Although you cannot see the electrons moving or the waves in the air, oscilloscopes make you feel the presence of a signal in your electronic circuit. The pictorial representation of the waveforms helps you understand, analyse and debug the waveforms and systems, thus making the oscilloscope a ‘must-have’ device in every lab, be it small or big.
Today, oscilloscopes, also referred to as ‘scopes’ in the industry, are designed not only for viewing signals but also as the solution providers. “The demand for high-speed serial links is increasing. Oscilloscopes have built-in functionalities for electrical compliance and inter-operability test for all different standards. The other wide application is in efficient power supply designs—scope tools help to identify and correct problems quickly and make a reliable product,” shares T. Rajendran, chief technology officer at Primeasure Technologies (a distributor for LeCroy).
“An oscilloscope is the only device that can diagnose the complete health of an electronic circuit by measuring important parameters like wave shape, voltage, frequency and phase angle at any given point of time,” says Chandeep Singh, director, Silicom Electronics.
James Huang, vice president of Goodwill Instruments, believes that the oscilloscope is one of the greatest inventions of the 20th century and the oscilloscope technology has migrated into a new era of digital storage oscilloscopes (DSOs) in the first decade of 21st century.
The scope of oscilloscopes has widened today to serve various technological developments. These have come a long way in terms of internal hardware design, interconnectivity choices for high speed and high bandwidth, and sampling. They have evolved from being a system dedicated to viewing waveforms to an operating-system-based device with various software features.
Bandwidth is the first and most important specification for an oscilloscope. Primarily, to view the waveform and analyse its characteristics, an oscilloscope should be able to cover all the frequency components of the test signal. Scopes available today have bandwidth up to 30 GHz. High-bandwidth scopes are used for microwave testing and RF testing of recent telecom systems like WiMAX, 3G and LTE.
Bandwidth can be defined as the maximum frequency of the signal that can pass though the front-end amplifiers. The bandwidth of the scope must be higher than the maximum frequency that you wish to measure (in real time). If the input is not a pure sine wave, it will contain higher harmonics. For example, a 20MHz square wave viewed on a 20MHz scope is attenuated and distorted.
“To decide the bandwidth, two things are important—fundamental frequency and rise time. The signal may be low-frequency, but if your interest is fast rise time, you need a higher-bandwidth scope than the fundamental frequency. The bandwidth is equivalent to 0.4/RT for DSOs,” shares Rajendran.
• Sampling frequency
• Memory depth
• Signal integrity
• Form factor
Vikram Bhansali, chief executive officer, Arun Enterprises, suggests, “The rule of thumb is to purchase a scope with a bandwidth five times higher than the maximum frequency of the signal you wish to measure.”
“Apart from bandwidth, sampling rate and memory depth are important considerations. These are critical when signal curves are very far away from trigger points. Hence, while selecting a scope, a balance between these three parameters is necessary,” shares P. Prabhu, general manager-technology, Scientific Mes Technik.
“Many a times, high-end scopes have DSP-boosted bandwidth, which contributes to noise. You must carefully look for true bandwidth of the scope while choosing one for your test application,” shares Sanchit Bhatia, digital applications consultant, Agilent Technologies.
Mixed signals have a scope
Mixed-signal oscilloscopes (MSOs) offer analogue analysis capabilities such as standard time, voltage and frequency measurements as well as histograms, waveform maths, Fast Fourier Transform and eye diagrams. “These are hybrid test instruments that combine the usability of an oscilloscope with the measurement capabilities of a logic analyser and some serial protocol analysis,” explains Bhansali.
MSOs are suitable in a mixed-signal environment that has slower analogue signals as well as faster digital control signals. Bhansali shares, “MSOs allow you to view a variety of time-aligned analogue and digital waveforms on the display. Typically, these have two to four analogue inputs and around 16 digital inputs.”
Although MSOs do not provide the full range of advanced digital measurement or full-fledged logic analysis, their less complex working makes them suitable for debugging such hybrid circuits as embedded systems, control systems, analogue-to-digital converters (ADCs) and digital-to-analogue converters (DACs).
“High-speed embedded designs require an oscilloscope to provide integrated high-bandwidth digital channels with analogue channels. Using MSO helps the designers to solve their mixed-signal debug challenges,” justifies Manish Dhruv, applications engineer, Tektronix.
“Today’s embedded boards, apart from analogue signals, have a variety of digital signals such as high-speed SERDES and low-speed serial data (I2C, SPI, UART and RS-232). Multiple inputs/outputs (I/Os) from 16- or 32-bit microcontrollers need to be captured in time correlation for timing analysis on 36 digital channels and four analogue channels that only an MSO can provide at an economical cost,” says Rajendran.
