Evaluating Oscilloscope Fundamentals

Evaluating Oscilloscope Fundamentals Application Note This application note provides an overview of oscilloscope fundamentals. You will learn what an oscilloscope is and how it operates. We will discuss oscilloscope applications and give you an overview of basic measurements and performance characteristics. We will also look at the different types of probes and discuss their advantages and disadvantages. Table of Contents Introduction .....................1 Electronic Signals ................2 Introduction Wave properties ................... 2 Waveforms ....................... 3 Analog versus digital signals ......... 4 Electronic technology permeates What is an Oscilloscope and Why Do You Need One? ...........5 our lives. Millions of people use Signal integrity .................... 5 electronic devices such as cell phones, What an oscilloscope looks like ...... 6 televisions, and computers on a daily An oscilloscope’s purpose ........... 7 basis. As electronic technology has Types of oscilloscopes .............. 8 advanced, the speeds at which these Where oscilloscopes are used ....... 10 devices operate have accelerated. Basic Oscilloscope Controls and Today, most devices use high-speed Measurements ..................11 digital technologies. which are malfunctioning. They can Basic front-panel controls .......... 11 also help you determine whether or Softkeys ......................... 14 Engineers need the ability to not a newly designed component Basic measurements .............. 15 accurately design and test the behaves the way you intended. Basic mathematical functions ....... 16 components in their high-speed Oscilloscopes are far more powerful Important Oscilloscope digital devices. The instrumentation than multimeters because they allow Performance Characteristics ......17 engineers use to design and test their you to see what the electronic signals Bandwidth ....................... 17 components must be particularly well- actually look like. Channels ........................ 17 suited to deal with high speeds and Sample rate ...................... 18 high frequencies. An oscilloscope is an Oscilloscopes are used in a wide range Memory depth .................... 19 example of just such an instrument. Update rate ...................... 20 of fields, from the automotive industry Oscilloscope connectivity .......... 20 to university research laboratories, Oscilloscopes are powerful tools that to the aerospace-defense industry. Oscilloscope Probes .............21 are useful for designing and testing Companies rely on oscilloscopes Loading ......................... 21 electronic devices. They are vital in Passive probes ................... 21 to help them uncover defects and determining which components of a Active probes .................... 21 produce fully-functional products. Current probes ................... 22 system are behaving correctly and Probe accessories ................ 22 Conclusion .....................23 Electronic Signals The main purpose of an oscilloscope Wave properties second is called the root-mean-square is to display electronic signals. By (RMS) amplitude. To calculate the RMS viewing signals displayed on an Electronic signals are waves or pulses. voltage of a waveform, square the oscilloscope you can determine Basic properties of waves include: waveform, find its average voltage and whether a component of an electronic take the square root. system is behaving properly. So, to Amplitude understand how an oscilloscope Two main definitions for amplitude For a sine wave, the RMS amplitude operates, it is important to understand are commonly used in engineering is equal to 0.707 times the peak basic signal theory. applications. The first is often referred amplitude. to as the peak amplitude and is defined as the magnitude of the maximum Phase shift displacement of a disturbance. The Phase shift refers to the amount of horizontal translation between two otherwise identical waves. It is measured in degrees or radians. For a sine wave, one cycle is represented by 360 degrees. Therefore, if two sine waves differ by half of a cycle, their relative phase shift is 180 degrees. peak amplitude Period RMS amplitude The period of a wave is simply the amount of time it takes for a wave to repeat itself. It is measured in units of seconds. Frequency Every periodic wave has a frequency. The frequency is simply the number of times a wave repeats itself within Figure 1. Peak amplitude and RMS amplitude for a sine wave one second (if you are working in units of Hertz). The frequency is also the reciprocal of the period. period Figure 2. The period of a triangular wave 2 Oscilloscope Fundamentals Electronic Signals (continued) Waveforms Sine waves Square/rectangular waves Sine waves are typically associated A square