Most electronic engineers use an oscilloscope, so they know what they are, how they work, and how to apply them (Fig. 1). Right? Well, sort of. Most of today's scopes are digital storage (or sampling) oscilloscopes (DSOs), which tend to be rather complex and sophisticated animals. Generally speaking, you needn't know how something works to use it effectively. DSOs are a different case, though. The more you understand how these devices work, as well as their specs and limitations, the better you'll be able to apply their awesome analysis capabilities.
DSO INNARDS
Digital oscilloscopes really are just specialized versions of what almost all other electronic products are today—an embedded controller with a large memory, an LCD screen, and a super-fast analog-to-digital converter (ADC) (Fig. 2).
Today's DSO is a set of amplifiers and attenuators at the front end along with a matching set of probes that take in the signal and scale it to be received by the ADC. The ADC can digitize at a rate up to 40 Gsamples/s. Most of these devices are 8-bit ADCs. The captured signal is then stored in a fast memory. A key specification is memory depth as expressed in megasamples.
A fast special processor or a dedicated PC runs a Pentium and the Windows operating system with its own memory. The big benchtops also contain an internal hard drive and multiple I/O-networking interfaces. The processor and its software perform the signal examination, analysis, and measurement. Finally, the signal is displayed on a large monochrome or color LCD thanks to an accompanying ASIC or another fast processor with its own display memory.
The key point is that the software drives the scope's performance. Once you capture the signal in RAM, you can store it on the hard drive or even on a USB thumb drive for later analysis, online or offline. It can even include some elements of logic and protocol analysis.
The modern DSO is a cool piece of technology out on the leading edge, which also happens to be very expensive. But just remember to calculate the return on investment on a scope. Newer and more powerful models can really improve your productivity by saving you time and aggravation on your project— but only if you have the right scope for your needs.
WHAT'S RIGHT FOR YOU?
While most scopes are general-purpose instruments, some are better for specific applications than others. Not only that, many scopes can be upgraded and supplemented with software-analysis packages that better fit what you're doing. Scopes vary widely, so when considering a new scope, you need to evaluate your choices carefully.
At the low end of scopes reside handheld portables for field service work. Coming in at the high end are huge, powerful, general-purpose benchtop scopes—some of which cost more than $100,000. In between those two extremes lie multiple midrange categories that include bench scopes as well as modular, virtual-instrument scopes.
Scope designs continue to evolve as the top manufacturers attempt to keep up with rapidly changing chips and applications. One great justification for a new scope would concern any work you might be doing on the latest high-speed serial I/O buses, such as PCI Express, Serial ATA, 10-Gigabit Ethernet, RapidIO, 1394a/b, Fibre Channel, InfiniBand, or FBD (fully buffered DIMM).
Slower serial interfaces like CAN, LIN, FlexRay, SPI, and I2C are exploding in designs everywhere, so your ability to test them is critical. RF and wireless work (like Ultra-Wideband measurements, timedomain reflectometry, and softwaredefined radio) is also growing, making new test instruments a necessity. Therefore, you should keep nine key features and specifications in mind when contemplating your next scope.
BANDWIDTH
Of all the specs, bandwidth is the most crucial. Every scope exhibits a low-pass filter response that's a combination of the probes, input amplifiers, and other circuits before the ADC. The bandwidth is the upper 3 dB down cut-off frequency. You can find scopes with a bandwidth from about 40 MHz to well over 15 GHz.
Just remember that if you're looking at pulses and digital signals or complex modulation, you need a much higher bandwidth than the basic frequency of operation of the signals you will observe. Digital signals have fast rise and fall times and consist of a fundamental sine wave plus lots of harmonics.
If you want to see how a digital signal really looks, your scope needs to pass as many harmonics as possible. Generally speaking, a bandwidth at least five times the basic clock rate of the digital pulses is the minimum for digital signals. For analog signals, a bandwidth of two to three times the basic upper signal operating frequency is minimum. And don't forget that bandwidth is also related to the (10% to 90%) rise time by:
rise time = 0.35/bandwidth
For a bandwidth of 200 MHz, the shortest rise time that can be displayed is:
0.35/200 × 106 = 1.75 ns
If an input pulse has a faster rise time, the scope will only display its fastest rise time set by the bandwidth. Some scopes have a bandwidth/rise time relationship given by:
rise time = 0.4/bandwidth
rise time = 0.5/bandwidth
Be sure to check with the scope manufacturer's specs to see which applies and whether it is based on a 10% to 90% or 20% to 80% rise time.
Finally, with bandwidth, always give yourself some extra margin. The accuracy of your measurement depends on the bandwidth, so buy as much as you can afford. To be sure, double or triple what you calculate you'll need. And don't forget that clock rates, bus speeds, operating frequencies, and circuit responses continue to go up each year. If you want your scope to have a longer life, always buy the most bandwidth you can afford.
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