Oscilloscope: A Power Measurement Tool

Power supply refers to a subsystem, component or system that transforms electrical power from a particular form to another, usually converting alternating current (AC) to direct current (DC) power. It has to be noted that the proper operation of electronic equipment, which range from personal computers to industrial machinery and military equipment, relies on the reliability and performance of DC power supplies.

Oscilloscope’s power supplies come in various kinds and sizes, from conventional analog types to highly-efficient switch-mode power supplies, with each of them facing a complex, dynamic operating environment. Device demands and loads can significantly change from one instant to the other. A commodity switch-mode power supply has to survive sudden peaks, exceeding its average operating levels.

Characterizing a power supply’s behavior means taking voltage measurements and static current with a digital multimeter and painstakingly making calculations on a PC or a calculator. However, most engineers nowadays consider oscilloscope as one of the preferred power measurement platform. Equipped with integrated analysis and power measurement software, the oscilloscope simplifies set up, making it easier for users to take measurements over time. The oscilloscope even allows users to automate calculations, customize critical parameters and view results.

Ideally the behavior of every power supply is similar to the mathematical model employed to design it. However, in reality, loads vary; components are not perfect; changes in the environment affect performance and line power could be distorted.

Thus, designers are required to create a power supply that occupies less space, meets tougher EMC/EMI standards, cuts manufacturing costs and reduces heat. Engineers can achieve these goals by using a rigorous regime of measurements.

With their relatively low frequencies, power measurements may appear simple to users used to taking high-bandwidth measurements using an oscilloscope. On the contrary, power measurement present users with a plethora of challenges that has never been confronted by a high-speed circuit designer. The voltage on a switching device can be rather huge, and could be “floating”, which means that it is not referenced to the ground. Variations could be observed in the signal’s period, pulse width, duty cycle and frequency. Waveforms should be faithfully captured as well as analyzed for imperfections.

The demands presented on the oscilloscope are exacting, with various probe types single-ended, current and differential required simultaneously. The device should have a deep memory in order that it can deliver the record length required by long, low-frequency acquisitions. It may even be required to capture signals of widely different scales in a single acquisition.

When preparing the oscilloscope for power measurements, users should ensure that it offers the basic sample rate and bandwidth required to handle the switching frequencies across a Switch-mode power supply (SMPS). Power measurements demands at least two channels, one for current and one for voltage. Users should also note that the facilities used to take power measurements are easy to operate and are reliable. Users should also have accurate voltage and dependable current probes, in order that they can align differing delays expediently.

Selecting the best active and passive oscilloscope probes

To achieve reliable oscilloscope measurements, users should first determine the proper probe for their application. There are two main categories of oscilloscope probes — passive and active probes.

Passive probes do not need external probe power, and can be grouped into two major types — low-impedance resistor-divider probes and high-impedance-input probes.

Shipped with most mid- and low-range oscilloscopes, the high-impedance-input passive probe that features a 10:1 division ratio is today’s most commonly utilized probe.

Unlike active probes, passive probes are less expensive and more rugged. They provide wide dynamic range as well as high input resistance to match the input impedance of the oscilloscope. However, high-impedance-input probes require heavier capacitive loading and offer lower bandwidths than low-impedance (z0) resistor-divider passive probes or active probes.

Meanwhile, the key benefits offered by low-impedance resistor-divider probe include very low capacitive loading and remarkably high bandwidth, which helps achieve high-accuracy timing measurements. The probe also costs less than an active probe with a comparable bandwidth range. It can also be utilized in various applications, such as probing 50 Ω transmission lines, electronic circuit logic (ECL) circuits and microwave devices. However, one crucial trade-off of this probe is its relatively heavy resistive loading that can affect the signal’s measured amplitude.

Active probes are employed when utilizing an oscilloscope with over 500 MHz of bandwidth. It is the tool of choice when users need high-bandwidth performance. Costing more than passive probes, active probes offer limited input voltage. Their significantly lower capacitive loading enables them to deliver more accurate insight into fast signals.

Active probes require external probe power for its active components, such as amplifiers and transistors. Most contemporary active probes depend on intelligent probe interface that not only provide power, but also serve as communication links between the oscilloscope and the compatible probes. The probe interface usually identifies the kind of probe attached and sets up the proper attenuation ratio, input impedance, offset range and probe power needed.

