kellylee
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Logic / Spectrum Analyzers
What is the definition of a Logic Analyzer and a Spectrum
Analyzer? What is their function and how do they work?
Analyzer? What is their function and how do they work?
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I agree with kschang's description of a logic analyzer, though many have more bells & whistles. In essence, they let you record/monitor the logic levels of a number of signals sampled at sequential points in time. Why would you want to use one? There are many potential reasons, but three typical reasons would be to see how a device is responding to stimuli (stimuli & responses, to be monitored by a logic analyzer, are interpreted as binary values) to 1) identify where a problem is located, 2) validate a device's behavior, or 3) classify a device's behavior so that it can be duplicated (i.e., reverse engineered.)
His description of a Spectrum Analyzer is not totally correct. He describes a particular type of Spectrum Analyzer called an FFT Analyzer. Most Spectrum Analyzers used in engineering labs are not FFT Analyzers; they are analog devices. However, they serve the same purpose: to identify the frequency content (spectrum) of a signal. For example, you would use a Spectrum Analyzer to verify compliance with FCC regulations on spurious signal outputs of an appliance (such as a computer or a telephone.) When used properly, the Spectrum Analyzer will tell you the frequency and power of signals that are present. They have a lot of adjustments for varying bandwith and viewing time, which allow you to "get the big picture" or search "down in the noise" for hard-to-find signals.
As to how a Spectrum Analyzer works, the main points are that the incoming signal is "mixed" (mulitplied) with a reference signal to produce a "baseband" (near dc) result that is filtered. For a narrow bandwith, this filter and the sweep range of the reference signal must be narrow. For a wide bandwith, less filtering and a wider sweep are required. The display shows, with some persistence, the recent history of this mixing: high levels indicate relatively high power, whereas low levels indicate low or no power at those particular frequencies as they are swept.
For a better understanding, it would be useful for you to "play" with these instruments and their settings. Unfortunately, they tend to be expensive and only a few people have them at home (generally hobbyists that like to build things like radios would have an old spectrum analyzer they might have bought when some company got rid of its old, outdated equipment.)
His description of a Spectrum Analyzer is not totally correct. He describes a particular type of Spectrum Analyzer called an FFT Analyzer. Most Spectrum Analyzers used in engineering labs are not FFT Analyzers; they are analog devices. However, they serve the same purpose: to identify the frequency content (spectrum) of a signal. For example, you would use a Spectrum Analyzer to verify compliance with FCC regulations on spurious signal outputs of an appliance (such as a computer or a telephone.) When used properly, the Spectrum Analyzer will tell you the frequency and power of signals that are present. They have a lot of adjustments for varying bandwith and viewing time, which allow you to "get the big picture" or search "down in the noise" for hard-to-find signals.
As to how a Spectrum Analyzer works, the main points are that the incoming signal is "mixed" (mulitplied) with a reference signal to produce a "baseband" (near dc) result that is filtered. For a narrow bandwith, this filter and the sweep range of the reference signal must be narrow. For a wide bandwith, less filtering and a wider sweep are required. The display shows, with some persistence, the recent history of this mixing: high levels indicate relatively high power, whereas low levels indicate low or no power at those particular frequencies as they are swept.
For a better understanding, it would be useful for you to "play" with these instruments and their settings. Unfortunately, they tend to be expensive and only a few people have them at home (generally hobbyists that like to build things like radios would have an old spectrum analyzer they might have bought when some company got rid of its old, outdated equipment.)
Spectrum Analyzer takes a wave form and decomposes it into individual frequency bands, and displays the result. How do they work? A DSP inside takes the input form, does a fast fourier transform, and displays the result.
Logic analyzer is something completely different. By linking the different channels to the various input and output pins (some input and some output), then define a test sequence, you can figure out what the chip/box/whatever will react to binary inputs and timing sequences. That's just a matter of programming inputs and capturing the outputs, and storing the result into memory.
Logic analyzers are easy to build. A few wires to your parallel port can do a relatively decent job, though it's only 8-bit. Commercial ones goes up to 300 channels and captures straight to hard drive with nanosecond resolutions.
Spectrum Analyzer requires DSP programming since most computers, even the latest Pentium 3's, can't do fast fourier transforms on a real-time basis yet unless the input waveform is really low frequency.
If that's not too clear... Ask. :-)