Tektronix Signal_integrity Signal Integrity
User Manual: signal_integrity Tektronix Oscilloscope Use
Open the PDF directly: View PDF 
.
Page Count: 7
| Download | |
| Open PDF In Browser | View PDF |
XYZs of Oscilloscopes Primer Signal Integrity Without some precautionary measures, high-speed problems can creep into otherwise conventional digital designs. If a circuit is The Significance of Signal Integrity experiencing intermittent failures, or if it encounters errors at voltage The key to any good oscilloscope system is its ability to accurately recon- and temperature extremes, chances are there are some hidden signal struct a waveform – referred to as signal integrity. An oscilloscope is integrity problems. These can affect time-to-market, product reliability, analogous to a camera that captures signal images that we can then EMI compliance, and more. 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 capabilities of an oscilloscope contribute to its ability to deliver the highest signal integrity possible. Probes also affect the signal integrity of a measurement system. Why is Signal Integrity a Problem? Let’s look at some of the specific causes of signal degradation 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 margins, loading effects, transmission line effects, bus termination, decoupling and power Signal integrity impacts many electronic design disciplines. But until a distribution. All of these rules still apply, but… 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. Bus cycle times are up to a thousand times faster than they were 20 years ago! Transactions that once took microseconds 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. 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 memory devices use 400-MHz clocks as well as data signals with 200-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. 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. 4 www.tektronix.com 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 components, and of course, the bus traces themselves. This real estate adds up to distance, and distance means time – the enemy of speed. It’s important to remember that the edge speed – rise time – of a digital signal can carry much higher frequency components than its repetition rate might imply. For this reason, some designers deliberately seek IC devices with relatively “slow” rise times. XYZs of Oscilloscopes Primer 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. Y (voltage) when driven with signals exhibiting edge rates below four to six X (time) nanoseconds, irrespective of the cycle rate. In effect, new signal paths are created. These intangible connections aren’t on the schematics, but Y (voltage) Circuit board traces just six inches long become transmission lines Z (intensity) nevertheless provide a means for signals to influence one another in X (time) unpredictable ways. Z (intensity) 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 Figure 2. X, Y, and Z components of a displayed waveform. 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. The Oscilloscope In addition, fast edge speeds require generally higher currents to produce What is an oscilloscope and how does it work? This section answers them. Higher currents tend to cause ground bounce, especially on wide these fundamental questions. buses in which many signals switch at once. Moreover, higher current The oscilloscope is basically a graph-displaying device – it draws a increases the amount of radiated magnetic energy and with it, crosstalk. graph of an electrical signal. In most applications, the graph shows how 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. 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 2.) 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 To track down the cause of the digital fault, it’s often necessary to turn The “moving parts” of a circuit represented by the signal to an oscilloscope, which can display waveform details, edges and noise; The frequency with which a particular portion of the signal is occurring relative to can detect and display transients; and can help you precisely measure other portions timing relationships such as setup and hold times. Whether or not a malfunctioning component is distorting the signal Understanding each of the systems within your oscilloscope and how to How much of a signal is direct current (DC) or alternating current (AC) apply them will contribute to the effective application of the oscilloscope How much of the signal is noise and whether the noise is changing with time to tackle your specific measurement challenge. www.tektronix.com 5 XYZs of Oscilloscopes Primer Sine Wave Damped Sine Wave Square Wave Rectangular Wave Sawtooth Wave Triangle Wave Step Pulse 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 Figure 3. Common waveforms. 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 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. 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. 6 www.tektronix.com a waveform indicate sudden change. Figure 3 shows common waveforms and Figure 4 displays sources of common waveforms. XYZs of Oscilloscopes Primer Sine Wave Damped Sine Wave Figure 5. Sine and damped sine waves. Sawtooth Wave Triangle Wave Figure 7. Sawtooth and triangle 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 Square Wave Rectangular Wave increase the amplitude of a square wave with minimum distortion. Television, radio and computer circuitry often use square waves for Figure 6. Square and rectangular waves. timing signals. The rectangular wave is like the square wave except that the high and Types of Waves You can classify most waves into these types: 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. Sine waves Square and rectangular waves Triangle and saw-tooth waves Step and pulse shapes 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 