Test and Measurement
Bandwidth isn’t as simple as it seems. All oscilloscopes tend to act as a low pass frequency filter, with a frequency response that declines at higher frequencies. Oscilloscopes with bandwidths of 1 GHz and below exhibit a Gaussian response, a slow roll-off beginning at approximately one-third the -3dB frequency. At bandwidth specifications greater than 1 GHz, oscilloscopes will typically have a maximally flat frequency response, showing a sharper roll-off characteristic.
In general, test engineers should apply the ‘five times rule’, where an oscilloscope’s bandwidth should offer five times the maximum signal bandwidth. This means that 1 GHz scopes are the instrument of choice for analysing components for many of today’s communication bands, such as broadcast radio and television, land and maritime mobile as well as aviation and space.
Rise time
Another factor to bear in mind is rise time. Oscilloscopes with a Gaussian type response will show a rise time of 0.35/bandwidth frequency; maximally flat responses will lead to a rise time of around 0.4/bandwidth frequency. In practice, this means that rise time for 1 GHz oscilloscopes can vary from 350ps to 450ps, which could be a factor if viewing digital signals is critical.
Noise
Noise also plays an important role. The noise floor of the oscilloscope determines how well it can measure low-level signals. It’s typically in the range of millivolts peak- to-peak, and some oscilloscopes may have average acquisition mode to cancel noise.
Additional functionality Finally, additional functionality might help ensure compliance to a standard, or even surpass it. Brad Odhner, technical marketing manager at Tektronix and Keithley Instruments, commented: “While you might be able to meet the compliance certification for some standard, exceeding it would enable you to get an edge on competitors.”
Sample rate matters
For 1 GHz oscilloscopes, sample rates can vary from 2.5GSPS to 6.25GSPS and many believe that higher sample rates give better measurement results. This is because the more closely-spaced the samples, the higher the timing resolution, giving a more continuous waveform display. However, nearly all of today’s digital storage oscilloscopes automatically perform filtering to provide very high-density reconstructed
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samples. This means that an oscilloscope with a bandwidth of 1 GHz, and which offers a minimum-sample-rate-to-bandwidth ratio of 2.5:1, can accurately capture both high frequency analog signals and fast transitions on digital signals.
Look beyond the numbers Test engineers should also consider some of the less tangible benefits of oscilloscopes, especially because many products on the market meet these technical specifications. The length of warranty and the rapidity of repairs are critical. Some manufacturers offer three-year warranties, with others providing up to five years as standard. The time to repair is also crucial. For example, Tektronix advertises a repair service that boasts fast turnaround time, with full repair costs covered and a 90-day service warranty. Another factor is software updates. As well as updates to the regular software suite, some vendors will offer options to perform added tasks. Tektronix offers a software suite that can turn its regular digital oscilloscope products into a vector signal analyser, pulse analyser, WiGig or WLAN tester.
Ease of use
The easier the oscilloscope is to use the more time can be spent capturing data. Familiarity is an important factor, and many labs will choose to buy from the same manufacturer to minimise training needs.
Manufacturers spend a lot of time trying to simplify their user interface to maximise customer familiarity. “If we can make our products easier to use, we can enable engineers to spend more time focusing on the design they are working on,” said Odhner. Perhaps the most important factor other than familiarity is screen size, as bigger screens make it easier to identify problems. Interacting with the oscilloscope can also be helped by touch technology that is often included on systems with larger screens, making screen and menu navigation easier.
It’s not only about the oscilloscope Accessories are another item on the tick list. For oscilloscopes, these mainly take the form of different probes, which can have significant implications for both measurement accuracy and operator safety. Different probes are available for specific tasks such as debugging of complex electronic circuits, or signal integrity measurements of high-speed serial bus signals.
Is brand a good guide? Some vendors have a very strong brand reputation. Selecting big-name brands is
attractive, and often used to infer better ease of use, more reliability or excellent technical support. Tektronix and Keysight, for example, are well known because they offer extremely high-quality products and have been leading innovators in the development of oscilloscope technology for many years.
Although the big brand names offer great features, there is often a perception that reputation can come at a cost. However, even if a project is price sensitive, this might not be a reason to shy away from a well-known brand.
“There is a need for low-cost yet performant oscilloscopes at the university level that can show a waveform on the screen. However, they don’t really need any ultra-high-performance features,” said Hoffman.
For example, Keysight’s product range includes oscilloscopes that start from as little as $500 USD. So, it is not necessarily the name that is important, but rather a consideration of whether the manufacturer can meet the requirements needed at the necessary price point.
Use a PC instead
Another option is a PC-based oscilloscope, which is effectively a standard oscilloscope that uses a personal computer for the display. Benefits include increased portability, enabling work from home if needed, more storage for data points and the potential for a much larger display. One of the leading manufacturers of PC-based oscilloscopes is Pico Technology. The company finds that this option is popular with universities.
Mike Purday, regional manager (EMEA), Pico Technology, said: “Universities are giving students their own scopes in much the same way they do with textbooks. You can now have your own oscilloscope that
you keep throughout the course, which would have been unimaginable just a matter of years ago.”
Portability is another major attraction. “I can simply pack up my PicoScope together with my laptop when travelling or working remotely, but of course, you can’t do that with a traditional benchtop oscilloscope,” said Purday.
Although Pico is well known as a PC-based oscilloscope specialist, some vendors of dedicated oscilloscopes also offer PC-based versions. Keysight supplies a two-channel scope with a memory depth of 4 Mpts, a sampling rate of 5GSPS and a rise time of 450 ps.
The best approach when selecting a 1 GHz oscilloscope
It’s easy to focus on the numbers in the specification sheet, however sheer technical performance is not the most important factor. Ease of use, technical support, service and warranty, accessories and brand should all be considered. However, when it comes to getting the right product, there is no substitute for personal experience. “You can find all the information you want on the internet, but at the end of the day, nothing beats actually test driving the product. Some of these products cost as much as cars so why not test drive the oscilloscope as well?” said Hoffman. Many manufacturers, as well as high service distributors such as Farnell, offer customers the opportunity to ‘test drive’ their products before purchase alongside a whole range of additional support services in the product selection process. Taking full advantage of these services is a great first step in choosing the right scope for your requirements.
https://uk.farnell.com/ Components in Electronics October 2022 45
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