Saving time with oscilloscopes

Whether in manufacturing, education and R&D, the oscilloscope is the preferred tool for troubleshooting and debugging.

In R&D, oscilloscopes are used to troubleshoot, debug and characterize designs from prototype via the initial production run. Aside from accurate waveform capture, designers at R&D also need advanced analysis features to quickly determine the root cause of problems and reduce time to market.

Meanwhile, oscilloscopes provide technicians and production engineers in the manufacturing sector with repeatable pass/fail debug and measurements of products that do not pass. They also require fast test and easy operation as well as waveform analysis to identify problems.

Educators, on the other hand, utilize oscilloscopes to teach scientific measurements and basic electronic principles, while students need a device that are easy to use.

With this in mind, oscilloscope manufacturers have continuously worked to provide users with various features that can help them save time.

As an example, most oscilloscopes nowadays come with an integrated function generator. Combining these two important instruments in a single device effectively saves users with precious bench space, especially when they are working in an environment where space is a premium.

Another time saving feature is the segmented memory. This feature allows users to digitize only information of interest, effectively using less memory while capturing at a higher sample rate and longer period of time. Segmented memory is ideally suited for capturing signals with periods of burst data in between long spans of idle time, such as serial packets/frames and radar bursts.

Mask testing is one of the valuable applications integrated to an oscilloscope in addition to its traditional functions. Enabling users to easily capture a “golden” waveform, mask testing defines tolerance limits to form a test envelope. With incoming signals compared to the allowable tolerance limit and flagged as either pass or fail, users can then choose the preferred action that the oscilloscope will perform once it identifies a violation of the mask.

In addition, the oscilloscope’s Fast Fourier Transform (FFT) function allows users to easily view the frequency content of the signal under test. Aside from saving time, this function is also very useful in determining the main cause of noise within a waveform, like harmonic distortion or fine-tuning a filter.

Aware of the changing needs of the project, today’s designers usually prefers an oscilloscope that can easily be upgraded. This enables them to save not only time but costs as well.

Other time saving features of oscilloscopes include high-resolution mode and averaging mode, noise reject mode and high-frequency mode.

High-resolution mode and averaging mode allows users to effectively raise the resolution of oscilloscopes by up to 12 bits, reducing noise while producing a smoother image on the screen. Adding hysteresis to the trigger circuit, noise reject makes the trigger circuit less sensitive to noise although it may need more amplitude waveform to properly trigger. High-frequency reject, on the other hand, provides the trigger path with a 50 kHz low-pass filter to remove unwanted noise like FM or AM broadcast signals.

Oscilloscope vertical noise characteristics explained

One of the undesirable characteristics of an oscilloscope is vertical noise found in its analog front-end as well as in its digitizing process. The measurement system noise degrades the actual signal measurement accuracy particularly when measuring low-level noise and signals.

In most cases, the vertical noise of an oscilloscope, which is a broadband measurement instrument, increases as the bandwidth rises. Thus, engineers should carefully evaluate the vertical noise characteristics before purchasing one.

Among other effects, vertical noise can induce uncertainty in the sin (x)/x waveform construction, induce errors in amplitude measurements, produce visually undesirable “fat” waveforms and induce jitter or timing errors as a function of input edge slew rates.

Although some oscilloscope vendors provide vertical noise characteristics/specifications in their data sheet, they are often incomplete and misleading.

This article aims to help oscilloscope users better understand the vertical noise characteristics of an oscilloscope.

White noise or random theoretically exhibits a Gaussian distribution or is unbounded. This implies that due to the random nature of the noise, the more data are collected from the noise characterization measurements will cause the peak-to-peak excursion to also grow higher. Hence, random phenomenon such as random jitter and vertical noise should be specified and measured as an RMS (one standard deviation) value.

