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, mouse, can be divided into several basic of the signals on the screen display. and keyboard inputs. sections: the channel inputs, the The trigger controls allow you to tell display, the horizontal controls, the the oscilloscope under what conditions vertical controls, and the trigger you want the timebase to start an controls. If your oscilloscope does not acquisition. have a Microsoft® Windows®-based operating system, it will probably have a set of softkeys to control on-screen menus. Display Horizontal control section

Trigger control section

Vertical control section

Softkeys Channel inputs

Figure 8. Front panel on the Agilent InfiniiVision 2000 X-Series oscilloscope

Figure 9. Rear panel on the Agilent Infiniium 9000 Series oscilloscope

6 Oscilloscope Fundamentals What Is an Oscilloscope and Why Do You Need One? (continued)

An oscilloscope’s purpose the x and y-axes are broken into voltage measured by channel 1 versus divisions by a graticule. The graticule the voltage measured by channel 2. An oscilloscope is a measurement enables you to make measurements This mode is called the XY-mode of and testing instrument used to display by visual estimation, although with an oscilloscope. It is useful when a certain variable as a function of modern oscilloscopes, most of graphing I-V plots or Lissajous patterns another. For example, it can plot on these measurements can be made where the shape of these patterns its display a graph of voltage (y-axis) automatically and more accurately by tells you the phase difference and versus time (x-axis). Figure 10 shows the oscilloscope itself. the frequency ratio between the two an example of such a plot. This is signals. Figure 11 shows examples useful if you want to test a certain An oscilloscope can also do more of Lissajous patterns and the phase electronic component to see if it is than plot voltage versus time. An difference/frequency ratio they behaving properly. If you know what oscilloscope has multiple inputs, represent. the waveform of the signal should called channels, and each one of these be after exiting the component, you acts independently. Therefore, you can use an oscilloscope to see if could connect channel 1 to a certain the component is indeed outputting device and channel 2 to another. the correct signal. Notice also that The oscilloscope could then plot the

Figure 10. An oscilloscope’s voltage versus time display of a square wave

180 degrees; 1:1 ratio 90 degrees; 1:1 ratio 90 degrees; 1:2 ratio 30 degrees; 1:3 ratio

Figure 11. Lissajous patterns

Oscilloscope Fundamentals 7 What Is an Oscilloscope and Why Do You Need One? (continued)

Types of oscilloscopes makes the display act as a three- The attenuator scales the waveform. dimensional plot (in other words, x-axis The vertical amplifier provides Analog oscilloscopes is time, y-axis is voltage, and z-axis is additional scaling while passing the The first oscilloscopes were analog intensity). waveform to the analog-to-digital oscilloscopes, which use cathode- converter (ADC). The ADC samples and ray tubes to display a waveform. The downside of an analog digitizes the incoming signal. It then Photoluminescent phosphor on oscilloscope is that it cannot “freeze” stores this data in memory. The trigger the screen illuminates when an the display and keep the waveform for looks for trigger events while the electron hits it, and as successive an extended period of time. Once the time-base adjusts the time display for bits of phosphor light up, you can phosphorus substance deluminates, the oscilloscope. The microprocessor see a representation of the signal. that part of the signal is lost. Also, you system performs any additional A trigger is needed to make the cannot perform measurements on the postprocessing you have specified displayed waveform look stable. waveform automatically. Instead you before the signal is finally displayed on When one whole trace of the display have to make measurements usually the oscilloscope. is completed, the oscilloscope waits using the grid on the display. Analog until a specific event occurs (for oscilloscopes are also very limited in Having the data in digital form enables example, a rising edge that crosses the types of signals they can display the oscilloscope to perform a variety a certain voltage) and then starts the because there is an upper limit to of measurements on the waveform. trace again. An untriggered display is how fast the horizontal and vertical Signals can also be stored indefinitely unusable because the waveform is not sweeping of the electron beam can in memory. The data can be printed or shown as a stable waveform on the occur. While analog oscilloscopes are transferred to a computer via a flash display (this is true for DSO and MSO still used by many people today, they drive, LAN, USB, or DVD-RW. In fact, oscilloscopes, which will be discussed are not sold very often. Instead, digital software now allows you to control below, as well.) oscilloscopes are the modern tool of and monitor your oscilloscope from a choice. computer using a virtual front panel. Analog oscilloscopes are useful because the illuminated phosphor does Digital storage oscilloscopes (DSOs) not disappear immediately. You can Digital storage oscilloscopes (often see several traces of the oscilloscope referred to as DSOs) were invented to overlapping each other, which allows remedy many of the negative aspects you to see glitches or irregularities of analog oscilloscopes. DSOs input a in the signal. Since the display of the signal and then digitize it through the waveform occurs when an electron use of an analog-to-digital converter. strikes the screen, the intensity of Figure 12 shows an example of one the displayed signal correlates to the DSO architecture used by Agilent intensity of the actual signal. This digital oscilloscopes.