From parallel to high-speed serial
Today, design and development engineers have started using serial architecture for test and debug. In the past, embedded designs used parallel architecture. This meant that each bus component had its own path. “You could use a pattern or state trigger to find an event of interest and then just visually decode the data on the bus with such an architecture,” explains Bhansali. “Modern embedded designs, however, use serial architecture, meaning the bus data is sent serially. Serial architecture is used because less board space is required, the cost is less, embedded clocks can be used and power requirement is less.”
• WaveMaster 830Zi oscilloscope by LeCroy
• Infiniium 9000 series oscilloscopes by Agilent Technologies
• DL/DLM6000 digital and mixed-signal oscilloscopes by Yokogawa Electric
• PicoScope 9211 PC oscilloscopes by Pico Technology
• ZT4420, ZT4430 and ZT4440 series modular oscilloscopes by ZTEC Instruments
Development of PC interconnect technologies has been the key driver for PC-based instrumentation. “PC-based digital oscilloscopes are growing in popularity as these offer a considerable cost saving over their bench-top equivalent,” informs Bhansali. “The reason for cost saving is obvious—by using a PC already sitting on your desk, you can leverage a large colour display, fast processor, disk drives and keyboard effectively for free,” he adds.
PC-based scopes are available in two different forms: internal and external. Internal PC-based scopes are usually in the form of PCI-format plug-in cards. Bhansali says that the biggest disadvantage of PC cards is the noise; the inside of a personal computer can be a very noisy electrical environment and some cards suffer from it. Another disadvantage is portability; PC cards are tied to use with one desktop PC.
External PC-based oscilloscopes take the form of a small box that connects to the PC via USB or parallel port. As all the analogue electronics is kept outside the PC, the issue of noise is resolved. These are portable and can be used with either desktop or laptop PCs.
A personal computer has many features in terms of hardware and software. PC-based scopes leverage all these to provide a large display, easy storage and sharing of waveforms, easy analysis, export of data samples to Excel and WordPress, high-speed USB connection and compact form factor. Apart from that, any new functionality can be implemented with software upgrade.
PC-based scopes can also offer high resolutions of 10, 12 and 14 bits, which are used for debugging high-fidelity applications like audio.
Displays for enhanced view
Oscilloscopes are meant to view signals, and if the viewing is not good, their primary work is not done! Scopes today have full-range colour displays, LCD and plasma displays with screen size ranging from 14 cm to 39 cm. Modular oscilloscopes without their own display screens, can even be hooked up to giant digital signage in shopping malls. High-speed sampling and high resolution compliment the display of waveforms. With advanced software tools, you can also view a 3-D graphical representation of the signal in time and frequency, and convolution and histograms, giving you a better understanding of the overall signal spectrum.
High-speed embedded designs require an oscilloscope to provide integrated high-bandwidth digital channels with analogue channels
“The early digital scopes did a great job of data capture. However, these lacked the unique persistence characteristics of analogue scopes’ phosphor display that showed how the signal changed over time,” shares Bhansali. He adds, “Introduction of high-definition XGA display and digital phosphor displays eliminated the last remaining advantage of analogue scopes. With a digital phosphor display, waveform after waveform is overlaid on the display, making the display more intense.” DSOs are continually growing as, like analogue real-time scopes, these allow you to view the glitches and rare spots.
The display plays an important role when it comes to simultaneous display of multiple channels and measurement of parameters. “When you want to view, say, four channel traces, a single grid screen is split into four units, one for each trace. However, you do not get full ADC resolution on a single-grid display as multiple attenuated waveforms are placed one above the other. So there is a trade-off between waveform display and parameter measurement. The introduction of multi-grid display utilises the full dynamic range of the ADC for each trace and helps to get more accurate measurements,” explains Rajendran.
New features in scopes
While scopes are still used for viewing waveform shape and measuring different parameters, it is becoming increasingly common for people to use them as an automated verification tool. These are used to make sure that the device meets the specification requirements of serial data standards. They also include several application protocols and compliance analysis, vector signal analysis, jitter analysis, noise figure analysis and complex triggering.
Compliance and protocol test. “Various application-specific compliance test software can be loaded on the oscilloscopes to cater to the needs of different market segments. Test software like certified wireless USB, FireWire iEEE 1394 a/b and USB 2 are used in computer electronics, whereas digital video interface (DVI) and high-definition multimedia interface (HDMI) find their use in media. Controller area network (CAN), LAN and FlexRay are used in the automotive industry,” shares Bhatia.
“Oscilloscopes are used to debug complex protocols through accurate and complex triggering mechanisms wherein the user gets the power to correlate protocol decode with the physical layer signal.”
