Electronic equipment can be classified into two categories: analog and digital. Analog equipment works with continuously variable voltages, while digital equipment works with discrete binary numbers that represent voltage samples. A conventional phonograph is an analog device, while a CD player is a digital device. Oscilloscopes can be classified similarly – as analog and digital types. 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 reassembles the waveform for display on the screen, as shown in Figure 11.

Figure 11: Analog oscilloscopes trace signals, while digital oscilloscopes sample signals and construct displays.

Types of Digital Oscilloscopes

Digital oscilloscopes can be classified into four types:

• Digital storage oscilloscopes (DSO)
• Digital phosphor oscilloscopes (DPO)
• Mixed signal oscilloscopes (MSO)
• Digital sampling oscilloscopes

This chapter describes these types of digital oscilloscopes in detail to help you zero in on the type of oscilloscope that’s right for your needs.

Digital Storage Oscilloscopes (DSO)

A conventional digital oscilloscope is known as a digital storage oscilloscope (DSO). Its display typically relies on a raster-type screen rather than the luminous phosphor found in older analog oscilloscopes.

DSOs allow you to capture and view events that may happen only once, known as transients. Because the waveform information exists in digital form as a series of stored binary values, it can be analyzed, archived, printed, and otherwise processed, within the oscilloscope itself or by an external computer.

The waveform need not be continuous; it can be displayed even when the signal disappears. Unlike analog oscilloscopes, DSOs provide permanent signal storage and extensive waveform processing. However, DSOs typically have no real-time intensity grading. Therefore they cannot express varying levels of intensity in the live signal.

Some of the subsystems in DSOs are similar to those in analog oscilloscopes. However, DSOs contain additional data-processing subsystems that are used to collect and display data for the entire waveform. A DSO employs a serial-processing architecture to capture and display a signal on its screen, as shown in Figure 12.

Figure 12: The serial-processing architecture of a digital storage oscilloscope (DSO).

Serial-processing Architecture

Like an analog oscilloscope, a DSO’s first (input) stage is a vertical amplifier. Vertical controls allow you to adjust the amplitude and position range at this stage. Next, the analog to digital converter (ADC) in the horizontal system samples the signal at discrete points in time and converts the signal’s voltage at these points into digital values called sample points. This process is referred to as digitizing a signal.

The horizontal system’s sample clock determines how often the ADC takes a sample. This rate is referred to as the sample rate and is expressed in samples per second (S/s).

The sample points from the ADC are stored in acquisition memory as waveform points. Several sample points may comprise one waveform point. Together, the waveform points comprise one waveform record. The number of waveform points used to create a waveform record is called the record length. The trigger system determines the start and stop points of the record.

The DSO’s signal path includes a microprocessor through which the measured signal passes on its way to the display. This microprocessor processes the signal, coordinates display activities, manages the front panel controls, and more. The signal then passes through the display memory and is displayed on the oscilloscope screen.

Depending on the capabilities of an oscilloscope, additional processing of the sample points may take place, which enhances the display. Pre-trigger may also be available so you can see events before the trigger point. Most digital oscilloscopes also provide a selection of automatic parametric measurements, simplifying the measurement process.

DSOs provide high performance in a single-shot, multi-channel instrument (Figure 13). They are ideal for low repetition-rate or single-shot, high-speed, multichannel design applications. In the real world of digital design, an engineer usually examines four or more signals simultaneously, making the DSO a critical companion.

Figure 13: The digital storage oscilloscope delivers high-speed, single-shot acquisition across multiple channels, increasing the likelihood of capturing elusive glitches and transient events

Digital Phosphor Oscilloscopes (DPO)

The digital phosphor oscilloscope (DPO) offers a new approach to oscilloscope architecture. This architecture enables it to deliver unique acquisition and display capabilities to accurately reconstruct a signal. While a DSO uses a serial-processing architecture to capture, display, and analyze signals, a DPO employs a parallel processing architecture to perform these functions (Figure 14).

Figure 14: The parallel-processing architecture of a digital phosphor oscilloscope (DPO).

The DPO architecture dedicates unique ASIC hardware to acquire waveform images, delivering high waveform capture rates, resulting in a higher level of signal visualization. This performance increases the probability of witnessing transient events that occur in digital systems, such as runt pulses, glitches and transition errors, and enables additional analysis capability.

Parallel-processing Architecture

A DPO’s first (input) stage is similar to that of an analog oscilloscope—a vertical amplifier—and its second stage is similar to that of a DSO—an ADC. But, the DPO differs significantly from its predecessors following the analog-to-digital conversion.

For any oscilloscope—analog, DSO, or DPO—there is always a hold-off time during which the instrument processes the most recently acquired data, resets the system, and waits for the next trigger event. During this time, the oscilloscope is blind to all signal activity. The probability of seeing an infrequent or low repetition event decreases as the hold off time increases.

It is impossible to determine the probability of capture by simply looking at the display update rate. If you rely solely on the update rate, it is easy to make the mistake of believing that the oscilloscope is capturing all pertinent information about the waveform when, in fact, it is not.

