Attach a digitizer to a computer via USB, add software, and you have an oscilloscope, but just because the idea is simple doesn’t mean choosing one is.

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.

DPScope-II USB oscilloscope

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.

High-end USB oscilloscope from Link Instruments

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.

USB oscilloscope selection chart

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
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Top 7 PC-based USB oscilloscopes of 2017: for hobbyists, makers,
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