Serial data equalisation and de-embedding. “Apart from the compliance testing for standards, there is also a strong need to embed or de-embed a fixture, channel or be able to apply an FFE/DFE equalisation on the data in order to analyse the complete serial data link,” shares Dhruv.
Equalisation techniques used in digital receivers are now being modelled in oscilloscopes. Bhatia adds, “Equalisation software are used to quickly verify equaliser chip tap values or provide custom tap values to help engineers open even the most tightly closed eyes. De-embedding software are used to combine measurements and models to view simulated scope measurement results at any location in a design. Design models (s-parameters or transfer functions) of interconnects, probes, traces, etc can be imported in the oscilloscope software and the acquired real-time scope data can be transformed to desired measurement locations.”
Jitter analysis. Oscilloscopes have in-built jitter measurement and analysis capability. Jitter is the unwanted part of the signal. It degrades the performance of the system and eludes troubleshooting efforts. It is mandatory for high-speed designs and serial data standards to comply with jitter tests.
“Today, total jitter (Tj) measurement is very important for all high-speed serial data. With the capability to analyse the entire record length and higher measurement accuracy, Tj BER at 10-12 provides design confidence for reliable data communication,” informs Rajendran.
Signal integrity (SI) analysis. A high-end user needs to look at noise-floor and trigger-jitter of a high-bandwidth oscilloscope. “The intrinsic noise of equipment becomes the most significant factor for high-bandwidth measurement. Some applications where you do jitter measurement, the intrinsic jitter of the scope may produce unreliable results,” says Bhatia.
Dhruv adds, “Signal integrity analysis mandates for accurate and repeatable measurements, which calls for better ‘effective number of bits’ specification of a scope at higher bandwidth.”
“New high-performance DSOs/MSOs now use reference-class ADCs that have a very low intrinsic noise,” shares Prabhu.
Complex triggering. Triggering is required for signal characterisation and stabilising repetitive waveforms. Bhatia shares, “A high-end user may look for triggering capabilities of an oscilloscope to trigger on complex signals. Examples of complex triggering include three-stage triggering, zone-qualify triggering and measurement triggering. These enable the user to trigger on highly complex signals.”
Price and value preposition
Oscilloscopes have evolved from analogue scopes to high-performance digital oscilloscopes. While the transition from analogue real-time oscilloscopes (AROs) to DSOs continues, in some application areas, AROs will still be used. “As the ARO market shrinks, the ARO price will go up due to an increase in the component price, especially for CRT,” explains Huang.
“As key components of DSOs are becoming commercially available, the DSO development no more relies on the expensive application-specific integrated circuit (ASIC) components. Also, with the evolution in IT technology, high-end DSOs are aiming at new applications. The gap between high-end and low-cost DSOs is becoming smaller every day,” adds Huang.
The competition among DSO suppliers will highly rely on better and cheaper new products. “The differentiation of banner specifications is becoming insignificant, as the cost of key components is the same for all the suppliers. So it’s all about value preposition—like the functional capabilities offered by different suppliers at a given cost,” says Huang.
Scope for improvement
Oscilloscopes have come a long way in terms of functionality, cost, size and variety of integration. These are available for as low as Rs 5000, while the high-end oscilloscopes have bandwidth as high as 30 GHz (real-time) with 80GS/s sampling rates. Oscilloscopes are available in all form factors—benchtop, handheld, portable and PC-based small boxes dedicated to different work environments. Still, there is a scope for improvement, not just in the equipment but in the usage of equipment too.
“From early valve versions of oscilloscopes, designs have updated to transistors, ICs and the latest embedded technology. Perhaps, the nanotechnology may open new frontiers,” adds Prabhu.
“All the hobbyists, budding engineers, professionals, designers and test engineers use oscilloscopes, but not the full functionalities,” says Rajendran. “The average use is 20-30 per cent of the features, but there are so many useful tools in the scope that can bail them out of troubled waters.”
Multigrid display utilises the full dynamic range of ADCs for each trace
Is it because they do not need all the features? Or because they don’t know how to use the oscilloscope?
Anand Bhushan, managing director of Bhushan & Bhushan, explains, “Education requires a basic oscilloscope. Engineers need to understand the concepts of triggering, sampling and measurement of frequency, amplitude, time period, etc. Oscilloscopes with automated measurements are useful in the industry to save time. These give ready results but do not help the budding engineers in learning the underlying processes leading to those results. Black boxes don’t educate!”
Test and measurement manufacturers provide user manual, hands-on training and demonstrations to the academia and the industry so that they can explore all the features of a scope. Manuals come handy when you use an oscilloscope for more than viewing a waveform. Following the buying guidelines and reviews on the Internet will help you buy the right scope for your application.
The author is a senior technology journalist at EFY