The DSO processes captured waveforms serially. The speed of its microprocessor is a bottleneck in this process because it limits the waveform capture rate. The DPO rasterizes the digitized waveform data into a digital phosphor database. Every 1/30th of a second—about as fast as the human eye can perceive it—a snapshot of the signal image that is stored in the database is pipelined directly to the display. This direct rasterization of waveform data, and direct copy to display memory from the database, removes the data-processing bottleneck inherent in other architectures. The result is an enhanced “real-time” and lively display update. Signal details, intermittent events, and dynamic characteristics of the signal are captured in real-time. The DPO’s microprocessor works in parallel with this integrated acquisition system for display management, measurement automation, and instrument control, so that it does not affect the oscilloscope’s acquisition speed.

A DPO faithfully emulates the best display attributes of an analog oscilloscope, displaying the signal in three dimensions: time, amplitude, and the distribution of amplitude over time. All in real-time.

Unlike an analog oscilloscope’s reliance on chemical phosphor, a DPO uses a purely electronic digital phosphor that’s actually a continuously updated database. This database has a separate “cell” of information for every single pixel in the oscilloscope’s display. Each time a waveform is captured—in other words, every time the oscilloscope triggers—it is mapped into the digital phosphor database’s cells. Each cell that represents a screen location and is touched by the waveform is reinforced with intensity information, while other cells are not. Thus, intensity information builds up in cells where the waveform passes most often.

When the digital phosphor database is fed to the oscilloscope’s display, the display reveals intensified waveform areas, in proportion to the signal’s frequency of occurrence at each point, much like the intensity grading characteristics of an analog oscilloscope. The DPO also allows the display of the varying frequency-of-occurrence information on the display as contrasting colors, unlike an analog oscilloscope. With a DPO, it is easy to see the difference between a waveform that occurs on almost every trigger and one that occurs, say, every 100th trigger.

A DPO break down the barrier between analog and digital oscilloscope technologies. They are equally suitable for viewing high and low frequencies, repetitive waveforms, transients, and signal variations in real-time. Only a DPO provides the Z-axis (intensity) in real-time that is missing from conventional DSOs.

A DPO is ideal for those who need the best general purpose design and troubleshooting tool for a wide range of applications (Figure 15). A DPO is exemplary for advanced analysis, communication mask testing, digital debug of intermittent signals, repetitive digital design. and timing applications.

Figure 15: Some DPOs can acquire millions of waveform in just seconds, significantly increasing the probability of capturing intermittent and elusive events and revealing dynamic signal behavior.

Mixed Domain Oscilloscopes (MDO)

Electronic equipment can be classified into two categories: analog and digital. Analog equipment works with continuously variable voltages, while digital equipment works with discrete binary numbers that represent voltage samples. A phonograph is an analog device, while an MP3 player is a digital device.

Figure 16: Time-correlated display of a Zigbee radio's microprocessor SPI (MOSI) and (MISO) control lines, with measurements of drain current and voltage to the radio IC and the spectrum during turn-on.

Mixed Signal Oscilloscopes (MSO)

The mixed signal oscilloscope (MSO) combines the performance of a DPO with the basic functionality of a 16-channel logic analyzer, including parallel/serial bus protocol decoding and triggering.

The MSO's digital channels view a digital signal as either a logic high or logic low, just like a digital circuit views the signal. This means as long as ringing, overshoot and ground bounce do not cause logic transitions, these analog characteristics are not of concern to the MSO. Just like a logic analyzer, a MSO uses a threshold voltage to determine if the signal is logic high or logic low.

The MSO is the tool of choice for quickly debugging digital circuits using its powerful digital triggering, high-resolution acquisition capability, and analysis tools. The root cause of many digital problems is quicker to pinpoint by analyzing both the analog and digital representations of the signal, as shown in Figure 17, making an MSO ideal for verifying and debugging digital circuits.

Figure 17: The MSO provides 16 integrated digital channels, enabling the ability to view and analyze time-correlated analog and digital signals.

Digital Sampling Oscilloscopes

In contrast to the digital storage and DPO architectures, the architecture of the digital sampling oscilloscope reverses the position of the attenuator/ amplifier and the sampling bridge (Figure 18). The input signal is sampled before any attenuation or amplification is performed. A low bandwidth amplifier can then be used after the sampling bridge because the signal has already been converted to a lower frequency by the sampling gate, resulting in a much higher bandwidth instrument.

Figure 18: The parallel-processing architecture of a digital phosphor oscilloscope (DPO).

The tradeoff for this high bandwidth, however, is that the sampling oscilloscope’s dynamic range is limited. Since there is no attenuator/amplifier in front of the sampling gate, there is no facility to scale the input. The sampling bridge must be able to handle the full dynamic range of the input at all times. Therefore, the dynamic range of most sampling oscilloscopes is limited to about 1 V peak-to-peak. Digital storage and DPOs, on the other hand, can handle 50 to 100 volts.

In addition, protection diodes cannot be placed in front of the sampling bridge, as this limits the bandwidth. This reduces the safe input voltage for a sampling oscilloscope to about 3 V, as compared to 500 V available on other oscilloscopes.