To measure single-ended signals, users would usually select a single-ended active probe, while picking a differential active probe for determining differential signals. The ground plane of differential probes can effectively connect the device-under-test (DUT) to the probe tip ground than the ground connections of single-ended probes. This implies that differential probes are more ideal than single-ended probes when measuring single-ended signals.

Another clear advantage of active probes over passive probes is higher bandwidth. However, probe users often ignore the effect of the connection to the target, known as the “connection bandwidth”. Thus, even if an active probe offers remarkable bandwidth specification, the published specified performance is provided as ideal for probing conditions.

In addition, most people believe that probe impedance is constant. It is not. In fact, input impedance decreases over frequency.

At low frequency ranges and DC, the input impedance of the probe begins at the rated input resistance, and as the frequency rises, the probe’s input capacitance starts to shorten, while its impedance begins to drop.

Oscilloscope Controls

Oscilloscopes are operated using the front-panel controls, which are divided into four main groups — the horizontal and vertical controls, input controls and the triggering controls.

Found in the front-panel section marked Horizontal, the horizontal controls of the oscilloscope allow users to adjust the display’s horizontal scale. This section includes the control for the horizontal delay (offset) as well as the control that indicates the time per division on the x-axis. The first control allows users to scan through a range of time, while the latter enables users to zoom in on a particular range of time by decreasing the time per division.

Meanwhile, the oscilloscope’s vertical controls are generally located in a section specifically marked Vertical. The controls found in this section allow users to adjust the display’s vertical aspect, and include the control that indicates the number of volts per division on the display grid’s y-axis. Also found in this section is the control for the waveform’s vertical offset, which translates the waveform up or down the display.

Triggering on the signal helps provide a usable and stable display and enables users to synchronize the oscilloscope’s acquisition on the waveform of interest. The trigger controls of the oscilloscope allow users to choose the vertical trigger level as well as the desired triggering capability. Common triggering types include glitch triggering, edge triggering and pulse-width triggering.

Useful for identifying random errors or glitches, glitch triggering enables users to trigger on a pulse or event whose width is less than or greater than some specified length of time. This triggering mode allows users to capture errors or glitches that do not occur very often, making them very hard to see.

The most famous triggering mode, edge triggering occurs when the voltage exceeds some set threshold value. This mode allows users to choose between triggering on a falling or rising edge.

Although pulse-width triggering is comparable to glitch triggering when users are looking for pulse width, it is, however, more general since it allows users to trigger on pulses of specified width. Users can also select the polarity of the pulses to be triggered and set the trigger’s horizontal position. This enables users to view what occurred during pre-trigger or post-trigger.

The input controls of an oscilloscope commonly comprise two or four analog channels. They are usually numbered and features a button associated with every channel that allows users to turn them on and off. This section may also include a selection that enables users to specify DC or AC coupling. Choosing the DC coupling implies that the entire signal will be input. The AC coupling, on the other hand, blocks the DC component and focuses the waveform around zero volts. Users can also specify the probe impedance of the channels through a selection button. In addition, the input controls allow users to select the type of sampling to be used.

Posital offers two new ACS inclinometer variants

Posital (www.posital.com) has announced that its ACCELENS (ACS) inclinometers are now offered in two major product variants. Fitted in a tough fiber-reinforced PBT plastic housing, the first variant features fully-encapsulated electronic components.

Primarily designed for industrial environments, the devices can withstand rough handling, prolonged exposure to dust, UV radiation, immersion and even high-pressure water jets (IP 69K). Ideally suited for factory environments, they are also suitable for use in medical equipments, transportation systems or lifts and cranes, whether indoors or outdoors.

The second variant of ACS inclinometers features extra-rugged die-cast aluminum housings that are specifically designed for harsh environments or the toughest off-road conditions. Its new housings are developed to withstand accidental impact loads from rocks, tools or pieces of unsecured equipment. They also come with reinforced connection points to support heavy-duty connectors and cables.

Just like the plastic-housed variant, the devices also offer IP 69K-rated protection from high-pressure sprays or water and dust. Its mounting holes for ¼-inch (M6) screws and solid flanges are built to withstand high bolt-tightening torques. Ideal for mounting on agricultural and construction machinery, these “armored” ACS inclinometers can also be used on mobile equipment, military vehicles, drilling/mining equipment as well as on cranes and lifts.