Meanwhile, what is often known as the oscilloscope’s “baseline noise floor” is actually the level of noise of the oscilloscope when it is at the lowest V/div or most sensitive V/div setting. However, most oscilloscopes offered on the market today feature reduced bandwidth characteristics when operated at their most sensitive V/div settings. Since oscilloscopes are broadband instruments — where noise floor increases as the bandwidth rises — comparing the base-line noise floor characteristics of the oscilloscopes at their most sensitive V/div setting, may not yield apples-to-apples comparison as users may be comparing lower bandwidth oscilloscope against a higher bandwidth oscilloscope.

Most oscilloscope evaluators often commit the mistake of testing the baseline noise floor characteristic at the most sensitive V/div setting of the oscilloscope, and erroneously assume that this amplitude noise is true to all V/div settings.

It has to be noted that the oscilloscope’s inherent noise actually has two components. The first is a fixed level of noise primarily attributed to the oscilloscope’s amplifier and front-end attenuator. A good approximation of this component is the base-line noise floor found at the most sensitive full-bandwidth V/div setting of the oscilloscope. Although this component of noise is negligible when the oscilloscope is used on the least sensitive settings, it dominates at the most sensitive settings.

Determined by the specific V/div settings, the second component is a relative level of noise that is based on the dynamic range of the oscilloscope. Unlike the first, this component dominates when used on the least sensitive settings and is negligible at the most sensitive settings. Despite the noisy appearance of the waveform when set at high V/div settings, the amplitude of the noise can be relatively high.

Verifying oscilloscope jitter

As one of the most important oscilloscope specification, jitter is a digital signal’s unwanted phase modulation, and is the combination of the oscilloscope’s noise floor and sample clock jitter.

However, oscilloscope vendors specify jitter measurement floor in various ways. To verify how an oscilloscope’s jitter is specified, users can use these simple steps.

The first step is to find a very low-jitter sine-wave source with much greater bandwidth than the oscilloscope.

The next step requires users to connect the sine wave generator to the oscilloscope’s input.

Beginning with one GHz on the sine wave generator, users should then input the sine wave into the oscilloscope.

Finally, after doing the three steps, users should then activate the time-interval error measurement of the oscilloscope and record the measurement result. It has to be noted that the clock recovery can be set by users to the constant clock recovery setting of the oscilloscope.

The above-mentioned steps correspond to the jitter measurement floor curve’s first measurement. Users should repeat Steps 2, 3 and 4 — in either 500 MHz or one GHz more than the bandwidth of the oscilloscope — to complete the curve.

Although these easy to perform steps are quite straightforward, users should bear in mind several additional issues.

First, users should note that increasing the sine wave will cause a decline in bandwidth. This is attributed to faster rise time, which results to reduced noise in the jitter measurement floor.

The noise contributes more to the oscilloscope’s jitter measurement floor rather than to its actual sample clock rate when measuring rise times that are slower than 30 ps.

Users should also consider that as the offset and amplitude of the sine wave alters, the results generated by the oscilloscope also change. Thus, an oscilloscope of a particular vendor may be very sensitive to offset changes, significantly worsening jitters when adding offset. Meanwhile, an oscilloscope from another vendor can be enhanced with 75 percent of the scale input. The jitter measurement floor can be increased by raising the scale input above 90 percent.

If there is time, users can also alter key measurement variables such as offset, amplitude and percentage of screen of input, while measuring similar jitter measurement floor curve. This exercise brings to mind another consideration – a decline in frequency causes rise time to increase, resulting to an increase in noise contribution to the oscilloscope’s jitter measurement.

Finally, users should also be aware of the measurement’s jitter being close to the jitter of the oscilloscope. When this happens, more jitter will be contributed by the oscilloscope to any measurement being made. Although users can view the oscilloscope’s lowest theoretical jitter, doing so requires the device to have a significantly lower jitter. Taking for instance an oscilloscope with a lowest jitter measurement floor of 150 fs and a device featuring 150 fs of jitter, an error of around 30 percent and 40 percent will be added to the jitter measurement. This leads the real-time oscilloscope to realize a jitter measurement of 200 fs, at best.

Understanding the oscilloscope’s jitter specifications

The digital signal’s unwanted phase modulation, jitter is one of the most important specifications for measuring the quality of a device. In fact, a high jitter measurement can significantly affect a product, forcing it to be redesigned or from being shipped altogether.

