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`Oscilloscope Fundamentals
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`PI 2009
`Semiconductor Components v. Power Integrations
`IPR2016-01600
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`Oscilloscope Fundamentals
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`Table of Contents
`
`Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
`
`Signal Integrity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 - 6
`The Significance of Signal Integrity . . . . . . . . . . . . . . . . 5
`Why is Signal Integrity a Problem? . . . . . . . . . . . . . . . . . 5
`Viewing the Analog Orgins of Digital Signals . . . . . . . . . 6
`
`The Oscilloscope . . . . . . . . . . . . . . . . . . . . . . . . . 7 - 11
`Understanding Waveforms & Waveform Measurements . .7
`Types of Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
`Sine Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
`Square and Rectangular Waves . . . . . . . . . . . . . . . . 9
`Sawtooth and Triangle Waves . . . . . . . . . . . . . . . . . 9
`Step and Pulse Shapes . . . . . . . . . . . . . . . . . . . . . . 9
`Periodic and Non-periodic Signals . . . . . . . . . . . . . 10
`Synchronous and Asynchronous Signals . . . . . . . . 10
`Complex Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
`Eye Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
`Constellation Diagrams . . . . . . . . . . . . . . . . . . . . . . 11
`Waveform Measurements . . . . . . . . . . . . . . . . . . . . . . .11
`Frequency and Period . . . . . . . . . . . . . . . . . . . . . . .11
`Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
`Amplitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
`Phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
`Waveform Measurements with Digital Oscilloscopes 12
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`Types of Oscilloscopes . . . . . . . . . . . . . . . . . . . .13 - 17
`Digital Oscilloscopes . . . . . . . . . . . . . . . . . . . . . . . . . . 13
`Digital Storage Oscilloscopes . . . . . . . . . . . . . . . . 14
`Digital Phosphor Oscilloscopes . . . . . . . . . . . . . . . 15
`Digital Sampling Oscilloscopes . . . . . . . . . . . . . . . 17
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`The Systems and Controls of an Oscilloscope .18 - 31
`Vertical System and Controls . . . . . . . . . . . . . . . . . . . . 19
`Position and Volts per Division . . . . . . . . . . . . . . . . 19
`Input Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
`Bandwidth Limit . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
`Bandwidth Enhancement . . . . . . . . . . . . . . . . . . . . 20
`Horizontal System and Controls . . . . . . . . . . . . . . . . . 20
`Acquisition Controls . . . . . . . . . . . . . . . . . . . . . . . . 20
`Acquisition Modes . . . . . . . . . . . . . . . . . . . . . . . . . 20
`Types of Acquisition Modes . . . . . . . . . . . . . . . . . . 21
`Starting and Stopping the Acquisition System . . . . 21
`Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
`Sampling Controls . . . . . . . . . . . . . . . . . . . . . . . . . 22
`Sampling Methods . . . . . . . . . . . . . . . . . . . . . . . . 22
`Real-time Sampling . . . . . . . . . . . . . . . . . . . . . . . . 22
`Equivalent-time Sampling . . . . . . . . . . . . . . . . . . 24
`Position and Seconds per Division . . . . . . . . . . . . . 26
`Time Base Selections . . . . . . . . . . . . . . . . . . . . . . . 26
`Zoom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
`XY Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
`Z Axis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
`XYZ Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
`Trigger System and Controls . . . . . . . . . . . . . . . . . . . . 27
`Trigger Position . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
`Trigger Level and Slope . . . . . . . . . . . . . . . . . . . . . 28
`Trigger Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
`Trigger Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
`Trigger Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
`Trigger Holdoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
`Display System and Controls . . . . . . . . . . . . . . . . . . . . 30
`Other Oscilloscope Controls . . . . . . . . . . . . . . . . . . . . . 31
`Math and Measurement Operations . . . . . . . . . . . . 31
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`The Complete Measurement System . . . . . . . . 32 - 34
`Probes
` . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
`Passive Probes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
`Active and Differential Probes . . . . . . . . . . . . . . . . . . . . 33
`Probe Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
`
`Performance Terms and Considerations . . . . . 35 - 43
`Bandwidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
`Rise Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
`Sample Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
`Waveform Capture Rate . . . . . . . . . . . . . . . . . . . . . . . . 38
`Record Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