Channel memory

Channel Attenuater Vertical ADC MegaZoom Micro- Display Input amplifier processor

Trigger Time-base

Figure 12. Digitizing oscilloscope architecture

8 Oscilloscope Fundamentals What Is an Oscilloscope and Why Do You Need One? (continued)

Mixed signal oscilloscopes (MSOs) a small number of analog channels Portable/handheld oscilloscopes In a DSO, the input signal is analog (2 or 4) and a larger number of digital As its name implies, a portable and the digital-to-analog converter channels (see Figure 13). oscilloscope is one that is small digitizes it. However, as digital enough to carry around. If you need electronic technology expanded, Mixed signal oscilloscopes have the to move your oscilloscope around it became increasingly necessary advantage of being able to trigger on to many locations or from bench to to monitor analog and digital a combination of analog and digital bench in your lab, then a portable signals simultaneously. As a result, signals and display them all, correlated oscilloscope may be perfect for oscilloscope vendors began producing on the same time base. you. Figure 14 shows an example mixed signal oscilloscopes that can of a portable instrument, the trigger on and display both analog Agilent InfiniiVision 2000 X-Series and digital signals. Typically there are oscilloscope.

The advantages of portable oscilloscopes are that they are lightweight and portable, they turn on and off quickly, and they are easy to use. They tend to not have as much performance power as larger oscilloscopes, but scopes like the Agilent InfiniiVision 2000 and 3000 8 digital X-Series are changing that. These channels 4 analog channels oscilloscopes offer all the portability and ease typically found in portable Figure 13. Front panel inputs for the four analog channels and eight digital oscilloscopes, but are also powerful channels on a mixed-signal oscilloscope enough to handle most of today’s debugging needs up to 500 MHz bandwidth.

Figure 14. Agilent InfiniiVision 2000 X-Series portable oscilloscope

Oscilloscope Fundamentals 9 What Is an Oscilloscope and Why Do You Need One? (continued) Types of oscilloscopes

Economy oscilloscopes High-performance oscilloscopes Where oscilloscopes are used Economy oscilloscopes are reasonably High-performance oscilloscopes priced, but they do not have as much provide the best performance If a company is testing or using performance capability as high- capabilities available. They are used electronic signals, it is highly likely performance oscilloscopes. These by people who require high bandwidth, they have an oscilloscope. For this oscilloscopes are typically found in fast sampling and update rates, large reason, oscilloscopes are prevalent in university laboratories. The main memory depth, and a vast array of a wide variety of fields: advantage of these oscilloscopes is measurement capabilities. Figure 15 their low price. For a relatively modest shows an example of • Automotive technicians use amount of money, you get a very useful a high-performance oscilloscope, oscilloscopes to diagnose electrical oscilloscope. the Agilent Infiniium 90000A Series problems in cars. oscilloscope. • University labs use oscilloscopes to The main advantages of a teach students about electronics. high-performance oscilloscope are that the scope enables you to properly • Research groups all over the world analyze a wide range of signals, have oscilloscopes at their disposal. and provides many applications and tools that make analyzing current • Cell phone manufacturers use technology simpler and faster. oscilloscopes to test the integrity of The main disadvantages of high- their signals. performance oscilloscopes are their price and size. • The military and aviation industries use oscilloscopes to test radar communication systems.

• R&D engineers use oscilloscopes to test and design new technologies.

• Oscilloscopes are also used for compliance testing. Examples include USB and HDMI where the output must meet certain standards.