Jitter can be accurately measured using an oscilloscope. Although most Windows-based oscilloscopes provide analysis software that helps users measure jitter, such are limited by the instruments’ jitter measurement floor, which can cause devices to unnecessarily fail tests while eroding crucial design margins. The jitter of the DUT (device under test) can not be measured if it is lower than the jitter measurement floor of the oscilloscope. Thus, it is important for engineers to understand the jitter specifications of an oscilloscope prior to making any purchase.

The most important jitter specification in an oscilloscope is the jitter measurement floor, which is a combination of the sample clock jitter and noise floor of the oscilloscope. This can result into random jitter in the oscilloscope, effectively affecting its accuracy.

It has to be noted, however, the oscilloscope vendors specify the jitter measurement floor in various ways. Hence, in order that engineers can make an educated decision when purchasing an oscilloscope, it is important that they understand the meaning of each specification, which include intrinsic jitter, jitter measurement floor as well as long-term jitter.

The intrinsic or sample clock jitter of an oscilloscope refers to the amount of jitter transmitted using an internal timing. For a clearer understanding of this definition, take for instance a real-time oscilloscope, which sample data at a relatively fast rate, up to 120 GSa/s. With this, ensuring that data points are aligned can be achieved in one of two ways — either by utilizing a chip or time-base system, which offers the tight-time correlation needed between the sampled input signals delivered to the internal clock and the A-to-D (analog-to-digital) converter. Much like the oscilloscope, the internal clock also has its own jitter specification and is characterized by how it can align the sample points of the oscilloscope through its time base. Consequently, sample clock jitter refers to the jitter specification of the whole oscilloscope time base.

Meanwhile, the jitter measurement floor of an oscilloscope is the combination of the slew rate’s noise influence and the sample clock jitter. It is also influenced by the oscilloscope’s noise floor. Generally, slower rise time (slew rate), implies greater influence of the noise floor. Taking into account the noise floor of the oscilloscope, this specification is critical since noise is one of the biggest contributors to oscilloscope jitter.

The last jitter specification of an oscilloscope is the long-term jitter. This specification measures the maximum change in the output transition of the clock from its ideal in a large number of data points.

In sum, an oscilloscope needs a low jitter measurement floor to obtain an accurate representation of what a design really looks like. However, reading the data sheet measurement is not enough in order that users can determine if the requirement is satisfied. Engineers should have a good understanding of what the oscilloscope vendor is specifying.

Tektronix’s Thunderbolt Oscilloscopes

Tektronix’s Thunderbolt is a new, high-speed, multi-protocol, four channel 10.3 Gbps  I/O technology designed to provide headroom for next generation display and I/O requirements. The new solution for support includes a 20GHz DSA70000 Series Oscilloscope, 12.5 Gb/s BSA Series BERTScope, and a DSA8300 Series Sampling Oscilloscope.  With its physical layer testing and spec conformance validation, it is the most significant advancement in PC I/O design ever introduced into the consumer level electronics industry and Tektronix is dedicated to delivery of a broad portfolio of tools to facilitate the successful deployment of this new technology working in concert with Intel.

More information can be found at http://www.tek.com/webinar/10gpbs-thunderbolt-conformance-testing .

Oscilloscope display quality affects ability to view signal details

The ability to effectively troubleshoot designs can be affected by the quality of the oscilloscope’s display. Users may not be able to view critical signal anomalies if they are using an oscilloscope with a low-quality display, while an oscilloscope that shows signal intensity gradations reveal vital waveform details, such as signal anomalies, in a vast variety of digital and analog signal applications.

Generally, engineers perceive DSOs (digital storage oscilloscopes) as two-dimensional device that display the graphical representation of voltage versus time. However, oscilloscopes actually have a third dimension, which is known as the z-axis. At a specific X-Y location, this dimension shows that constant waveform intensity gradation is a function of the signal’s frequency of occurrence. While intensity modulation, or the third dimension, is a natural phenomenon in analog oscilloscopes’ vector-type display, this is missing in digital oscilloscopes due to early limitations in digital display technology.