`Triggering Capabilities . . . . . . . . . . . . . . . . . . . . . . . . . 39
`Effective Bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
`Frequency Response . . . . . . . . . . . . . . . . . . . . . . . . . . 39
`Vertical Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
`Sweep Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
`Gain Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
`Horizontal Accuracy (Time Base) . . . . . . . . . . . . . . . . . 40
`Vertical Resolution (Analog-to-digital Converter) . . . . . . 40
`Connectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
`Expandability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
`Ease-of-use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
`Probes
` . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
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`Oscilloscope Fundamentals
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`Operating the Oscilloscope . . . . . . . . . . . . . . . . 44 - 46
`Setting Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
`Ground the Oscilloscope . . . . . . . . . . . . . . . . . . . . . . . 44
`Ground Yourself . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
`Setting the Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
`Instrument Calibration . . . . . . . . . . . . . . . . . . . . . . . . . 45
`Using Probes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
`Connecting the Ground Clip . . . . . . . . . . . . . . . . . . . . . 45
`Compensating the Probe . . . . . . . . . . . . . . . . . . . . . . . 46
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`Oscilloscope Measurement Techniques . . . . . . 47 - 51
`Voltage Measurements . . . . . . . . . . . . . . . . . . . . . . . . . 47
`Time and Frequency Measurements . . . . . . . . . . . . . . 48
`Pulse Width and Rise Time Measurements . . . . . . . . . 48
`Phase Shift Measurements . . . . . . . . . . . . . . . . . . . . . . 49
`Other Measurement Techniques . . . . . . . . . . . . . . . . . . 49
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`Written Exercises . . . . . . . . . . . . . . . . . . . . . . . . 50 - 55
`Part I
`A. Vocabulary Exercises . . . . . . . . . . . . . . . . . . . . . 50
`B. Application Exercises . . . . . . . . . . . . . . . . . . . . . 51
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`Part II
`A. Vocabulary Exercises . . . . . . . . . . . . . . . . . . . . . 52
`B. Application Exercises . . . . . . . . . . . . . . . . . . . . .53
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`Answer Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55
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`Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 - 59
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`Oscilloscope Fundamentals
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`Introduction
`Nature moves in the form of a sine wave, be it an ocean
`wave, earthquake, sonic boom, explosion, sound through air,
`or the natural frequency of a body in motion. Energy, vibrating
`particles and other invisible forces pervade our physical uni-
`verse. Even light – part particle, part wave – has a fundamen-
`tal frequency, which can be observed as color.
`
`Sensors can convert these forces into electrical
`signals that you can observe and study with an
`oscilloscope. Oscilloscopes enable scientists,
`engineers, technicians, educators and others to
`“see” events that change over time.
`
`Oscilloscopes are indispensable tools for anyone designing,
`manufacturing or repairing electronic equipment. In today’s
`fast-paced world, engineers need the best tools available to
`solve their measurement challenges quickly and accurately.
`As the eyes of the engineer, oscilloscopes are the key to
`meeting today’s demanding measurement challenges.
`
`The usefulness of an oscilloscope is not limited to the world
`of electronics. With the proper sensor, an oscilloscope can
`measure all kinds of phenomena. A sensor is a device that
`creates an electrical signal in response to physical stimuli,
`such as sound, mechanical stress, pressure, light, or heat. A
`microphone is a sensor that converts sound into an electrical
`signal. Figure 1 shows an example of scientific data that can
`be gathered by an oscilloscope.
`
`Oscilloscopes are used by everyone from physicists to
`television repair technicians. An automotive engineer uses
`an oscilloscope to correlate analog data from sensors
`with serial data from the engine control unit. A medical
`researcher uses an oscilloscope to measure brain waves.
`The possibilities are endless.
`
`The concepts presented in this primer will provide you with
`a good starting point in understanding oscilloscope basics
`and operation.
`
`Light Source
`
`Photo Cell
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`Figure 1. An example of scientific data gathered by an oscilloscope.
`
`The glossary in the back of this primer will give you definitions
`of unfamiliar terms. The vocabulary and multiple-choice
`written exercises on oscilloscope theory and controls make
`this primer a useful classroom aid. No mathematical or elec-
`tronics knowledge is necessary.