This is just a small subset of the possible uses of an oscilloscope. It truly is a versatile and powerful instrument.

Figure 15. Agilent Infiniium 90000A Series oscilloscope

10 Oscilloscope Fundamentals Basic Oscilloscope Controls and Measurements

Basic front-panel controls Vertical controls Horizontal controls Vertical controls on an oscilloscope An oscilloscope's horizontal controls Typically, you operate an oscilloscope typically are grouped in a section typically are grouped in a front- using the knobs and buttons on the marked Vertical. These controls allow panel section marked Horizontal. front panel. In addition to controls you to adjust the vertical aspects of These controls enable you to make found of the front panel, many high- the display. For example, there will be adjustments to the horizontal scale of end oscilloscopes now come equipped a control that designates the number the display. There will be a control that with operating systems, and as a of volts per division (scale) on the designates the time per division on the result, they behave like computers. You y-axis of the display grid. You can x-axis. Again, decreasing the time per can hook up a mouse and keyboard to zoom in on a waveform by decreasing division enables you to zoom in on a the oscilloscope and use the mouse to the volts per division or you can zoom narrower range of time. There will also adjust the controls through drop down out by increasing this quantity. There be a control for the horizontal delay menus and buttons on the display as also is a control for the vertical offset (offset). This control enables you to well. In addition, some oscilloscopes of the waveform. This control simply scan through a range of time. You can have touch screens so you can use translates the entire waveform up or see the horizontal control section for a stylus or fingertip to access the down on the display. You can see the Agilent InfiniiVision 2000 X-Series menus. the vertical control section for oscilloscope in Figure 17. an Agilent InfiniiVision 2000 X-Series Before you begin . . . oscilloscope in Figure 16. When you first sit down at your oscilloscope, check that the input channel you are using is turned on. Then press [Default Settings] if there is one. This will return the oscilloscope to its original default state. Then Turns channel 1 on Adjusts the vertical scaling for channel 4 press [Autoscale] if there is one. This will automatically set the vertical and horizontal scale such that your waveform can be nicely viewed on the display. Use this as a starting point and then make needed adjustments. If you ever lose track of your waveform or you are having a hard time displaying it, repeat these steps. Most oscilloscope front panels contain at least four main sections: vertical and horizontal controls, triggering controls, Vertically positions the waveform on channel 2 and input controls. Figure 16. Front panel vertical control section on an Agilent InfiniiVision 2000 X-Series oscilloscope

Adjusts the horizontal scaling Horizontally positions the waveform

Figure 17. Front panel horizontal control section on an Agilent InfiniiVision 2000 X-Series oscilloscope

Oscilloscope Fundamentals 11 Basic Oscilloscope Controls and Measurements (continued)

Trigger controls As we mentioned earlier, triggering on your signal helps provide a stable, Trigger voltage usable display and allows you to synchronize the scope’s acquisition on the part of the waveform you are interested in viewing. The trigger Rising edge triggering controls let you pick your vertical trigger level (for example, the voltage Figure 18. When you trigger on a rising at which you want your oscilloscope to edge, the oscilloscope triggers when the trigger) and choose between various trigger threshold is reached triggering capabilities. Examples of common triggering types include:

Edge triggering – Edge triggering is the most popular triggering mode. The trigger occurs when the voltage surpasses some set threshold value. You can choose between triggering on a rising or a falling edge. Figure 18 shows a graphical representation of triggering on a rising edge.

Glitch triggering – Glitch triggering mode enables you to trigger on an event or pulse whose width is greater than or less than some specified length of time. This capability is very useful for finding random glitches or errors. If these glitches do not occur very often, it may be very difficult to see them. However, glitch triggering allows you to catch many of these errors. Figure 19 shows a glitch caught Figure 19. An infrequent glitch caught on an Agilent InfiniiVision by an Agilent InfiniiVision 6000 Series 6000 Series oscilloscope oscilloscope.