When searching for signal anomalies, display intensity gradation plays an important role specifically when viewing complex-modulated analog signals like read-write disk head signals, video and digitally controlled motor drive signals. In addition, intensity gradation is of much use also in various mixed-signal applications fitted in embedded microcontroller and microprocessor technologies common in industrial, automotive and consumer markets. Moreover, intensity gradation can also show statistical data about vertical noise, edge jitter as well as the relative occurrence of anomalies even when viewing purely digital waveforms.

When debugging digital circuitry, intensity gradation is very helpful in uncovering signal anomalies. It also helps users view waveforms containing noise, jitter and infrequent events.

In an effort to emulate the display quality of analog oscilloscopes, digital oscilloscope vendors have recently offered z-axis intensity gradation but with varying levels of success.

Complex-modulated signals require an oscilloscope with adequate display quality to allow users to view large pictures and then zoom in to view the small details.

Taking for instance the NSTC or PAL composite video signal, which is a complex-modulated analog signal, capturing one using an analog oscilloscope shows that although its display may flicker, important information are embedded in the displayed waveform envelope. On the other hand, a look at the display of an older digital oscilloscope, which represents some of the entry-level DSOs offered in the market today, reveals that the waveform detail of similar signal is visually lost. Thus, it comes as no surprise that today’s video labs are littered with analog oscilloscopes.

However, several digital oscilloscopes in the market today have finally matched the visual quality offered by analog oscilloscopes. This latest breed of digital oscilloscopes can display repetitive analog signal with the same quality as that of analog oscilloscopes. With the same visual resolution, the oscilloscopes can also capture, display as well as store complex single-shot signals — a capability that analog oscilloscopes fall short of, as it is limited to displaying repetitive signals only.

Mixed Signal Oscilloscope Explained

An MSO (mixed signal oscilloscope) is a hybrid test device that seamlessly combines some of the measurement capabilities found in a logic analyzer with the measurement capabilities found in a DSO (digital storage oscilloscope), such as trigger holdoff, autoscale, probe/channel de-skew and infinitepersistence on digital and analog channels — in one compact instrument. An MSO allows users to view multiple time-aligned digital and analog waveforms on a single display.

Compared to a full-fledged logic analyzer, an MSO does not have large number of digital acquisition channels. It also lacks most of the advanced digital measurement functions offered by logic analyzers. Despite this, an MSO still offers some unique advantages over both logic analyzers and traditional oscilloscopes.

One of its primary advantages is its use model. An MSO can be used in almost the same manner that users use an oscilloscope. Because of the complexity and time needed to learn or relearn a logic analyzer, design and test engineers generally avoid using one. Another reason why engineers avoid a logic analyzer is the much longer time needed to set one up to make measurements. Moreover, the logic analyzer’s advanced measurement capabilities add complexity and are commonly regarded as an overkill for most of MCU- (microcontrollers) and DSP-based (digital signal processors) designs.

As one of R&D’s most commonly utilized test instruments, embedded hardware design engineers must have a basic knowledge on how to operate an oscilloscope to achieve signal-quality and timing measurements. However, the measurements of dual-channel and four-channel oscilloscope are insufficient to test and monitor critical timing interactions between digital and analog signals. This is the time when an MSO proves to be of much help.

Since an MSO offers “just enough” logic analyzer measurement capability, minus the complexity, it is usually regarded as the appropriate tool for debugging embedded designs. With the use-model similar to that of an oscilloscope, an MSO can also be thought of as a multi-channel oscilloscope with some channels (logic/digital) offering low-resolution measurements (one bit), while several channels (analog) offers lots of vertical resolution, usually eight-bits.

Unlike a loosely-tethered two-box, mixed-signal measurement solution, a highly integrated MSO is user-friendly, delivers fast waveform update rates while operating more like an oscilloscope and not like a logic analyzer.