`
`After reading this primer, you will be able to:
`Describe how oscilloscopes work
`Describe the differences between analog, digital storage,
`digital phosphor, and digital sampling oscilloscopes
`Describe electrical waveform types
`Understand basic oscilloscope controls
`Take simple measurements
`
`The manual provided with your oscilloscope will give you
`more specific information about how to use the oscilloscope
`in your work. Some oscilloscope manufacturers also provide
`a multitude of application notes to help you optimize the
`oscilloscope for your application-specific measurements.
`
`Should you need additional assistance, or have any
`comments or questions about the material in this primer,
`simply contact your Tektronix representative, or visit
`www.tektronix.com.
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`Signal Integrity
`The Significance of Signal Integrity
`The key to any good oscilloscope system is its ability to
`accurately reconstruct a waveform – referred to as signal
`integrity. An oscilloscope is analogous to a camera that
`captures signal images that we can then observe and
`interpret. Two key issues lie at the heart of signal integrity.
`When you take a picture, is it an accurate picture of what
`actually happened?
`Is the picture clear or fuzzy?
`How many of those accurate pictures can you take per
`second?
`
`Taken together, the different systems and performance capa-
`bilities of an oscilloscope contribute to its ability to deliver the
`highest signal integrity possible. Probes also affect the signal
`integrity of a measurement system.
`
`Signal integrity impacts many electronic design disciplines.
`But until a few years ago, it wasn’t much of a problem for
`digital designers. They could rely on their logic designs to
`act like the Boolean circuits they were. Noisy, indeterminate
`signals were something that occurred in high-speed designs
`– something for RF designers to worry about. Digital systems
`switched slowly and signals stabilized predictably.
`
`Processor clock rates have since multiplied by orders of
`magnitude. Computer applications such as 3D graphics,
`video and server I/O demand vast bandwidth. Much of
`today’s telecommunications equipment is digitally based,
`and similarly requires massive bandwidth. So too does
`digital high-definition TV. The current crop of microprocessor
`devices handles data at rates up to 2, 3 and even 5 GS/s
`(gigasamples per second), while some DDR3 memory
`devices use clocks in excess of 2 GHz as well as data signals
`with 35-ps rise times.
`
`Importantly, speed increases have trickled down to the
`common IC devices used in automobiles, VCRs, and
`machine controllers, to name just a few applications.
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`Oscilloscope Fundamentals
`
`A processor running at a 20-MHz clock rate may well have
`signals with rise times similar to those of an 800-MHz
`processor. Designers have crossed a performance threshold
`that means, in effect, almost every design is a high-speed
`design.
`
`Without some precautionary measures, high-speed problems
`can creep into otherwise conventional digital designs. If a
`circuit is experiencing intermittent failures, or if it encounters
`errors at voltage and temperature extremes, chances are
`there are some hidden signal integrity problems. These can
`affect time-to-market, product reliability, EMI compliance,
`and more. These high speed problems can also impact the
`integrity of a serial data stream in a system, requiring some
`method of correlating specific patterns in the data with the
`observed characteristics of high-speed waveforms.
`
`Why is Signal Integrity a Problem?
`Let’s look at some of the specific causes of signal degrada-
`tion in today’s digital designs. Why are these problems so
`much more prevalent today than in years past?
`
`The answer is speed. In the “slow old days,” maintaining
`acceptable digital signal integrity meant paying attention to
`details like clock distribution, signal path design, noise mar-
`gins, loading effects, transmission line effects, bus termina-
`tion, decoupling and power distribution. All of these rules still
`apply, but…
`
`Bus cycle times are up to a thousand times faster than they
`were 20 years ago! Transactions that once took microsec-
`onds are now measured in nanoseconds. To achieve this
`improvement, edge speeds too have accelerated: they are
`up to 100 times faster than those of two decades ago.
`
`This is all well and good; however, certain physical realities
`have kept circuit board technology from keeping up the pace.
`The propagation time of inter-chip buses has remained
`almost unchanged over the decades. Geometries have
`shrunk, certainly, but there is still a need to provide circuit
`board real estate for IC devices, connectors, passive compo-
`nents, and of course, the bus traces themselves. This real
`estate adds up to distance, and distance means time – the
`enemy of speed.