Both variants of the ACS inclinometers are offered as two-axis (+/- 80°) or single axis (360°) models. Featuring up to +/- 0.01° resolution, both variants are individually calibrated and temperature compensated at the factory to guarantee accurate measurements in any environment. Available instrument interfaces include serial (SSI), analogue (voltage or current), DeviceNet and CANopen.

LeCroy’s WaveAce 102 Oscilloscope

LeCroy (www.lecroy.com) has released a 60 MHz oscilloscope that features 32 standard automatic measurements that helps users simplify work. This dual channel oscilloscope also delivers a sampling rate of 250 MS/s on both channels, four kpts memory and a vast array of advanced timing parameters that allows users to gain insight on the relationship among two different signals.

Knowing that different applications demand different acquisition modes, the WaveAce 102 has been specifically designed with various acquisition modes, including Equivalent Time, Real Time, Averaging and Peak Detect modes, to ensure that the oscilloscope can capture and display all types of waveforms.

The oscilloscope also comes with five math functions — subtract, add, divide, multiply and FFT. The FFT function comes with a selection of two vertical scales and four windows.

Since edge triggering is not always the most suitable choice for every signal, the oscilloscope offers more than the basic edge trigger by providing a set of trigger capabilities that include Video, Pulse Width and Slope (Rise Time) triggers.

Its integrated Pass/Fail Mask testing allows users to easily identify problems by letting users know whenever they occur. The oscilloscope also displays a history of the P/F results on the screen.

The large internal storage of WaveAce 102 allows users to conveniently save and recall up to 20 waveforms, two reference waveforms and 20 setups, effectively saving precious time during test and debug.

Its Waveform Sequence Recorder enables users to capture and replay a maximum sequence of 2500 waveforms to easily isolate glitch or runt that is causing problems in the system.

TMS 9250 Torque Measurement System features digital telemetry technology

Honeywell (www.honeywell.com), one of the leading suppliers of test and measurement sensors in the world, has launched its latest TMS 9250 Torque Measurement System, which employs non-contact digital telemetry technology.

Built to deliver unrivalled performance and precision, the system has been specifically designed to meet the evolving demands of the industry for wireless torque measurement systems. It provides flexible configurations; optimized accuracy in performance and torque measurement data, simplified installation, setup and operation; as well as long-term reliability.

Offering new options for installing, mounting, controlling, sensing, collecting data, actuating and reporting data, the TMS 9250 is accurate, fast, simple to install, flexible and requires less maintenance. It features a wireless design that allows higher integrity of torque data capture through faster response, higher sensitivity, higher resolution, and no mechanical interferences. This generates a more accurate representation of the actual torque experienced, including the increase in reliability.

The TMS 9250’s non-contact digital telemetry design allows the system to measure the torque wirelessly to effectively eliminate mechanical interference.

The system’s modular design can be easily customized or adapted to conveniently fit into various types of test stands to meet certain test application requirements. Its multiple mechanical configuration includes SAE, DIN, shaft-to-shaft, integral coupling and custom mounting.

Its quick-attaching wiring connectors allow for fast and easy installation of the TMS 9250. Its simple setup parameters can be easily accessed via the software system, allowing for quick and remote test parameters adjustments.

Since the system is non-contact for torque measurement, its design yields lesser sensor and instrumentation wiring concerns, offering reduced maintenance needs.

Determining oscilloscope amplitude frequency measurement

Electrical engineers consider the oscilloscope as one of the most important equipment in their lab. Working without it is just like working in blind fold since the oscilloscope provides engineers a clear view of the electronic signal’s hidden world.

Aside from presenting a visual display of the circuit’s voltage level at a particular point, the oscilloscope also offers a visual display of the sensor’s signal output. Since most of the signals are cyclical in nature or are repetitive at a specified period time, knowing the frequency when signals repeat is of utmost importance.

To measure oscilloscope amplitude frequency, a user must first put the signal into the oscilloscope’s input port. Signals can be used through a dedicated probe or through a less sophisticated cable.

The user must then set a trigger source to enable the oscilloscope to start scanning. This is the time when the oscilloscope begins to display a trace. Coming from a threshold level, the trigger can alter in slope of the signal itself. It can also come from a different source.