Waveform update is one of the important characteristics of an oscilloscope since it directly impacts the instrument’s usability. With sluggish response limiting the usability, users may find it frustrating to operate an unresponsive and slow oscilloscope. This applies not only to DSOs but to MSOs as well. Thus, oscilloscope vendors should ensure that waveform update rate is not compromised when they create an MSO by porting logic acquisition channels into a DSO. Otherwise, this would lead to sacrificing the use-model of traditional oscilloscope. Moreover, mixed-signal measurement solutions that are based on external logic pods and/or two-box solutions connected through an external communication bus like USB are generally difficult to use and unresponsive. On the other hand, MSOs that are based on highly integrated hardware architecture are easier to use and more responsive.

More pointers for better oscilloscope probing

To help oscilloscope users select the right probe for their application, this article will offer more hints to guide them from committing the most common probing pitfalls.

The first hint is to make safe floating measurements with a differential probe.

Oscilloscope users commonly make floating measurements where neither of the measurement’s point is at earth ground potential.

A standard oscilloscope measurement that attaches the probe to a signal point while the probe tip ground lead is connected to the circuit ground is a measurement of signal difference between the earth ground and the test point. The signal ground terminals (BNC interface’s outer shells) of most oscilloscopes are attached to the protective earth ground system to provide all the signals of the oscilloscope a common connection point. Generally, oscilloscope measurements are with attached to ‘earth’ ground. Linking the ground connector to the floating points pulls the probed point to earth ground resulting to malfunctions or spikes on the circuit.

One of the more popular solutions to avoiding floating measurement problems is the A-B technique. This technique uses the oscilloscope’s subtract function and two single-ended probes to get the difference in the differential signal. If the differential signal on each probe is very small while the common mode signal is much larger, the two sides’ gain difference will alter their A-B or differential result significantly. Thus, users will have to double probe the signal and see what the A-B indicates.

Utilizing a high-voltage differential probe with sufficient bandwidth and dynamic range offers a much better solution for making accurate and safe floating measurements with oscilloscopes.

Another hint is to check the common mode rejection. Users often have the misconception that common mode rejection limits the measurement’s quality. With either a differential or a singe-ended probe, it always pays to have both probe tips attached to the ground of the DUT and check if any signal is displayed on the screen.

The next hint is to check the probe coupling. With the probe attached to the signal, users should move the probe cable around and then grab it. Significant variation on the waveform on display implies energy being coupled on the probe shield. Utilizing a ferrite core on the probe cable helps improve probing accuracy by lowering the common noise currents within the cable shield.

The ferrite core’s position on the cable is important. Placing the ferrite core at the end of the oscilloscope makes the probe head easier to handle and lighter, while placing it at the cable’s probe interface end significantly reduce its effectiveness.

Users should also note that the probe cable environment can alter their measurements, particularly at higher frequencies, leading to frustrations with quality and repeatability of measurements.

The final hint is to damp the resonance.

Probe connection affects the performance of a probe. When probes are connected to a device they create a resonant circuit. If the resonance is within the oscilloscope probe’s bandwidth being used, it will be hard to determine if the measured perturbations are caused by the probe or the circuit.

If users need to add wires to the probe’s tip, the resonance of the added wire can be damped by placing a resistor at the tip.

Pointers for better oscilloscope probing

Choosing the most appropriate probe for the application is the first step to making reliable measurements. How the probe is used also affects the oscilloscope user’s ability to gain useful measurement results and make accurate measurements. To help users select the perfect probe for their application and make probing better, this article will discuss some of the useful probing tips.

Users should first determine whether they need a passive or an active probe. Passive high-impedance resistor divider probes are ideal for general-purpose mid-to-low-frequency (below 600 MHz) measurements. However, although these inexpensive and rugged tools provide high input resistance, they offer lower bandwidths than low-impedance (z0) active probes or passive probes, while imposing heavier capacitive loading.

On the other hand, active probes are suitable for high-frequency applications (more than 600 MHz) that require precision in a broad frequency range. Despite their limited input voltage, these expensive probes deliver accurate insight of fast signals due to their significantly lower capacitive loading.

