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Application Notes
When you combine many circuit elements with similar frequency responses, you get a Gaussian response. Traditional analog oscilloscopes chain many analog amplifiers from the input to the cathode ray tube (CRT) display,1 and therefore exhibit a Gaussian response. The properties of a Gaussianresponse oscilloscope are fairly well understood in the industry.
Less familiar, though, is the flat response that is now more commonly exhibited by modern, highbandwidth digital oscilloscopes. A digital oscilloscope has a shorter chain of analog amplifiers, and it can use digital signal processing techniques to optimize the response for accuracy. More importantly, a digital oscilloscope can be subject to sampling alias errors,2 which is not an issue with analog scopes. Compared to a Gaussian response, a flat-response reduces sample alias errors, an important requirement in the design and operation of a digital oscilloscope.
This application note reviews the properties of both Gaussian- and flat-response oscilloscopes, then discusses rise-time accuracy for each response type. It shows that a flat-response oscilloscope gives more accurate rise-time measurements than a Gaussian-response oscilloscope of equal bandwidth, and how you can estimate the oscilloscope bandwidth you need. This discussion refers to using a 1 GHz oscilloscope, but this analysis is scalable to other bandwidths with the same validity.
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