When measuring high-frequency signals, the DSO or DPO may not be able to collect enough samples in one sweep. A digital sampling oscilloscope is an ideal tool for accurately capturing signals whose frequency components are much higher than the oscilloscope’s sample rate (Figure 19). This oscilloscope is capable of measuring signals of up to an order of magnitude faster than any other oscilloscope. It can achieve bandwidth and high-speed timing ten times higher than other oscilloscopes for repetitive signals. Sequential equivalent-time sampling oscilloscopes are available with bandwidths to 80 GHz.

Figure 19: Time domain reflectometry (TDR) display from a digital sampling oscilloscope.

Once you've determined the type of oscilloscope you need, there are still many models to choose from, including portable and hand-held. And when choosing an oscilloscope, there are a number of things to consider, such as the ease-of use, sample rate the probes used to bring data into it, and all the elements of an oscilloscope that affect its ability to achieve the required signal integrity.

To understand these considerations, we'll look briefly at ease-of-use and probes, and then describe some useful measurement and oscilloscope performance terms. These terms cover the criteria essential to choosing the right oscilloscope for your application.

Ease-of-Use

Oscilloscopes should be easy to learn and easy to use, helping you work at peak efficiency and productivity. This means you can focus on your design, rather than the measurement tools. Just as there is no one typical car driver, there is no one typical oscilloscope user. Regardless of whether you prefer a traditional instrument interface or a Windows® software interface, it is important to have flexibility in your oscilloscope's operation. Many oscilloscopes offer a balance between performance and simplicity by providing many ways to operate the instrument. A typical oscilloscope's front-panel layout (Figure 60) provides dedicated vertical, horizontal and trigger controls.

Figure 60: Traditional, analog-style knobs control position, scale, intensity, etc. – precisely as you would expect.

The Complete Measurement System Probes

Even the most advanced instrument can only be as precise as the data that goes into it. A probe works in conjunction with an oscilloscope as part of the measurement system. Precision measurements start at the probe tip. The right probes matched to the oscilloscope and the device under test (DUT) not only allow the signal to be brought to the oscilloscope cleanly, they also amplify and preserve the signal for the greatest signal integrity and measurement accuracy. Please refer to the Tektronix ABCs of Probes Primer for more information about probes and probe accessories.

Bandwidth

Bandwidth determines an oscilloscope's fundamental ability to measure a signal. As signal frequency increases, the capability of an oscilloscope to accurately display the signal decreases. The bandwidth specification indicates the frequency range that the oscilloscope can accurately measure.

Oscilloscope bandwidth is specified as the frequency at which a sinusoidal input signal is attenuated to 70.7% of the signal's true amplitude, known as the –3 dB point, a term based on a logarithmic scale, as shown in Figure 44.

Figure 44: Oscilloscope bandwidth is the frequency at which a sinusoidal input signal is attenuated to 70.7% of the signal's true amplitude, known as the -3 dB point.

Without adequate bandwidth, an oscilloscope cannot resolve high-frequency changes. Amplitude is distorted. Edges vanish. Details are lost. All the features, bells and whistles in your oscilloscope will mean nothing.

To determine the oscilloscope bandwidth needed to accurately characterize signal amplitude in your specific application, apply the “5 Times Rule”:

5 Times Rule

An oscilloscope selected using the “5 Times Rule” provides less than ±2% error in your measurements. This is typically sufficient for today's applications. However, as signal speeds increase, it may not be possible to achieve this rule of thumb. Keep in mind that higher bandwidth will likely provide more accurate reproduction of a signal, as shown in Figure 45.

Figure 45: The higher the bandwidth, the more accurate the reproduction of your signal, as illustrated with a signal captured at 250 MHz, 1 GHz and 4 GHz bandwidth levels.

Some oscilloscopes provide a method of enhancing the bandwidth through digital signal processing (DSP). A DSP arbitrary equalization filter can be used to improve the oscilloscope channel response. This filter extends the bandwidth, flattens the oscilloscope's channel frequency response, improves phase linearity, and provides a better match between channels. It also decreases rise time and improves the time domain step response.

Analog and digital oscilloscopes have some basic controls that are similar, and some that are different. We’ll look at the basic systems and controls that are common to both. Understanding these systems and controls is key to using an oscilloscope to tackle your specific measurement challenges. Note that your oscilloscope probably has additional controls not discussed here.

The Three Systems

A basic oscilloscope consists of three different systems – the vertical system, horizontal system, and trigger system. Each system contributes to the oscilloscope’s ability to accurately reconstruct a signal.

The front panel of an oscilloscope is divided into three sections labeled Vertical, Horizontal, and Trigger. Your oscilloscope may have other sections, depending on the model and type.

When using an oscilloscope, you adjust settings in these areas to accommodate an incoming signal:

• Vertical: This is the attenuation or amplification of the signal. Use the volts/div control to adjust the amplitude of the signal to the desired measurement range.
• Horizontal: This is the time base. Use the sec/div control to set the amount of time per division represented horizontally across the screen.
• Trigger: This is the triggering of the oscilloscope. Use the trigger level to stabilize a repeating signal, or to trigger on a single event.

We’ll look at each of these systems and controls, in this chapter.

Figure 20: Front-panel control section of an oscilloscope.