Periodic and non-periodic signals Synchronous and asynchronous signals these waves change at a constant rate. These transitions are called ramps. Figure 7 shows examples of saw-tooth and triangle waves. Complex waves 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. www.tektronix.com 7 XYZs of Oscilloscopes Primer Step Pulse Pulse Train Figure 8. Step, pulse and pulse train shapes. Complex Figure 9. An NTSC composite video signal is an example of a complex wave. Step and Pulse Shapes Synchronous and Asynchronous Signals Signals such as steps and pulses that occur rarely, or non-periodically, When a timing relationship exists between two signals, those signals are are called single-shot or transient signals. A step indicates a sudden referred to as synchronous. Clock, data and address signals inside a change in voltage, similar to the voltage change you would see if you computer are an example of synchronous signals. turned on a power switch. Asynchronous is a term used to describe those signals between which no A pulse indicates sudden changes in voltage, similar to the voltage timing relationship exists. Because no time correlation exists between the changes you would see if you turned a power switch on and then off act of touching a key on a computer keyboard and the clock inside the again. A pulse might represent one bit of information traveling through computer, these are considered asynchronous. 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 Complex Waves components in a computer communicate with each other using pulses. Some waveforms combine the characteristics of sines, squares, steps, Pulses are also common in x-ray and communications equipment. and pulses to produce waveshapes that challenge many oscilloscopes. Figure 8 shows examples of step and pulse shapes and a pulse train. The signal information may be embedded in the form of amplitude, phase, and/or frequency variations. For example, although the signal in Figure 9 Periodic and Non-periodic Signals is an ordinary composite video signal, it is composed of many cycles of Repetitive signals are referred to as periodic signals, while signals that higher-frequency waveforms embedded in a lower-frequency envelope. constantly change are known as non-periodic signals. A still picture is In this example, it is usually most important to understand the relative analogous to a periodic signal, while a moving picture can be equated to levels and timing relationships of the steps. To view this signal, you need a non-periodic signal. 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. Analog and 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. 8 www.tektronix.com XYZs of Oscilloscopes Primer 1 2 3 0° 90° 180° 270° 360° +1 V Frequency 3 Cycles per Second = 3 Hz 2V 0 period –1 V 1 second Figure 10. Frequency and period of a sine wave. Figure 11. Amplitude and degrees of a sine wave. Waveform Measurements Voltage Many terms are used to describe the types of measurements that you Voltage is the amount of electric potential – or signal strength – between make with your oscilloscope. This section describes some of the most two points in a circuit. Usually, one of these points is ground, or zero common measurements and terms. volts, but not always. You may want to measure the voltage from the Frequency and Period maximum peak to the minimum peak of a waveform, referred to as the peak-to-peak voltage. 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 Amplitude one second, referred to as cycles per second. A repetitive signal also Amplitude refers to the amount of voltage between two points in a circuit. has a period – this is the amount of time it takes the signal to complete Amplitude commonly refers to the maximum voltage of a signal measured one cycle. Period and frequency are reciprocals of each other, so that from ground, or zero volts. The waveform shown in Figure 11 has an 1/period equals the frequency and 1/frequency equals the period. For amplitude of 1 V and a peak-to-peak voltage of 2 V. example, the sine wave in Figure 10 has a frequency of 3 Hz and a period of 1/3 second. www.tektronix.com 9 XYZs of Oscilloscopes Primer Waveform Measurements with Digital Oscilloscopes Voltage Modern digital oscilloscopes have functions that make waveform measurements easier. They have front-panel buttons and/or screen-based Current menus from which you can select fully automated measurements. These include amplitude, period, rise/fall time, and many more. Many digital instruments also provide mean and RMS calculations, duty cycle, and 0 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. Phase = 90° Fully automated waveform measurements available on some digital phosphor oscilloscopes include: Period Duty cycle + High Frequency Duty cycle – Low Width + Delay Minimum Phase Width – Phase Maximum Phase is best explained by looking at a sine wave. The voltage level of Rise time Burst width Overshoot + sine waves is based on circular motion. Given that a circle has 360°, one Fall time Peak-to-peak Overshoot – cycle of a sine wave has 360°, as shown in Figure 11. Using degrees, you Amplitude Mean RMS can refer to the phase angle of a sine wave when you want to describe Extinction ratio Cycle mean Cycle RMS Mean optical power Cycle area Figure 12. Phase shift. 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. 10 www.tektronix.com
Source Exif Data:
File Type : PDF File Type Extension : pdf MIME Type : application/pdf PDF Version : 1.3 Linearized : Yes Encryption : Standard V1.2 (40-bit) User Access : Print, Copy, Annotate, Fill forms, Extract, Assemble, Print high-res Modify Date : 2001:05:30 10:41:05-07:00 Create Date : 2001:05:24 15:52:22-07:00 Producer : Acrobat Distiller 4.0 for Windows Title : Tektronix: Primer > XYZs of Oscilloscopes : Signal Integrity Author : Tektronix, Inc. Page Count : 7EXIF Metadata provided by EXIF.tools