Logic analyzers, which came into existence after the earliest microprocessors were created in order to find their faults during design, are commonly used to test digital or logic circuits; these instruments need access to a large number of lines — more than one could see with a standard oscilloscope. Because the complexity of circuits has grown greatly, the use and improvement of logic analyzers has grown apace.

Logic analyzers are better suited to operation in digital environments than oscilloscopes, despite the fact that oscilloscopes perform many of the same functions, because analyzers are capable of displaying the relative timing of a large amount of signals. One can monitor and investigate several lines in a digital circuit with a logic analyzer, which enables one to trace logic signals conveniently.

There are several types of logic analyzers — one of the most popular is comes as a standard test instrument case. One can also utilize a computer’s processing power by using a PC-based logic analyzer. Budget and testing requirements will dictate what kind of logic analyzer one chooses to use. PC-based logic analyzers are especially cost-effective, but the downside is that their functionality and features aren’t as extensive as stand-alone logic analyzers, which is hardly surprising, considering the difference in price.


Logic Analyzer Basics

Logic analyzers have a horizontal time axis and a vertical axis that indicates a high or low logical state. Because logic analyzers are made to monitor a large number of digital circuits, they usually have enough channels to monitor between thirty-two and one hundred, thirty-two lines (one channel, one line). There are also higher-powered analyzers with many more channels designed to track and troubleshoot much more complex systems.

Something to note is that logic analyzers don’t give full analog displays of waveforms; while they display a waveform’s high and low logical states, analyzers only check for whether a line’s state is too high or low at the trigger voltage. After the analyzer measures whether a line is high or low it displays the relevant level, meaning that it isn’t possible to observe small amplitude variations (e.g., ringing in the signal). One can, however, observe the state of the lines and their respective timings. Using this information one can trace any issues in the circuit’s design.


Logic Analyzer Probes

Logic analyzer probe design can be a major issue, given the extensive number of signals that need to be monitored, frequently from a small area on a board or even a single integrated circuit. Logic analyzer probes have internal comparators in which the board’s waveform’s voltage is compared to the threshold voltage.

There are different types of logic analyzer probes, most of which fall into three categories: 1) Multichannel probes use dedicated connectors on the PCB and enable access to a large number of points using a high-density connector; 2) high-density compression probes use a compression contact without a dedicated connector, meaning this type of logic analyzer probe requires that the PCB have contacts; and 3) flying lead probes are connected to a small electronic unit designed to detect high and low line levels and are used to monitor points that might not be included on other access points.


George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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