Vertical System and Controls

Vertical controls are used to position and scale the waveform vertically, set the input coupling, and adjust other signal conditioning. Common vertical controls include:

• Position
• Coupling: DC, AC, and GND
• Bandwidth: Limit and Enhancement
• Termination: 1M ohm and 50 ohm
• Offset
• Invert: On/Off
• Scale: Fixed Steps and Variable

Some of these controls are described next.

Position and Volts per Division

The vertical position control allows you to move the waveform up and down so it’s exactly where you want it on the screen.

The volts-per-division setting (usually written as volts/div) is a scaling factor that varies the size of the waveform on the screen. If the volts/div setting is 5 volts, then each of the eight vertical divisions represents 5 volts and the entire screen can display 40 volts from bottom to top, assuming a graticule with eight major divisions. If the setting is 0.5 volts/div, the screen can display 4 volts from bottom to top, and so on.

The maximum voltage you can display on the screen is the volts/div setting multiplied by the number of vertical divisions. Note that the probe you use, 1X or 10X, also influences the scale factor.

You must divide the volts/div scale by the attenuation factor of the probe if the oscilloscope does not do it for you. Often the volts/div scale has either a variable gain or a fine gain control for scaling a displayed signal to a certain number of divisions. Use this control to assist in taking rise time measurements.

Input Coupling

Coupling refers to the method used to connect an electrical signal from one circuit to another. In this case, the input coupling is the connection from your test circuit to the oscilloscope.

The coupling can be set to DC, AC, or ground. DC coupling shows all of an input signal. AC coupling blocks the DC component of a signal so that you see the waveform centered around zero volts. Figure 21 illustrates this difference.

The AC coupling setting is useful when the entire signal (alternating current + direct current) is too large for the volts/div setting.

Figure 21: AC and DC input coupling.

The ground setting disconnects the input signal from the vertical system, which lets you see where zero volts is located on the screen.

With grounded input coupling and auto trigger mode, you see a horizontal line on the screen that represents zero volts. Switching from DC to ground and back again is a handy way of measuring signal voltage levels with respect to ground.

Bandwidth Limit

Most oscilloscopes have a circuit that limits the bandwidth of the oscilloscope. By limiting the bandwidth, you reduce the noise that sometimes appears on the displayed waveform, resulting in a cleaner signal display.

Note that while eliminating noise, the bandwidth limit can also reduce or eliminate high frequency signal content.

Bandwidth Enhancement

Some oscilloscopes may provide a DSP arbitrary equalization filter that can be used to improve the oscilloscope channel response. This filter extends the bandwidth, flattens the oscilloscope channel frequency response, improves phase linearity, and provides a better match between channels. It also decreases rise time and improves the time domain step response.

Horizontal System and Controls

An oscilloscope’s horizontal system is most closely associated with its acquisition of an input signal. Sample rate and record length are among the considerations here. Horizontal controls are used to position and scale the waveform horizontally. Common horizontal controls include:

Some of these controls are described next.

Acquisition Controls

Digital oscilloscopes have settings that let you control how the acquisition system processes a signal. Figure 22 shows an example of an acquisition menu.

Look over the acquisition options on your digital oscilloscope while you read this section.

Figure 22: Example of an acquisition menu.

Acquisition Modes

Acquisition modes control how waveform points are produced from sample points. Sample points are the digital values derived directly from the analog-to-digital converter (ADC). The sample interval refers to the time between these sample points.

Waveform points are the digital values that are stored in memory and displayed to construct the waveform. The time-value difference between waveform points is referred to as the waveform interval.

The sample interval and the waveform interval may or may not be the same. This fact leads to the existence of several different acquisition modes in which one waveform point is comprised of several sequentially acquired sample points.

Additionally, waveform points can be created from a composite of sample points taken from multiple acquisitions, which provides another set of acquisition modes. A description of the most commonly used acquisition modes follows.

Sample Mode: This is the simplest acquisition mode. The oscilloscope creates a waveform point by saving one sample point during each waveform interval.

Peak Detect Mode: The oscilloscope saves the minimum and maximum value sample points taken during two waveform intervals and uses these samples as the two corresponding waveform points.

Digital oscilloscopes with peak detect mode run the ADC at a fast sample rate, even at very slow time base settings (slow time base settings translate into long waveform intervals) and are able to capture fast signal changes that would occur between the waveform points if in sample mode (Figure 23).

Figure 23: Sample rate varies with time base settings - the slower the time based setting, the slower the sample rate. Some digital oscilloscopes provide peak detect mode to capture fast transients at slow sweep speeds.

Peak detect mode is particularly useful for seeing narrow pulses spaced far apart in time, as shown in Figure 24.

Figure 24: Advanced analysis and productivity software, such as MATLAB®, can be installed in Windows-based oscilloscopes to accomplish local signal analysis.

Hi-Res Mode: Like peak detect, hi-res mode is a way of getting more information in cases when the ADC can sample faster than the time base setting requires. In this case, multiple samples taken within one waveform interval are averaged together to produce one waveform point.

The result is a decrease in noise and an improvement in resolution for low-speed signals. The advantage of Hi-Res Mode over Average is that Hi-Res Mode can be used even on a single shot event.