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`Oscilloscope Fundamentals
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`It’s important to remember that the edge speed – rise time –
`of a digital signal can carry much higher frequency compo-
`nents than its repetition rate might imply. For this reason,
`some designers deliberately seek IC devices with relatively
`“slow” rise times.
`
`The lumped circuit model has always been the basis of
`most calculations used to predict signal behavior in a circuit.
`But when edge speeds are more than four to six times faster
`than the signal path delay, the simple lumped model no
`longer applies.
`
`Circuit board traces just six inches long become transmission
`lines when driven with signals exhibiting edge rates below
`four to six nanoseconds, irrespective of the cycle rate.
`In effect, new signal paths are created. These intangible
`connections aren’t on the schematics, but nevertheless
`provide a means for signals to influence one another in
`unpredictable ways.
`
`Sometimes even the errors introduced by the
`probe/instrument combination can provide
`a significant contribution to the signal being
`measured. However, by applying the “square
`root of the sum of the squares” formula to the
`measured value, it is possible to determine
`whether the device under test is approaching
`a rise/fall time failure. In addition, recent
`oscilloscope tools use special filtering tech-
`niques to de-embed the measurement system’s
`effects on the signal, displaying edge times
`and other signal characteristics.
`
`At the same time, the intended signal paths don’t work the
`way they are supposed to. Ground planes and power planes,
`like the signal traces described above, become inductive
`and act like transmission lines; power supply decoupling
`is far less effective. EMI goes up as faster edge speeds
`produce shorter wavelengths relative to the bus length.
`Crosstalk increases.
`
`In addition, fast edge speeds require generally higher currents
`to produce them. Higher currents tend to cause ground
`bounce, especially on wide buses in which many signals
`switch at once. Moreover, higher current increases the
`amount of radiated magnetic energy and with it, crosstalk.
`
`Viewing the Analog Origins of Digital Signals
`What do all these characteristics have in common? They
`are classic analog phenomena. To solve signal integrity
`problems, digital designers need to step into the analog
`domain. And to take that step, they need tools that can
`show them how digital and analog signals interact.
`
`Digital errors often have their roots in analog signal integrity
`problems. To track down the cause of the digital fault, it’s
`often necessary to turn to an oscilloscope, which can display
`waveform details, edges and noise; can detect and display
`transients; and can help you precisely measure timing
`relationships such as setup and hold times. Modern oscillo-
`scopes can help to simplify the troubleshooting process by
`triggering on specific patterns in serial data streams and
`displaying the analog signal that corresponds in time with
`a specified event.
`
`Understanding each of the systems within your oscilloscope
`and how to apply them will contribute to the effective
`application of the oscilloscope to tackle your specific
`measurement challenge.
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`Oscilloscope Fundamentals
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`Figure 2b. Two offset clock patterns with Z axis intensity grading.
`
`Understanding Waveforms and Waveform
`Measurements
`The generic term for a pattern that repeats over time is
`a wave – sound waves, brain waves, ocean waves, and
`voltage waves are all repetitive patterns. An oscilloscope
`measures voltage waves. Remember as mentioned earlier,
`that physical phenomena such as vibrations or temperature
`or electrical phenomena such as current or power can be
`converted to a voltage by a sensor. One cycle of a wave is
`the portion of the wave that repeats. A waveform is a graphic
`representation of a wave. A voltage waveform shows time
`on the horizontal axis and voltage on the vertical axis.
`
`Y (voltage)
`
`Y (voltage)
`
`X (time)
`
`Z (intensity)
`
`X (time)
`
`Z (intensity)
`
`Figure 2a. X, Y, and Z components of a displayed waveform.
`
`The Oscilloscope
`What is an oscilloscope and how does it work? This section
`answers these fundamental questions.