12 Oscilloscope Fundamentals Basic Oscilloscope Controls and Measurements (continued)

Pulse-width triggering – Pulse usually have a button associated with acquisition. Some of today’s higher width triggering is similar to glitch each particular channel that enables performance oscilloscopes can capture triggering when you are looking for you to turn them on or off. There may up to 32-GHz bandwidth signals in a specific pulse widths. However, it is also be a selection that allows you single-shot utilizing real-time sampling more general in that you can trigger to specify AC or DC coupling. If DC on pulses of any specified width and coupling is selected, the entire signal Equivalent-time sampling – you can choose the polarity (negative will be input. On the other hand, AC Equivalent time sampling builds up the or positive) of the pulses you want to coupling blocks the DC component and waveform over several acquisitions. It trigger. You can also set the horizontal centers the waveform about 0 volts samples part of the signal on the first position of the trigger. This allows (ground). In addition, you can specify acquisition, then another part on the you to see what occurred pre-trigger the probe impedance for each channel second acquisition, and so on. It then or post-trigger. For instance, you can through a selection button. The input laces all this information together to execute a glitch trigger, find the error, controls also let you choose the type recreate the waveform. Equivalent time and then look at the signal pre-trigger of sampling. There are two basic ways sampling is useful for high-frequency to see what caused the glitch. If you to sample the signal: signals that are too fast for real-time have the horizontal delay set to zero, sampling (>32 GHz). your trigger event will be placed in Real-time sampling – Real-time the middle of the screen horizontally. sampling samples the waveform often Events that occur right before the enough that it captures a complete trigger will be to the left of the screen image of the waveform with each and events that occur directly after the trigger will be to the right of the screen. You also can set the coupling of the trigger and set the input source you want to trigger on. You do not always have to trigger on your signal, but can instead trigger a related signal. Figure 20 shows the trigger control section of an oscilloscope’s front panel.

Input controls There are typically two or four analog channels on an oscilloscope. They will be numbered and they will also

Adjusts the trigger level

These keys allow you to select the trigger mode

Figure 20. Front panel trigger control section on an Agilent InfiniiVision 2000 X-Series oscilloscope

Oscilloscope Fundamentals 13 Basic Oscilloscope Controls and Measurements (continued)

Softkeys like when a softkey is pressed. The specific menu shown in the figure is Softkeys are found on oscilloscopes for selecting the trigger mode. You can that do not have Windows-based continually press the softkey to cycle operating systems (refer to Figure 8 for through the choices, or there may be a a picture of softkeys). These softkeys knob on the front panel that allows you allow you to navigate the menu system to scroll to your selection. on the oscilloscope’s display. Figure 21 shows what a popup menu looks

Figure 21. The Trigger Type menu appears when you push the softkey underneath the trigger menu

14 Oscilloscope Fundamentals Basic Oscilloscope Controls and Measurements (continued)

Basic measurements Risetime negative pulse width measurement This measurement calculates the computes the width of a pulse by Digital oscilloscopes allow you to amount of time it takes for the signal calculating the time it takes for the perform a wide range of measurements to go from a low voltage to a high wave to go from 50% of the peak-to- on your waveform. The complexity voltage. It is usually calculated by peak voltage to the minimum voltage and range of measurements available computing the time it takes to go and then back to the 50% mark. depends on the feature set of your from 10% to 90% of the peak-to-peak oscilloscope. Figure 22 shows the voltage. Period blank display of an Agilent 8000 Series This measurement calculates the oscilloscope. Notice the measurement Pulse width period of the waveform. buttons/icons lined up on the far-left A positive pulse width measurement side of the screen. Using a mouse, computes the width of a pulse by Frequency you can drag these icons over to a calculating the time it takes for the This measurement calculates the waveform and the measurement will wave to go from 50% of the peak-to- frequency of your waveform. be computed. They are also convenient peak voltage to the maximum voltage because the icons give you an and then back to the 50% mark. A This list is intended to give you an idea indication of what the measurement of the kinds of measurements available computes. on many oscilloscopes. However, most oscilloscopes can perform many more Basic measurements found on many measurements. oscilloscopes include:

Peak-to-peak voltage This measurement calculates the voltage difference between the low voltage and high voltage of a cycle on your waveform.

RMS voltage This measurement calculates the RMS voltage of your waveform. This quantity can then be used to compute the power.