Envelope Mode: Envelope mode is similar to peak detect mode. However, in envelope mode, the minimum and maximum waveform points from multiple acquisitions are combined to form a waveform that shows min/max accumulation over time.

Peak detect mode is usually used to acquire the records that are combined to form the envelope waveform.

Average Mode: In average mode, the oscilloscope saves one sample point during each waveform interval as in sample mode. However, waveform points from consecutive acquisitions are then averaged together to produce the final displayed waveform.

Average mode reduces noise without loss of bandwidth, but requires a repeating signal.

Waveform Database Mode: In waveform database mode, the oscilloscope accumulates a waveform database that provides a three-dimensional array of amplitude, time, and counts.

Starting and Stopping the Acquisition System

One of the greatest advantages of digital oscilloscopes is their ability to store waveforms for later viewing.

To this end, there are usually one or more buttons on the front panel that allow you to start and stop the acquisition system so you can analyze waveforms at your leisure.

Additionally, you may want the oscilloscope to automatically stop acquiring after one acquisition is complete or after one set of records has been turned into an envelope or average waveform.

This feature is commonly called single sweep or single sequence and its controls are usually found either with the other acquisition controls or with the trigger controls.

Sampling

Sampling is the process of converting a portion of an input signal into a number of discrete electrical values for the purpose of storage, processing, and/or display. The magnitude of each sampled point is equal to the amplitude of the input signal at the instant in time in which the signal is sampled.

Sampling is like taking snapshots. Each snapshot corresponds to a specific point in time on the waveform. These snapshots can then be arranged in the appropriate order in time to reconstruct the input signal.

In a digital oscilloscope, an array of sampled points is reconstructed on a display with the measured amplitude on the vertical axis and time on the horizontal axis (Figure 25).

The input waveform in Figure 25 appears as a series of dots on the screen. If the dots are widely spaced and difficult to interpret as a waveform, the dots can be connected using a process called interpolation.

Interpolation connects the dots with lines or vectors. A number of interpolation methods are available that can be used to produce an accurate representation of a continuous input signal.

Figure 25: Basic sampling, showing sample points are connected by interpolation to produce a continuous waveform.

Sampling Controls

Some digital oscilloscopes provide you with a choice in sampling method, either real-time sampling or equivalent time sampling. The acquisition controls available with these oscilloscopes allow you to select a sample method to acquire signals.

Note that this choice makes no difference for slow time base settings and only has an effect when the ADC cannot sample fast enough to fill the record with waveform points in one pass. Each sampling method has distinct advantages, depending on the kind of measurements being made.

Controls are typically available to give you the choice of three horizontal time base modes of operations. If you are simply doing signal exploration and want to interact with a lively signal, you use the Automatic or Interactive Default mode that provides you with the liveliest display update rate.

If you want a precise measurement and the highest real-time sample rate that will give you the most measurement accuracy, then you use the Constant Sample Rate mode. It maintains the highest sample rate and provides the best real-time resolution.

The last mode is called the Manual mode because it ensures direct and independent control of the sample rate and record length.

Real-time Sampling Method

Real-time sampling is ideal for signals whose frequency range is less than half the oscilloscope’s maximum sample rate.

Here, the oscilloscope can acquire more than enough points in one “sweep” of the waveform to construct an accurate picture, as shown in Figure 26. Real-time sampling is the only way to capture fast, single-shot, transient signals with a digital oscilloscope.

Figure 26: Advanced analysis and productivity software, such as MATLAB®, can be installed in Windows-based oscilloscopes to accomplish local signal analysis.

Real-time sampling presents the greatest challenge for digital oscilloscopes because of the sample rate needed to accurately digitize high-frequency transient events, as shown in Figure 27.

These events occur only once, and must be sampled in the same time frame that they occur.

Figure 27: Real-time sampling method.

If the sample rate isn’t fast enough, high-frequency components can “fold down” into a lower frequency, causing aliasing in the display, as demonstrated in Figure 28. In addition, real-time sampling is further complicated by the high-speed memory required to store the waveform once it is digitized.

Please refer to the Sample Rate and Record Length sections in Chapter 3—Evaluating Oscilloscopes for additional detail about the sample rate and record length needed to accurately characterize high-frequency components.

Figure 28: Undersampling of a 100 MHz sine wave introduces aliasing effects.

For real-time sampling with interpolation, digital oscilloscopes take discrete samples of the signal that can be displayed. However, it can be difficult to visualize the signal represented as dots, especially because there can be only a few dots representing high-frequency portions of the signal.

To aid in the visualization of signals, digital oscilloscopes typically have interpolation display modes.

Interpolation is a processing technique used to estimate what the waveform looks like based on a few points. In simple terms, interpolation “connects the dots” so that a signal that is sampled only a few times in each cycle can be accurately displayed.

Using real-time sampling with interpolation, the oscilloscope collects a few sample points of the signal in a single pass in real-time mode and uses interpolation to fill in the gaps. Linear interpolation connects sample points with straight lines. This approach is limited to reconstructing straight- edged signals (Figure 29), which better lends itself to square waves. The more versatile sin x/x interpolation connects sample points with curves (Figure 29).