Prior to probing a circuit, users should then check the probe loading effect using two oscilloscope probes. This can be achieved by connecting the probe tip to a point in the circuit and attaching the second point to that same point. Ideally, no change would be visible on the signal. Any change observed is caused by the probe loading.

Next, users should check the probe compensation before connecting the probe to an oscilloscope input as it may have been previously adjusted to match a different input. Passive probes generally integrate compensation RC divider networks. Probe compensation refers to the process of adjusting the RC divider to maintain the probe’s attenuation ratio over its rated bandwidth.

It makes sense to employ that feature if the oscilloscope can automatically compensate for the probe’s performance. Otherwise, users should adjust the variable capacitance of the probe using manual compensation.

Oscilloscope users should carefully evaluate the noise characteristics of their device prior to making measurements.

With the advent of mobile phones and various battery-powered devices, engineers nowadays need higher-sensitivity current measurement that will ensure that the current consumption of their device is kept within acceptable limits. The simplest way to make current measurement without breaking the circuit is by using an oscilloscope with a clamp-on current probe. However, this process becomes quite tricky when the current level drop to low milliampere range or below.

The inherent noise of the oscilloscope turns into a real issue as the current level declines, revealing one undesirable characteristic of the oscilloscope — vertical noise. When measuring low-level signals, the noise of the measurement system may undermine the accuracy of the actual signal measurement. Given that oscilloscopes are broadband measurement devices, the higher their bandwidth, the higher their vertical noise will also be. It has to be noted that the oscilloscope’s acquisition memory can affect the noise floor when making low-level measurements.

A look at sampling and real-time oscilloscopes

It is critical for oscilloscope users to choose the oscilloscope that will meet all of their application needs. Oscilloscopes are grouped into two main types — the equivalent-time (ET) oscilloscopes and the real-time (RT) oscilloscopes. For every type of oscilloscope there are various specifications that users need to consider, and one of which is bandwidth. If the oscilloscope does not have sufficient bandwidth to meet the Nyquist criterion, users will encounter significant aliasing in their signal.

Although there is no simple way to determine how much bandwidth a user will need, oscilloscope vendors offers a “fifth harmonic” rule of thumb. Under this rule, oscilloscope users are advised to acquire an oscilloscope that has adequate bandwidth to capture the signal’s fifth harmonic. There are instances, however, that even if the oscilloscope features sufficient bandwidth to capture the fifth harmonic, it may not capture any fifth harmonic at all. Thus, it will be unwise to buy the oscilloscope, when it can only capture up to the third harmonic. Users could buy a lower-bandwidth oscilloscope that delivers similar harmonic content for reduced costs. This is also the reason why it is important for users to determine how much bandwidth they truly need as well as the required features.

The high-bandwidth oscilloscope’s higher price is not the only downside of purchasing more bandwidth than what is actually needed. Higher-bandwidth oscilloscopes also produce more noise, which in turn cause more distortion. The noise affects amplitude measurements as well as the accuracy of timing measurements, defeating the very purpose of the added bandwidth. The best solution is to use a measurement system that offers just enough bandwidth to accurately measure the signal while reducing the extra noise generated by the measurement system.

Generally, ET oscilloscopes deliver a lower noise floor compared to RT oscilloscopes. With proper research, however, users will discover that there are several RT oscilloscopes in the market that feature low noise floors.

Real-time oscilloscopes also provide various features that are not offered in ET oscilloscopes. The most important difference is that RT oscilloscope provide significantly higher sampling rates and memory depths than that offered by ET oscilloscopes.

With its capability to trigger itself, RT oscilloscopes also eliminate the need for an external trigger. In contrast, an ET oscilloscope needs an outside trigger while requiring a number of singlepoint acquisitions to display the data.

In addition, RT oscilloscopes commonly offer more automated compliance software applications, making it easier and faster for users to take measurements.

Meanwhile, ET oscilloscopes provide less noise and more bandwidth.

In fine, although most engineers employ the fifth harmonic to measure how much bandwidth is needed, RT oscilloscopes will not capture any fifth harmonic due to its high noise floor.

For it to be part of the measurement, users need to have an ET oscilloscope, which features low noise floor.

Next Page »