Figure 23. Peak-to-peak voltage

Figure 22. The blank display of an Agilent 8000 Series oscilloscope Figure 24. An example of risetime (0% to 100% of peak-to-peak voltage is shown instead of the usual 10% to 90%) Oscilloscope Fundamentals 15 Basic Oscilloscope Controls and Measurements (continued)

Basic mathematical functions Integration This math function computes the In addition to the measurements integral of your waveform. discussed above, there are many mathematical operations you can Addition or subtraction perform on your waveforms. Examples These math functions enable you to include: add or subtract multiple waveforms and display the resulting signal. Fourier transform This math function allows you to see Again, this is a small subset of the frequencies that compose your the possible measurements and signal. mathematical functions available on an oscilloscope. Absolute value This math function shows the absolute value (in terms of voltage) of your waveform.

16 Oscilloscope Fundamentals Important Oscilloscope Performance Properties

Many oscilloscope properties Bandwidth Channels dramatically affect the instrument’s performance and, in turn, your ability Bandwidth is the single most A channel refers to an independent to accurately test devices. This section important characteristic of an input to the oscilloscope. The number covers the most fundamental of these oscilloscope, as it gives you an of oscilloscope channels varies properties. It also will familiarize you indication of its range in the frequency between two and twenty. Most with oscilloscope terminology and domain. In other words, it dictates the commonly, they have two or four describe how to make an informed range of signals (in terms of frequency) channels. The type of signal a channel decision about which oscilloscope will that you are able to accurately display carries also varies. Some oscilloscopes best suit your needs. and test. Bandwidth is measured in have purely analog channels (these Hertz. Without sufficient bandwidth, instruments are called DSOs – digital your oscilloscope will not display an signal oscilloscopes). Others, called accurate representation of the actual mixed signal oscilloscopes (MSOs), signal. For example, the amplitude of have a mixture of analog and digital the signal may be incorrect, edges channels. For example, the Agilent may not be clean, and waveform InfiniiVision Series MSOs are available details may be lost. The bandwidth with twenty channels, where up to of an oscilloscope is the lowest sixteen of them are digital and four are frequency at which an input signal analog. is attenuated by 3 dB. Another way to look at bandwidth: If you input a Ensuring that you have enough pure sine wave into the oscilloscope, channels for your applications is the bandwidth will be the minimum essential. If you have two channels, frequency where the displayed but you need to display four signals amplitude is 70.7% of the actual signal simultaneously, then you obviously amplitude. have a problem.

For details about oscilloscope bandwidth, see application note, Evaluating Oscilloscope Bandwidths for Your Applications.

8 digital channels 4 analog channels

Figure 25. Analog and digital channels on an Agilent MSO 2000 X-Series oscilloscope

Oscilloscope Fundamentals 17 Important Oscilloscope Performance Properties (continued)

Sample rate channels are used simultaneously, the If we kept increasing the sample sample rate may decrease. Therefore, rate for the waveform in this above The sample rate of an oscilloscope it is wise to check how many channels example, the sampled points would is the number of samples the you can use while still maintaining eventually look almost continuous. In oscilloscope can acquire per second. It the specified maximum sample rate. fact, oscilloscopes usually use sin(x)/x is recommended that your oscilloscope If the sample rate of an oscilloscope interpolation to fill in between the have a sample rate that is at a least is too low, the signal you see on the sampled points. 2.5 times greater than its bandwidth. scope may not be very accurate. As However, ideally the sample rate an example, assume you are trying to For more information about should be 3 times the bandwidth or view a waveform, but the sample rate oscilloscope sampling rates, greater. only produces two points per period see application note, Evaluating (Figure 26). Oscilloscope Sample Rates vs. You need to be careful when you Sampling Fidelity: How to Make the evaluate an oscilloscope’s sample rate Now consider the same waveform, Most Accurate Digital Measurements. banner specifications. Manufacturers but with an increased sample rate that typically specify the maximum sample samples seven times per period rate an oscilloscope can attain, (Figure 27). and sometimes this maximum rate is possible only when one or two It is clear that the greater the samples channels are being used. If more per second, the more clearly and accurately the waveform is displayed.