The voltage scale must be set so that the signal’s full vertical range can be displayed as big as possible while still fitting within the screen of the oscilloscope. The timebase must also be set to easily spread the display of the signal’s full cycle across the screen – from left to right.

Then the user must decide on the starting and finish point for a full cycle. The point can be the maximum voltage to the next maximum, or the point where the voltage passes zero on its way to negative from positive, or any other easily identifiable feature on the signal.

It has to be noted that signals come in various waveform shapes. Thus, users must “mark” or identify the start and finish point. This marking can be achieved by taking a picture of the screen, by placing “markers” on the oscilloscope or by simply looking at the oscilloscope display, depending on the oscilloscope. It can also be performed internally by the oscilloscope that will measure the frequency automatically.

The number of horizontal divisions among the start and finish point must then be measured by the user. To determine the period of signal, the number of divisions must be multiplied with the timebase.

Taking the inverse of the period yields to the frequency. For instance, a measured value of 2.5 ms (2.5 millionths of a second, or 2.5 milliseconds) represent a frequency of 1/(2.5 X 10^-6 sec) or 400,000 cycles per second, or 400 kiloHertz. Depending on the oscilloscope, the calculation can be performed with pencil and paper, or instantly determined and analyzed statistically for thousands of pulses and offered as part of the display.

Prior to settling on a particular frequency measurement, users must ensure that they have thoroughly examined the signal to determine if the measurement is an accurate representation of the whole signal and not an artifact of a certain trigger or selected signal feature.

Advantages of modular oscilloscopes over modular digitizers

A powerful option within the tool kit of a test equipment designer, particularly in automatic test equipment (ATE) applications, modular instruments offers all the needed test functions without the added size and expense of benchtop instruments. However, modular digitizers are not as efficient as benchtop oscilloscopes, especially in terms of breadth of measurement functions and features. Thus, ATE designers are usually faced with a challenging situation when using modular digitizers. With the advent of modular oscilloscopes, ATE designers easily overcame these limitations.

Utilized in ATE systems, both modular oscilloscopes and modular digitizers convert analog input signals into digital signals for processing and analysis. Aside from featuring similar basic specifications such as sample rate, vertical resolution, memory capacity and analog bandwidth, both modular oscilloscopes and modular digitizers also have similar power consumption, size and cost. Although both need to transfer data to the controller or the system PC for further processing, modular oscilloscopes offer better on-board processing than modular digitizers. Modular oscilloscopes can also provide information relating to waveform characteristics quickly and with less data transfer.

Modular oscilloscopes provide significant advantages over modular digitizers, as they offer ATE designers all the functionality, flexibility and rich set of measurement features of benchtop oscilloscope. Using minimal amount of processing power and external resources, these features effectively speeds up and simplifies the ATE design task while ensuring that the ATE conforms to all its design requirements.

Providing a vast array of features and functions, including those offered in benchtop oscilloscopes, modular oscilloscopes also have extended features and functions that are not offered in modular oscilloscopes, such as flexible analog signal conditioning, advanced triggering, onboard waveform measurement and analysis, advanced acquisition modes, an intuitive graphical user interface and flexible segmented memory.

Unlike digitizers, modular oscilloscopes offer numerous triggering options, including rising/falling edge, pulse width, event count, pattern, video, glitch and multi-event cascading triggers. These triggers help users easily capture elusive events missed by digitizers.

Modular oscilloscopes offer various flexible input signal conditioning ranges that are similar to those on benchtop oscilloscopes. They are also designed with advanced signal acquisition modes, including envelop detection, averaging, peak detect, equivalent time and high resolution. These acquisition modes deliver more waveform acquisition flexibility than those offered by modular digitizers.

With on-board waveform math and waveform analysis, modular oscilloscopes deliver faster waveform analysis and math while eliminating the need to transfer huge amount of waveform data to an external controller or PC. Math and analysis function may include subtract, add, FFT, multiply, integral, derivative, limit testing, histogram, waveform parameter trending and mask testing.

Unlike modular digitizers, which may need additional programming to establish a quality graphical user interface (GUI), modular oscilloscopes are integrated with a software that creates an intuitive GUI, providing manual instrument control and a user experience comparable to that of a benchtop oscilloscope.