`
`The oscilloscope is basically a graph-displaying device – it
`draws a graph of an electrical signal. In most applications,
`the graph shows how signals change over time: the vertical
`(Y) axis represents voltage and the horizontal (X) axis
`represents time. The intensity or brightness of the display
`is sometimes called the Z axis. (See Figure 2a) In DPO
`oscilloscopes, the Z axis can be represented by color grading
`of the display. (See Figure 2b)
`
`This simple graph can tell you many things about a signal,
`such as:
`The time and voltage values of a signal
`The frequency of an oscillating signal
`The “moving parts” of a circuit represented by the signal
`The frequency with which a particular portion of the signal
`is occurring relative to other portions
`Whether or not a malfunctioning component is distorting
`the signal
`How much of a signal is direct current (DC) or alternating
`current (AC)
`How much of the signal is noise and whether the noise
`is changing with time
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`Oscilloscope Fundamentals
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`Sine Wave
`
`Damped Sine Wave
`
`Square Wave
`
`Rectangular Wave
`
`Sawtooth Wave
`
`Triangle Wave
`
`Step
`
`Pulse
`
`Figure 3. Common waveforms.
`
`Figure 4. Sources of common waveforms.
`
`Complex
`
`Waveform shapes reveal a great deal about a signal. Any
`time you see a change in the height of the waveform, you
`know the voltage has changed. Any time there is a flat
`horizontal line, you know that there is no change for that
`length of time. Straight, diagonal lines mean a linear
`change – rise or fall of voltage at a steady rate. Sharp
`angles on a waveform indicate sudden change. Figure 3
`shows common waveforms and Figure 4 displays sources
`of common waveforms.
`
`Types of Waves
`You can classify most waves into these types:
`Sine waves
`Square and rectangular waves
`Triangle and saw-tooth waves
`Step and pulse shapes
`Periodic and non-periodic signals
`Synchronous and asynchronous signals
`Complex waves
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`Oscilloscope Fundamentals
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`Sine Wave
`
`Damped Sine Wave
`
`Sawtooth Wave
`
`Triangle Wave
`
`Figure 5. Sine and damped sine waves.
`
`Figure 7. Sawtooth and triangle waves.
`
`Square Wave
`
`Rectangular Wave
`
`Step
`
`Pulse
`
`Pulse Train
`
`Figure 6. Square and rectangular waves.
`
`Figure 8. Step, pulse and pulse train shapes.
`
`Sine Waves
`The sine wave is the fundamental wave shape for several
`reasons. It has harmonious mathematical properties – it is
`the same sine shape you may have studied in high school
`trigonometry class. The voltage in your wall outlet varies as
`a sine wave. Test signals produced by the oscillator circuit
`of a signal generator are often sine waves. Most AC power
`sources produce sine waves. (AC signifies alternating current,
`although the voltage alternates too. DC stands for direct
`current, which means a steady current and voltage, such
`as a battery produces.)
`
`The damped sine wave is a special case you may see in
`a circuit that oscillates, but winds down over time. Figure 5
`shows examples of sine and damped sine waves.
`
`Square and Rectangular Waves
`The square wave is another common wave shape. Basically,
`a square wave is a voltage that turns on and off (or goes
`high and low) at regular intervals. It is a standard wave for
`testing amplifiers – good amplifiers increase the amplitude of
`a square wave with minimum distortion. Television, radio and
`computer circuitry often use square waves for timing signals.
`
`The rectangular wave is like the square wave except that
`the high and low time intervals are not of equal length.
`It is particularly important when analyzing digital circuitry.
`Figure 6 shows examples of square and rectangular waves.
`
`Sawtooth and Triangle Waves
`Sawtooth and triangle waves result from circuits designed
`to control voltages linearly, such as the horizontal sweep
`of an analog oscilloscope or the raster scan of a television.
`The transitions between voltage levels of these waves change
`at a constant rate. These transitions are called ramps.
`Figure 7 shows examples of saw-tooth and triangle waves.
`
`Step and Pulse Shapes
`Signals such as steps and pulses that occur rarely, or non-
`periodically, are called single-shot or transient signals. A step
`indicates a sudden change in voltage, similar to the voltage
`change you would see if you turned on a power switch.