Figure 26. Waveform where the Figure 27. Waveform where the sample rate yields two data points sample rate yields seven data per period points per period

18 Oscilloscope Fundamentals Important Oscilloscope Performance Properties (continued)

Memory depth memory depth an oscilloscope has, the more time you can spend capturing As we mentioned earlier, a digital waveforms at full sampling speed. oscilloscope uses an A/D (analog-to- Mathematically, this can be digital) converter to digitize the input seen by: waveform. The digitized data is then stored in the oscilloscope’s high-speed Memory depth = memory. Memory depth refers to (sample rate)(time across display) exactly how many samples or points and, therefore, what length of time can So, if you are interested in looking be stored. at long periods of time with high resolution between points, you will Memory depth plays an important role need deep memory. It is also important in the sampling rate of an oscilloscope. to check the responsiveness of the In an ideal world, the sampling rate oscilloscope when it is in the deepest would remain constant no matter what memory depth setting. Scopes the settings were on an oscilloscope. usually have a severe drop in update However, this kind of an oscilloscope rate performance in this mode and, would require a huge amount of therefore, many engineers only use memory at a large time/division deep memory when it is essential for setting and would have a price that their purposes. would severely limit the number of customers that could afford it. Instead, To learn more about oscilloscope the sampling rate decreases as you memory depth, see application increase the range of time. Memory note, Demystifying Deep Memory depth is important because the more Oscilloscopes.

Oscilloscope Fundamentals 19 Important Oscilloscope Performance Properties (continued)

Update rate oscilloscope has an update rate of Oscilloscope connectivity 100,000 waveforms per second, then Update rate refers to the rate at you will capture this glitch twice per Oscilloscopes come with a wide range which an oscilloscope can acquire second on average. If, however, your of connectivity features. Some are and update the display of a waveform. oscilloscope has an update rate of 800 equipped with USB ports, DVD-RW While it may appear to the human waveforms per second, then it would drives, external hard drives, external eye that the scope is displaying a take you one minute on average. This monitor ports, and much more. All “live” waveform, it is because the is a long time to be watching. of these features make it easier to updates are occurring so fast that use your oscilloscope and transfer the human eye cannot detect the Update rate specifications need to be data. Some oscilloscopes also come changes. In actuality, there is some read with care. Some manufacturers equipped with operating systems that dead-time in between acquisitions of require special acquisition modes to allow your oscilloscope to behave like the waveform (Figure 28). During this attain the banner specification update a personal computer. With an external dead-time, a portion of the waveform rates. These acquisition modes can monitor, a mouse, and a keyboard, you is not displayed on the oscilloscope. severely limit the performance of the can view your oscilloscope’s display As a result, if some infrequent event oscilloscope in areas such as memory and control your oscilloscope as if it or glitch occurs during one of these depth, sample rate, and waveform were embedded in your PC’s tower. moments, you will not see it. reconstruction. Therefore, it is wise You can also transfer data from an to check the performance of the oscilloscope to a PC via a USB or LAN It is easy to see why having a fast oscilloscope when it is displaying connection in many instances. update rate is important. Faster update waveforms with this maximum update rates mean shorter dead-times, which rate. Good connectivity features can means a higher probability of catching save you a great deal of time and infrequent events or glitches. make completing your job easier. For instance, it can allow you to Say for example you are displaying a quickly and seamlessly transfer data signal that has a glitch which occurs to your laptop or share data with once every 50,000 cycles. If your geographically dispersed colleagues. It can also allow you to remotely control your oscilloscope from your PC. In a world where the efficient transfer of data is a requirement in many situations, purchasing an oscilloscope Display “Effective” Display with quality connectivity features is a Window dead-time Window very good investment.

Acquisition “Real” Acquisition time dead-time time

Figure 28. Visual depiction of dead-time. The circles highlight two infrequent events that would not be displayed

