By Richard Quinnell, editor-in-chief
Oscilloscopes are one of the fundamental pieces of test equipment that an electronics developer can have, and they have been in use for more than 50 years. One of the more recent innovations was stripping down the instrument to a signal acquisition front end that connects to a personal computer through the USB, using the PC’s resources to eliminate the bulk and cost of a power supply and display. The cost and size advantages of such USB oscilloscopes make them a tempting addition to the developer’s toolkit, but they are now available in a bewildering range of performance and price points. Understanding the key specifications can help in making the right decision.
It wasn’t long after the Universal Serial Bus (USB) became available that someone figured out how to use the bus to power and attach a digitizer whose data the computer could use to emulate the operation of a traditional oscilloscope. Early devices were both crude and had limited performance but had the advantage of being relatively inexpensive. For the examination of signals that fell within the limited range that these first USB oscilloscopes could handle, they provided an interesting alternative to a more traditional (and expensive) oscilloscope.
Fast forward to today and you’ll find that USB oscilloscopes need no longer be low-performance or low-cost. Devices are on the market today that run the gamut from $20 to $20,000, with performance ranges equally widespread but not necessarily correlated. Choosing among the USB oscilloscope options has thus become much more complex.
Entry-level USB oscilloscopes such as the DPScope-II are suitable for modest signal requirements such as audio and serial buses. (Photo: DPScope)
The place to begin is understanding the oscilloscope’s key specifications and features and evaluating them against your specific needs. Things to look at include the following:
Number of channels – While you may only want to look at one signal at a time, having more than one channel carries some significant potential benefits. Multiple channels permit viewing the timing relationships among signals, and allow one signal to serve as the trigger to start viewing the other. Depending on the software available, one channel can also be subtracted from another in order to see small differences, such as across a load resister, that might otherwise be lost in a large base signal.
Number of channels. While you may only want to look at one signal at a time, having more than one channel carries some significant potential benefits. Multiple channels permit viewing the timing relationships among signals and allow one signal to serve as the trigger to start viewing the other. Depending on the software available, one channel can also be subtracted from another in order to see small differences, such as across a load resistor, that might otherwise be lost in a large base signal.
Bandwidth. This is the analog bandwidth of the channel leading to the digitizer, measured at its 3-dB point. It is effectively the highest-frequency sinusoidal signal that the oscilloscope can accurately capture.
ADC bits. The number of bits that the analog-to-digital converter uses sets the limit on vertical resolution during capture and display. Some devices offer an “enhanced resolution” greater than the ADC’s bit count, which is achieved through processing. The technique requires substantial oversampling of the signal, which reduces the oscilloscope’s effective sampling rate.
Sample rate. Digitizing an analog signal requires a sampling rate at least twice the highest sinusoidal frequency present in the signal (the Nyquist rate) or else higher-frequency signals will appear to be lower-frequency signals (aliasing) in the display. Most oscilloscope vendors recommend that the sample rate be at least five times the channel’s analog bandwidth to avoid distortions.
A method exists for enhancing the oscilloscope’s effective sample rate (ESR) when dealing with a repeating waveform such as a clock signal. By inserting randomly interleaved samples into the data stream, and sampling for many cycles of the signal, it is possible through processing to create a captured signal description with time resolution far finer than the actual sample rate would suggest. Some devices can use this technique to boost the effective sample rate on repeating waveforms by 25 times.
Memory depth. One advantage of digital oscilloscopes is that they can capture a signal for subsequent display and measurement. How long the capture interval will be depends on the sample rate and the device’s memory depth. When multiple channels are available, look for the depth per channel. A few devices also offer the option to stream signal data directly to mass storage for later analysis and display.
Operating system. Much of a USB oscilloscope’s functionality comes from software. The operating system that the vendor supports will determine which host computers can run the drivers and application software that accompany the hardware.