`
`A pulse indicates sudden changes in voltage, similar to the
`voltage changes you would see if you turned a power switch
`on and then off again. A pulse might represent one bit of
`information traveling through a computer circuit or it might be
`a glitch, or defect, in a circuit. A collection of pulses traveling
`together creates a pulse train. Digital components in a
`computer communicate with each other using pulses. These
`pulses may be in the form of serial data stream or multiple
`signal lines may be used to represent a value in a parallel
`data bus. Pulses are also common in x-ray and communica-
`tions equipment. Figure 8 shows examples of step and pulse
`shapes and a pulse train.
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`Oscilloscope Fundamentals
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`Complex
`
`Figure 9. An NTSC composite video signal is an example of a complex wave.
`
`Periodic and Non-periodic Signals
`Repetitive signals are referred to as periodic signals, while
`signals that constantly change are known as non-periodic
`signals. A still picture is analogous to a periodic signal, while
`a moving picture can be equated to a non-periodic signal.
`
`Synchronous and Asynchronous Signals
`When a timing relationship exists between two signals,
`those signals are referred to as synchronous. Clock, data
`and address signals inside a computer are an example of
`synchronous signals.
`
`Asynchronous is a term used to describe those signals
`between which no timing relationship exists. Because no
`time correlation exists between the act of touching a key
`on a computer keyboard and the clock inside the computer,
`these are considered asynchronous.
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`Complex Waves
`Some waveforms combine the characteristics of sines,
`squares, steps, and pulses to produce waveshapes that
`challenge many oscilloscopes. The signal information may
`be embedded in the form of amplitude, phase, and/or
`frequency variations. For example, although the signal
`in Figure 9 is an ordinary composite video signal, it is
`composed of many cycles of higher-frequency waveforms
`embedded in a lower-frequency envelope.
`
`In this example, it is usually most important to understand
`the relative levels and timing relationships of the steps. To
`view this signal, you need an oscilloscope that captures the
`low-frequency envelope and blends in the higher-frequency
`waves in an intensity-graded fashion so that you can see
`their overall combination as an image that can be visually
`interpreted. Digital phosphor oscilloscopes are most
`suited to viewing complex waves, such as video signals,
`illustrated in Figure 9. Their displays provide the necessary
`frequency-of-occurrence information, or intensity grading,
`that is essential to understanding what the waveform is
`really doing. Some oscilloscopes allow for displaying certain
`types of complex waveforms in special ways. For example,
`Telecommunications data may be displayed as an eye
`pattern or a constellation diagram.
`
`Eye Patterns
`Telecommunications digital data signals can be displayed on
`an oscilloscope as a special type of waveform referred to as
`an eye pattern. The name comes from the similarity of the
`waveform to a series of eyes. (Figure 10a) Eye patterns are
`produced when digital data from a receiver is sampled and
`applied to the vertical input, while the data rate is used to
`trigger the horizontal sweep. The eye pattern displays one bit
`or unit interval of data with all possible edge transitions and
`states superimposed in one comprehensive view.
`(Figure 10a)
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`Oscilloscope Fundamentals
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`1
`
`2
`
`3
`
`Frequency
`3 Cycles per
`Second = 3 Hz
`
`period
`
`1 second
`
`Figure 10b. Frequency and period of a sine wave.
`
`second. A repetitive signal also has a period – this is the
`amount of time it takes the signal to complete one cycle.
`Period and frequency are reciprocals of each other, so that
`1/period equals the frequency and 1/frequency equals the
`period. For example, the sine wave in Figure 10 has a fre-
`quency of 3 Hz and a period of 1/3 second.
`
`Voltage
`Voltage is the amount of electric potential – or signal strength
`– between two points in a circuit. Usually, one of these points
`is ground, or zero volts, but not always. You may want to
`measure the voltage from the maximum peak to the minimum
`peak of a waveform, referred to as the peak-to-peak voltage.
`
`Figure 10a. 622 Mb/s serial data eye pattern.
`
`Constellation Diagram
`A constellation diagram is a representation of a signal modu-
`lated by a digital modulation scheme such as quadrature
`amplitude modulation or phase-shift keying.
`
`Waveform Measurements
`Many terms are used to describe the types of measurements
`that you make with your oscilloscope. This section describes
`some of the most common measurements and terms.