20 Oscilloscope Fundamentals Oscilloscope Probes

The oscilloscope is just one piece Inductive loading―appears as ringing Active probes of the system that determines how in your signal. It occurs because of the accurately you are able to display and inductive effects of the probe ground To operate an active probe, the probe analyze your signals. Probes, which lead, so use the shortest lead possible. requires a power supply to power- are used to connect the oscilloscope up active devices within the probe to your device under test (DUT), are Passive probes itself. The required power supply is crucial in terms of signal integrity. If sometimes supported with a USB you have a 1-GHz oscilloscope but Passive probes contain only passive cable connection, an external “box”, only have a probe that supports a components and do not require a and sometimes it is supplied by bandwidth of 500 MHz, you are not power supply for their operation. They the scopes mainframe itself. Active fully utilizing the bandwidth of your are useful for probing signals with probes use active components to oscilloscope. This section discusses bandwidths less than 600 MHz. Once amplify or condition a signal. They are the types of probes and when you this frequency is surpassed, a different able to support much higher signal should use each one. kind of probe is required (an active bandwidths and are, therefore, the probe). probes of choice for high-performance Loading applications. Passive probes are typically No probe is able to perfectly reproduce inexpensive, easy to use, and rugged. Active probes are considerably more your signal, because when you They are a versatile and accurate expensive than passive probes. Active connect a probe to a circuit, the probe type of probe. Types of passive probes probes also tend to be less rugged and becomes part of that circuit. Part of the include low-impedance resistor-divider the probe tip on active probes tend to electrical energy in the circuit flows probes, compensated, high-resistance be heavier. However, they provide the through the probe. This phenomenon passive divider probes, and high- best overall combination of resistive is called loading. There are three types voltage probes. and capacitive loading and allow you of loading: resistive, capacitive, and to test much higher-frequency signals. inductive. Passive probes usually produce relatively high capacitive loading and The Agilent InfiniiMax Series probes Resistive loading―can cause the low resistive loading. are high-performance probes. They use amplitude of your displayed signal to a damping resistor in the probe tips be incorrect. It can also cause a circuit to significantly reduce loading effects. that is malfunctioning to start working They also have very high bandwidths. when the probe is attached. It is a good idea to make sure the resistance of your probe is greater than ten times the resistance of the source in order to get an amplitude reduction of less than ten percent.

Capacitive loading―causes rise times to be slowed and bandwidth to be reduced. To reduce capacitive loading, choose a probe with at least five times the bandwidth of your signal.

Figure 29. A passive probe Figure 30. An active probe

Oscilloscope Fundamentals 21 Oscilloscope Probes (continued)

Current probes Probe accessories

Current probes are used to measure Probes also come with a variety of the current flowing through a circuit. probe tips. There are many different They tend to be big and have limited types of probes tips, everything from bandwidth (100 MHz). bulky tips that can wrap around cables to tips the size of several hairs. These tips make it easier for you to access various parts of a circuit or a device under test.

Figure 31. A current probe

22 Oscilloscope Fundamentals Conclusion

Related literature Oscilloscopes are a powerful tool in the technological world we currently Publication title Publication type Publication number live. They are used in a wide range Evaluating Oscilloscope Bandwidths for Your Application note 5989-5733EN of fields and offer many advantages Applications over other measurement and testing devices. After reading this application Evaluating Oscilloscope Sample Rates vs. Sampling Application note 5989-5732EN Fidelity note, you should have a good feel for oscilloscope fundamentals. Take this Demystifying Deep Memory Oscilloscopes Application note 5989-4501EN knowledge and continue to read more Agilent Technologies Oscilloscopes Brochure 5989-7650ENUS advanced topics so you can make the Agilent U1600A Series handheld oscilloscope Data sheet 5989-5576EN most of your time with an oscilloscope. Agilent Technologies InfiniiVision 2000 X-Series Data sheet 5990-6618EN oscilloscopes Agilent Technologies InfiniiVision 3000 X-Series Data sheet 5990-6619EN oscilloscopes Agilent Technologies InfiniiVision 6000 Series oscil- Data sheet 5989-2000EN loscopes Agilent Technologies InfiniiVision 6000L Series oscil- Data sheet 5989-5470EN loscopes Agilent Technologies InfiniiVision 7000 Series oscil- Data sheet 5989-7736EN loscopes Agilent Technologies Infiniium 9000 Series Data sheet 5990-3746EN oscilloscopes Agilent Technologies Infiniium DSO/DSA90000A Series Data sheet 5989-7819EN oscilloscopes Agilent Technologies Infiniium DSO/DSA90000X Series Data sheet 5990-5271EN oscilloscopes Agilent Technologies 86100D Infiniium DCA-X Data sheet 5989-5235EN wide-bandwidth oscilloscopes Learn more about Agilent oscilloscopes at www.agilent.com/find/scopes

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Oscilloscope Fundamentals 23 www.agilent.com

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