USB oscilloscopes can also tackle high-end applications with equivalent sample rates (ESR) in the GHz range, like the Link Instruments MSO-9412. (Photo: Link Instruments)
Evaluating a candidate USB oscilloscope based on the above parameters, along with cost, will go a long way toward narrowing the list of prospects to meet your needs. There are several other attributes worth considering, however, that might sway your final decision. For example, check to see what kinds of probes the device supports. While most USB oscilloscopes can accept standard BNC probe connections, for instance, some only offer header pins for attaching simple wires. Other probe attributes to look for are support for attenuating probes and differential probes.
External trigger and clocking inputs are another feature to consider. Having an external trigger, for instance, would allow a two-channel scope to examine two independent signals rather than requiring one to be used solely to trigger capture of the other.
Similarly, look for any output signals the device makes available. Some, for instance, can generate the clock and logic signals needed to stimulate the device under test. Others provide outputs useful for synchronizing the operation of multiple devices. A few devices provide enough output signals that they allow devices to be stacked to form the equivalent of one larger, multichannel device.
Many USB oscilloscopes are multi-purpose instruments in and of themselves, which can be handy. Some have digital input lines in addition to the analog oscilloscope inputs, allowing the device to be both an oscilloscope and a logic analyzer with the two capture domains synchronized. Arbitrary waveform generation and spectrum analysis are two other functions frequently combined in a USB oscilloscope.
Once you have narrowed down a candidate list based on the hardware specifications, don’t forget to evaluate the software that is actually what makes the device an oscilloscope. See what kind of control over the device’s operation is available. Evaluate how easy it is to configure the device, capture and display data, make measurements, and the like. Look also for special features such as serial protocol decoding (IC2, CAN) in logic analyzer functions, automatic frequency or amplitude measurement, and the like.
At one time, the idea of a USB oscilloscope was virtually synonymous with cheap and low-grade. Although such options still exist, that is no longer necessarily the case. Meanwhile, the advantages of smaller size, lower relative cost, and often enhanced software functionality of USB oscilloscopes promised over to their benchtop cousins has expanded. For the most demanding applications, benchtop instruments still outshine USB devices. But for many applications, a suitable USB device is highly practical.
EP's chart of representative USB oscilloscopes can help you get started in finding the right device for your needs. Registered EP readers can download a copy using the link below.
To help you find candidates, Electronic Products has prepared a PDF selection guide summarizing representative offerings and hyperlinking to their vendor’s pages, available to registered readers by clicking on the “Download Guides” button at the bottom of this article . Note that this guide is representative, not comprehensive. Many of the vendors listed have more models available than described in the chart, and some vendors may have been missed. The chart will provide a starting place, however, and, for many, will be all you need to find the right USB oscilloscope.
For more information, check out these related articles from around the AspenCore network:
USB Oscilloscopes from Pro to Hobbyist — These 17 companies offer two- and four-channel oscilloscopes at prices from less than $100 to nearly $10,000.
Top 7 PC-based USB oscilloscopes of 2017: for hobbyists, makers, and pros — The oscilloscope has proven to be one of the most useful tools for designing and working with electronics.
Oscilloscopes and RF Test at ESC Boston 2017
Hands-on review: Xprotolab Plain, a $20 breadboard oscilloscop e — After staring at various cheap oscilloscope modules until I was bleary-eyed, I’ve nailed down the list that I would recommend to just a few, and the “Xprotolab Plain” is my top choice at the moment.
LabNation SmartScope Tryout: Is 30 MHz Enough? — LabNation claims that its USB SmartScope will operate under Windows, Linux, Android, Mac, and iOS. But, is 30 MHz enough for any real work?
Build your own oscilloscope probes for power measurements — When bandwidth and cost are limiting your measurements, consider building your own probes. In Part 1, learn what it takes to build a 50-Ω, 1:1 voltage probe. In Part 2, learn how to design and construct an n:1 voltage probe and two high-frequency current probes.
$199 wireless oscilloscope needs a few enhancements — The Aeroscope 100A lets you troubleshoot and monitor circuit performance from a smartphone or tablet but needs a few more features in its app.
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