Finally, modular oscilloscopes offer a remarkable amount of flexible segmented on-board memory, which allows users to view memory segments individually or overlaid, providing valuable insight into the behavior of the waveforms. This capability is not offered in modular digitizers.

PicoScope 3425 oscilloscope suitable for measuring non-ground signals

Pico Technology (www.picotech.com) has launched a four channel oscilloscope that allows users to measure non-ground or floating referenced signals without the need for costly differential probes or preamplifiers.

The PicoScope 3425 features a common mode and differential range of up to 400 V, allowing it to measure both low level and high voltage signals. High voltage applications typically include capturing waveforms from telephone cables, from switch mode power supplies, hybrid vehicles and motor inverters. The oscilloscope’s high impedance differential inputs allow users to measure sensitive amplifiers and bridge type sensors for load, pressure and strain.

Offering five MHz bandwidth, a maximum sample rate of 20 MS/s, and a buffer memory of 512 kS, the USB oscilloscope is housed in a tough carry case, which comes complete with leads, probes, adaptors and clips needed for taking differential measurements. The oscilloscope requires no complicated set up — users simply have to install the software and connect the oscilloscope to the Windows PC’s USB.

The PicoScope 3425’s lightweight and compact design makes it ideal for users who are always on the run. Its anti-slip case allows users to use the oscilloscope either vertically or horizontally, making it suitable for work environments with limited space.

Designed with feature-packed PicoScope 6 oscilloscope software, the PicoScope 3425 allows users to store multiple events in its waveform memory with its familiar Windows interface, while data from the oscilloscope can be exported in binary, text and graphical formats. The oscilloscope package also comes with a five year warranty.

The oscilloscope’s frequency domain measurements

The oscilloscope’s frequency domain measurements translate a time-domain waveform with a FFT (fast Fourier transform) and measures the frequency domain’s distortion and noise characteristics. Frequency domain measurements offer magnitude and phase characteristics over frequency.

Utilizing the FFT to transform a signal quickly into its frequency components is powerful since it reveals the characteristics of signal that cannot be viewed in the time-domain.

Similar to horizontal- and vertical-axis measurements, the FFT’s accuracy can be enhanced by analyzing longer waveforms. Considering the calculation’s nature, the resolution is limited to half of the onboard processor’s resolution.

When a signal is converted to the frequency-domain, it allows the oscilloscope to perform five measurements, which assume that the input signal comes in the form of a perfect single-frequency sine wave, while the rest of the frequency components are noise or harmonics. Except for ENOB (bits), all are expressed in decibels relative to carrier (dBc). The only negative value is THD.

The Signal-to-Noise Ratio (SNR) refers to the ratio of the fundamental frequency’s RMS amplitude to all non-harmonic noise sources’ RMS amplitude. The first nine harmonics are not considered by SNR as noise. SNR is usually used when the narrow-band around the fundamental frequency is the only concern and the harmonics will not affect the system under test.

A concern when utilizing active components such as mixers and amplifiers, the Total Harmonic Distortion (THD) refers to the ratio of the total of the first nine harmonics’ RMS amplitude to the fundamental’s RMS amplitude.

The Spurious-Free Dynamic Range (SFDR) refers to the ratio of the fundamental’s RMS amplitude to the largest spurious signal’s RMS amplitude. The spurious signal can be noise or harmonic frequency component. SFDR is commonly utilized when there is a dominant spurious signal relevant to the other distortion and noise components.

Generally utilized in broad-band applications where all noise and harmonics affect the signal, the Signal-to-Noise and Distortion (SINAD) refers to the ratio of the fundamental’s RMS amplitude to the total of all distortion and noise sources’ RMS amplitude, which is equal to the sum of the THD and SNR.

SINAD can also be expressed in Effective Number of Bits (ENOB), which offers a way of measuring the input signal dynamic range as if the signal was converted using the ADC.

A high-speed Analog to Digital Converters’ (ADCs) specification and test procedures are usually expressed in terms of frequency domain measurements. Some oscilloscopes offer frequency measurements that can be utilized to imitate a more expensive spectrum analyzer to complete the tests. One of the often used tests is a multi-tone or two-tone distortion test. It is completed due to the intermodulation distortion produced by the ADC sampling of a signal comprised of two or more sine wave. When the FFT is created, measurements such as SINAD and THD can be utilized to characterize the ADC’s performance.

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