`
`Frequency and Period
`If a signal repeats, it has a frequency. The frequency is
`measured in Hertz (Hz) and equals the number of times the
`signal repeats itself in one second, referred to as cycles per
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`Oscilloscope Fundamentals
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`0°
`
`90°
`
`180°
`
`270°
`
`°
`
`360
`
`Voltage
`
`+1 V
`
`2 V
`
`0
`
`–1 V
`
`Current
`
`0
`
`Phase = 90°
`
`Figure 11. Amplitude and degrees of a sine wave.
`
`Figure 12. Phase shift.
`
`Fully automated waveform measurements available
`on some digital phosphor oscilloscopes include:
`
`Period
`
`Duty Cycle +
`
`Frequency
`
`Duty Cycle -
`
`Width +
`
`Width -
`
`Rise time
`
`Fall time
`
`High
`
`Low
`
`Minimum
`
`Maximum
`
`Delay
`
`Phase
`
`Burst width
`
`Overshoot +
`
`Peak-to-peak
`
`Overshoot -
`
`Amplitude
`
`Mean
`
`RMS
`
`Extinction ratio
`
`Cycle mean
`
`Cycle RMS
`
`Mean optical power
`
`Cycle area
`
`instruments also provide mean and RMS calculations, duty
`cycle, and other math operations. Automated measurements
`appear as on-screen alphanumeric readouts. Typically these
`readings are more accurate than is possible to obtain with
`direct graticule interpretation.
`
`Amplitude
`Amplitude refers to the amount of voltage between two
`points in a circuit. Amplitude commonly refers to the
`maximum voltage of a signal measured from ground, or
`zero volts. The waveform shown in Figure 11 has an
`amplitude of 1 V and a peak-to-peak voltage of 2 V.
`
`Phase
`Phase is best explained by looking at a sine wave. The
`voltage level of sine waves is based on circular motion.
`Given that a circle has 360°, one cycle of a sine wave has
`360°, as shown in Figure 11. Using degrees, you can refer to
`the phase angle of a sine wave when you want to describe
`how much of the period has elapsed.
`
`Phase shift describes the difference in timing between two
`otherwise similar signals. The waveform in Figure 12 labeled
`“current” is said to be 90° out of phase with the waveform
`labeled “voltage,” since the waves reach similar points in their
`cycles exactly 1/4 of a cycle apart (360°/4 = 90°). Phase
`shifts are common in electronics.
`
`Waveform Measurements with Digital Oscilloscopes
`Modern digital oscilloscopes have functions that make
`waveform measurements easier. They have front-panel
`buttons and/or screen-based menus from which you can
`select fully automated measurements. These include ampli-
`tude, period, rise/fall time, and many more. Many digital
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`Oscilloscope Fundamentals
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`1010
`0001
`0010
`0101
`
`ADC
`
`Analog Oscilloscopes
`Trace Signals
`
`Digital Oscilloscopes Samples
`Signals and Construct Displays
`
`Figure 15. Analog oscilloscopes trace signals, while digital oscilloscopes sample signals and construct displays.
`
`The Types of Oscilloscopes
`Electronic equipment can be classified into two categories:
`analog and digital. Analog equipment works with continu-
`ously variable voltages, while digital equipment works with
`discrete binary numbers that represent voltage samples. A
`conventional phonograph is an analog device, while a
`compact disc player is a digital device.
`
`Oscilloscopes can be classified similarly – as
`analog and digital types. For many applications,
`either an analog or digital oscilloscope will do.
`However, each type has unique characteristics
`that may make it more or less suitable for
`specific applications. Digital oscilloscopes
`can be further classified into digital storage
`oscilloscopes (DSOs), digital phosphor oscillo-
`scopes (DPOs) and sampling oscilloscopes.
`
`Digital Oscilloscopes
`In contrast to an analog oscilloscope, a digital oscilloscope
`uses an analog-to-digital converter (ADC) to convert the
`measured voltage into digital information. It acquires the
`waveform as a series of samples, and stores these samples
`until it accumulates enough samples to describe a waveform.
`The digital oscilloscope then re-assembles the waveform for
`display on the screen. (see Figure 15)
`
`Digital oscilloscopes can be classified into digital storage
`oscilloscopes (DSOs), digital phosphor oscilloscopes (DPOs),
`and sampling oscilloscopes.
`
`The digital approach means that the oscilloscope can display