,000 glucose units.[20]](https://upload.wikimedia.org/wikipedia/commons/thumb/4/47/Glycogen_structure.svg/303px-Glycogen_structure.svg.png)
![Schematic 2-D cross-sectional view of glycogen. A core protein of glycogenin is surrounded by branches of glucose units. The entire globular granule may contain approximately 30,000 glucose units.[20]](https://en.wikipedia.org/wiki/File:Glycogen_structure.svg)
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