Sin x/x interpolation is a mathematical process in which points are calculated to fill in the time between the real samples. This form of interpolation lends itself to curved and irregular signal shapes, which are far more common in the real world than pure square waves and pulses. Because of this, sin x/x interpolation is the preferred method for applications where the sample rate is three to five times the system bandwidth.

If the sample rate isn’t fast enough, high-frequency components can “fold down” into a lower frequency, causing aliasing in the display, as demonstrated in Figure 28. In addition, real-time sampling is further complicated by the high-speed memory required to store the waveform once it is digitized.

Please refer to the Sample Rate and Record Length sections in Chapter 3—Evaluating Oscilloscopes for additional detail about the sample rate and record length needed to accurately characterize high-frequency components.

Figure 29: Linear and sin x/x interpolation.

Equivalent-time Sampling Method

When measuring high-frequency signals, the oscilloscope may not be able to collect enough samples in one sweep. Equivalent-time sampling can be used to accurately acquire signals whose frequency exceeds half the oscilloscope’s sample rate (Figure 30).

Figure 30: Some oscilloscopes use equivalent-time sampling to capture and display very fast, repetitive signals.

Equivalent-time digitizers (samplers) take advantage of the fact that most naturally occurring and man-made events are repetitive. Equivalent-time sampling constructs a picture of a repetitive signal by capturing a little bit of information from each repetition.

The waveform slowly builds up like a string of lights, illuminating one-by-one. This allows the oscilloscope to accurately capture signals whose frequency components are much higher than the oscilloscope’s sample rate. There are two types of equivalent-time sampling methods: random and sequential. Each has its advantages:

• Random equivalent-time sampling allows display of the input signal prior to the trigger point, without the use of a delay line.
• Sequential equivalent-time sampling provides much greater time resolution and accuracy.

Both require that the input signal be repetitive.

Random Equivalent-time Sampling

Random equivalent-time digitizers (samplers) use an internal clock that runs asynchronously with respect to the input signal and the signal trigger (Figure 31).

Figure 31: In random equivalent-time sampling, the sampling clock runs asynchronously with the input signal and the trigger.

Samples are taken continuously, independent of the trigger position, and are displayed based on the time difference between the sample and the trigger. Although samples are taken sequentially in time, they are random with respect to the trigger, hence the name “random” equivalent-time sampling. Sample points appear randomly along the waveform when displayed on the oscilloscope screen.

The ability to acquire and display samples prior to the trigger point is the key advantage of this sampling technique, eliminating the need for external pre-trigger signals or delay lines.

Depending on the sample rate and the time window of the display, random sampling may also allow more than one sample to be acquired per triggered event. However, at faster sweep speeds, the acquisition window narrows until the digitizer cannot sample on every trigger.

It is at these faster sweep speeds that very precise timing measurements are often made, and where the extraordinary time resolution of the sequential equivalent-time sampler is most beneficial. The bandwidth limit for random equivalent-time sampling is less than that for sequential-time sampling.

Sequential Equivalent-time Sampling

The sequential equivalent-time sampler acquires one sample per trigger, independent of the time/div setting, or sweep speed, as shown in Figure 32.

Figure 32: In sequential equivalent-time sampling, the single sample is taken for each recognized trigger after a time delay which is incremented after each cycle.

When a trigger is detected, a sample is taken after a very short, but well-defined, delay. When the next trigger occurs, a small time increment—delta t—is added to this delay and the digitizer takes another sample.

This process is repeated many times, with “delta t” added to each previous acquisition, until the time window is filled. Sample points appear from left to right in sequence along the waveform when displayed on the oscilloscope screen.

Technologically speaking, it is easier to generate a very short, very precise “delta t” than it is to accurately measure the vertical and horizontal positions of a sample relative to the trigger point, as required by random samplers. This precisely measured delay is what gives sequential samplers their unmatched time resolution.

With sequential sampling, the sample is taken after the trigger level is detected, so the trigger point cannot be displayed without an analog delay line. This may, in turn, reduce the bandwidth of the instrument. If an external pretrigger can be supplied, bandwidth will not be affected.

Position and Seconds per Division

The horizontal position control moves the waveform left and right to exactly where you want it on the screen. The seconds-per-division setting (usually written as sec/div) lets you select the rate at which the waveform is drawn across the screen (also known as the time base setting or sweep speed).

This setting is a scale factor. If the setting is 1 ms, each horizontal division represents 1 ms and the total screen width represents 10 ms, or ten divisions. Changing the sec/div setting enables you to look at longer and shorter time intervals of the input signal.

As with the vertical volts/div scale, the horizontal sec/div scale may have variable timing, allowing you to set the horizontal time scale between the discrete settings.

Time Base Selections

Your oscilloscope has a time base, which is usually referred to as the main time base. Many oscilloscopes also have what is called a delayed time base. This is a time base with a sweep that can start (or be triggered to start) relative to a pre-determined time on the main time base sweep.

Using a delayed time base sweep allows you to see events more clearly and to see events that are not visible solely with the main time base sweep.

The delayed time base requires the setting of a time delay and the possible use of delayed trigger modes and other settings not described in this primer. Refer to the manual supplied with your oscilloscope for information on how to use these features.