waveform periodically jumps A waveform is the shape or with alternating current (AC) sources between two different values such representation of a wave. Waveforms such as an electrical outlet in your that the lengths of the high and low can provide you with a great deal of house. A sine wave does not always segments are equivalent. A rectangular information about your signal. For have a constant peak amplitude. If the waveform differs in that the lengths example, it can tell you if the voltage peak amplitude continually decreases of the high and low segments are not changes suddenly, varies linearly, or as time progresses, we call the equal. remains constant. There are many waveform a damped sine wave. standard waveforms, but this section will cover the ones you will encounter most frequently. Figure 3. A sine wave Figure 4. A square wave Oscilloscope Fundamentals 3 Electronic Signals (continued) Triangular/sawtooth waves Analog versus digital signals is, therefore, discretized into minute In a triangular wave, the voltage varies intervals. One second it might be linearly with time. The edges are called Analog signals are able to take on any 11:54 and then it jumps to 11:55 ramps because the waveform is either value within some range. It is useful suddenly. Digital signals are likewise ramping up or ramping down to certain to think of an analog clock. The clock discrete and quantized. Typically, voltages. A sawtooth wave looks hands spin around the clock face discrete signals have two possible similar in that either the front or back every twelve hours. During this time, values (high or low, 1 or 0, etc.). The edge has a linear voltage response the clock hands move continuously. signals, therefore, jump back and forth with time. However, the opposite edge There are no jumps or discreteness between these two possibilities. has an almost immediate drop. in the reading. Now, compare this to a digital clock. A digital clock simply Pulses tells you the hour and the minute. It A pulse is a sudden single disturbance in an otherwise constant voltage. Imagine flipping the switch to turn the lights on in a room and then quickly turning them off. A series of pulses is called a pulse train. To continue our analogy, this would be like quickly turning the lights on and off over and over again. Pulses are the common waveform of glitches or errors in your signal. A pulse might also be the waveform if Figure 5. A triangular wave the signal is carrying a single piece of information. Complex waves Waves can also be mixtures of the above waveforms. They do not necessarily need to be periodic and can take on very complex wave shapes. Figure 6. A sawtooth wave Figure 7. A pulse 4 Oscilloscope Fundamentals What Is an Oscilloscope and Why Do You Need One? Signal integrity however, important to remember that the waveform on an oscilloscope will The main purpose of an oscilloscope never be an exact representation of is to give an accurate visual the true signal, no matter how good representation of electrical signals. the oscilloscope is. This is because For this reason, signal integrity when you connect an oscilloscope to a is very important. Signal integrity circuit, the oscilloscope becomes part refers to the oscilloscope’s ability to of the circuit. In other words, there reconstruct the waveform so that it are some loading effects. Instrument is an accurate representation of the makers strive to minimize loading original signal. An oscilloscope with effects, but they always exist to some low signal integrity is useless because degree. it is pointless to perform a test when the waveform on the oscilloscope does not have the same shape or characteristics as the true signal. It is, Oscilloscope Fundamentals 5 What Is an Oscilloscope and Why Do You Need One? (continued) What an oscilloscope looks like You send your signals into the An example of what the back panel of oscilloscope via the channel inputs, an oscilloscope looks like is seen in In general, modern digitizing which are connectors for plugging Figure 9. oscilloscopes look similar to the one in your probes. The display is simply seen in Figure 8. However, there are the screen where these signals are As you can see, many oscilloscopes a wide variety of oscilloscope types, displayed. The horizontal and vertical have the connectivity features found and yours may look very different. control sections have knobs and on personal computers. Examples Despite this, there are some basic buttons that control the horizontal axis include CD-ROM drives, CD-RW drives, features that most oscilloscopes have. (which typically represents time) and DVD-RW drives, USB ports, serial The front panel of most oscilloscopes vertical axis (which represents voltage) ports, and external monitor,

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