Zoom/Pan

Your oscilloscope may have special horizontal magnification settings that let you display a magnified section of the waveform on-screen. Some oscilloscopes add pan functions to the zoom capability. Knobs are used to adjust zoom factor or scale and the pan of the zoom box across the waveform.

Search

Some oscilloscopes offer search and mark capabilities, enabling you to quickly navigate through long acquisitions looking for user-defined events.

XY Mode

Most oscilloscopes have an XY mode that lets you display an input signal, rather than the time base, on the horizontal axis. This mode of operation opens up a whole new area of phase shift measurement techniques, as explained in the Oscilloscope Measurement Techniques section of Chapter 5—Setting Up and Using an Oscilloscope.

Z Axis

A digital phosphor oscilloscope (DPO) has a high display sample density and an innate ability to capture intensity information. With its intensity axis (Z-axis), the DPO is able to provide a three-dimensional, real-time display similar to that of an analog oscilloscope.

As you look at the waveform trace on a DPO, you can see brightened areas. These are the areas where a signal occurs most often.

This display makes it easy to distinguish the basic signal shape from a transient that occurs only once in a while—the basic signal appears much brighter. One application of the Z-axis is to feed special timed signals into the separate Z input to create highlighted “marker” dots at known intervals in the waveform.

XYZ Mode with DPO and XYZ Record Display

Some DPOs can use the Z input to create an XY display with intensity grading. In this case, the DPO samples the instantaneous data value at the Z input and uses that value to qualify a specific part of the waveform.

Once you have qualified samples, these samples can accumulate, resulting in an intensity-graded XYZ display.

XYZ mode is especially useful for displaying the polar patterns commonly used in testing wireless communication devices, such as a constellation diagram.

Another method of displaying XYZ data is XYZ record display. In this mode the data from the acquisition memory is used rather than the DPO database.

Once you have an oscilloscope, there are basic things you need to do to set it up and begin using it. This chapter briefly describes these things. In particular, proper grounding is very important for safety reasons; not just for yourself but also for the integrated circuits (ICs) you are testing. Setting oscilloscope controls, calibrating the oscilloscope, connecting probes, and compensating the probes are also described, along with basic oscilloscope measurement techniques.

Proper Grounding

Proper grounding is an important step when you set up to take measurements or work on a circuit:

• Properly grounding the oscilloscope protects you from a hazardous shock.
• Properly grounding yourself protects your ICs from damage.

To ground the oscilloscope means to connect it to an electrically neutral reference point, such as earth ground. Ground your oscilloscope by plugging its three-pronged power cord into an outlet grounded to earth ground. Grounding the oscilloscope is necessary for safety. If a high voltage contacts the case of an ungrounded oscilloscope—any part of the case, including knobs that appear insulated—it can give you a shock. However, with a properly grounded oscilloscope, the current travels through the grounding path to earth ground rather than through you to earth ground.

Grounding is also necessary for taking accurate measurements with your oscilloscope. The oscilloscope needs to share the same ground as any circuits you are testing. Some oscilloscopes do not require separate connection to earth ground. These oscilloscopes have insulated cases and controls, which keeps any possible shock hazard away from the user.

If you are working with ICs, you also need to ground yourself. ICs have tiny conduction paths that can be damaged by static electricity that builds up on your body. You can ruin an expensive IC simply by walking across a carpet or taking off a sweater and then touching the leads of the IC. To solve this problem, wear a grounding strap, as shown in Figure 64. This strap safely sends static charges on your body to earth ground.

Figure 64: Typical wrist-type grounding strap.

Setting the Controls

After plugging in the oscilloscope, take a look at the front panel. As described at the beginning of Chapter 4—Oscilloscope Systems and Controls, the front panel is typically divided into three main sections labeled vertical, horizontal, and trigger. Your oscilloscope may have other sections, depending on the model and type. Notice the input connectors on your oscilloscope—this is where you attach the probes. Most oscilloscopes have at least two input channels and each channel can display a waveform on the screen. Multiple channels are useful for comparing waveforms. The front panel of a Mixed Signal Oscilloscope (MSO) willalso have digital inputs.

Some oscilloscopes have AUTOSET and/or DEFAULT buttons that can set up the controls in one step to accommodate a signal. If your oscilloscope does not have this capability, it is helpful to set the controls to standard positions before taking measurements.

Oscilloscope Instructions

1. Set the oscilloscope to display channel 1.
2. Set the vertical volts/division scale and position controls to mid–range positions.
3. Turn off the variable volts/division.
4. Turn off all magnification settings.
5. Set the channel 1 input coupling to DC.
6. Set the trigger mode to auto.
7. Set the trigger source to channel 1.
8. Turn trigger holdoff to minimum or off.
9. Set the horizontal time/division and position controls to mid-range positions.
10. Adjust channel 1 volts/division such that the signal occupies as much of the 10 vertical divisions as possible without clipping or signal distortion.

Calibrating the Instrument

In addition to proper oscilloscope setup, periodic instrument self-calibration is recommended for accurate measurements. Oscilloscope calibration is needed if the ambient temperature has changed more than 5° C (9° F) since the last self-calibration or once per week. In the oscilloscope menu, this can sometimes be initiated as Signal Path Compensation. Refer to the manual that accompanied your oscilloscope for more detailed instructions.

Connecting the Probes

Once you have properly grounded the oscilloscope and yourself, and you’ve set up the oscilloscope in standard positions, you are ready to connect a probe to your oscilloscope. A probe, if well-matched to the oscilloscope, enables you to access all of the power and performance in the oscilloscope and will ensure the integrity of the signal you are measuring. Measuring a signal requires two connections:

• The probe tip connection
• The ground connection

Probes often come with a clip attachment for grounding the probe to the circuit under test. In practice, you attach the grounding clip to a known ground in the circuit, such as the metal chassis of a product you are repairing, and touch the probe tip to a test point in the circuit.

Compensating the Probes

Passive attenuation voltage probes must be compensated to the oscilloscope. Before using a passive probe, you need to compensate it to balance its electrical properties to a particular oscilloscope. You should get into the habit of compensating the probe every time you set up your oscilloscope. A poorly adjusted probe can make your measurements less accurate. Figure 65 illustrates the effects on a 1 MHz test signal when using a probe that is not properly compensated.

Most oscilloscopes have a square wave reference signal available at a terminal on the front panel used to compensate the probe. General instructions to compensate the probe are as follows:

1. Attach the probe to a vertical channel.
2. Connect the probe tip to the probe compensation, i.e. square wave reference signal.
3. Attach the ground clip of the probe to ground.
4. View the square wave reference signal.
5. Make the proper adjustments on the probe so that the corners of the square wave are square.

Oscilloscope Measurement Techniques

The two most basic measurements you can make are:

• Voltage measurements
• Time measurements

Just about every other measurement is based on one of these two fundamental techniques.

This section discusses methods for taking measurements visually with the oscilloscope screen. This is a common technique with analog instruments, and also may be useful for “at-a-glance” interpretation of digital oscilloscope displays.

Note that most digital oscilloscopes include automated measurement tools that simplify and accelerate common analysis tasks, thus improving the reliability and confidence of your measurements. However, knowing how to make measurements manually as described here will help you understand and check the automatic measurements.

Voltage Measurements

Voltage is the amount of electric potential, expressed in volts, between two points in a circuit. Usually one of these points is ground (zero volts), but not always. Voltages can also be measured from peak-to-peak. That is, from the maximum point of a signal to its minimum point. You must be careful to specify which voltage you mean. The oscilloscope is primarily a voltage-measuring device. Once you have measured the voltage, other quantities are just a calculation away. For example, Ohm’s law states that voltage between two points in a circuit equals the current times the resistance. From any two of these quantities you can calculate the third using the formula shown below.

Voltage = Current x Resistance

Another handy formula is the power law, which states that the power of a DC signal equals the voltage times the current. Calculations are more complicated for AC signals, but the point here is that measuring the voltage is the first step toward calculating other quantities. Figure 66 shows the voltage of one peak (V_p) and the peak-to-peak voltage (V_p–p).

Figure 66: Voltage peak (V_p) and peak-to-peak voltage (V_p-p).

The most basic method of taking voltage measurements is to count the number of divisions a waveform spans on the oscilloscope’s vertical scale. Adjusting the signal to cover most of the display vertically makes for the best voltage measurements, as shown in Figure 67. The more display area you use, the more accurately you can read the measurement.

Figure 67: Measure voltage on the center vertical graticule line.

Many oscilloscopes have cursors that let you make waveform measurements automatically, without having to count graticule marks. A cursor is simply a line that you can move across the display. Two horizontal cursor lines can be moved up and down to bracket a waveform’s amplitude for voltage measurements, and two vertical lines move right and left for time measurements. A readout shows the voltage or time at their positions.

Time and Frequency Measurements

You can make time measurements using the horizontal scale of the oscilloscope. Time measurements include measuring the period and pulse width of pulses. Frequency is the reciprocal of the period, so once you know the period, the frequency is one divided by the period. Like voltage measurements, time measurements are more accurate when you adjust the portion of the signal to be measured to cover a large area of the display, as illustrated in Figure 68.

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Figure 68: Measure time on the center horizontal graticule line.

Pulse Width and Rise Time Measurements

In many applications, the details of a pulse’s shape are important. Pulses can become distorted and cause a digital circuit to malfunction, and the timing of pulses in a pulse train is often significant.

Standard pulse measurements are pulse rise time and pulse width. Rise time is the amount of time a pulse takes to go from a low to high voltage. By convention, the rise time is measured from 10% to 90% of the full voltage of the pulse. This eliminates any irregularities at the pulse’s transition corners.

Pulse width is the amount of time the pulse takes to go from low to high and back to low again. By convention, the pulse width is measured at 50% of full voltage. Figure 69 illustrates these measurement points.

Figure 69: Rise time and pulse width measurement points.

Pulse measurements often require fine-tuning the triggering. To become an expert at capturing pulses, you should learn how to use trigger hold-off and how to set the digital oscilloscope to capture pre-trigger data, as described in Chapter 4—Oscilloscope Systems and Controls.. Horizontal magnification is another useful feature for measuring pulses, since it allows you to see fine details of a fast pulse.