Is the 5th Harmonic Still Useful for Predicting Data Signal Bandwidth?

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  • Author: Mark Johnson
As we attempt to push data faster and faster through serial interfaces, bit rates get larger and bit periods get smaller. A common assumption made is that as the bit rate increases, the signal contains more energy at higher frequencies and, as a result, higher bandwidth is required for the test equipment such as oscilloscopes used to measure the signals. While this can be true it is not always the case, as we shall see. The 5th harmonic of a data signal (or 2.5 times the bit rate for binary, NRZ data) is often used as a guide for selecting the required bandwidth for test equipment. In reality, the 5th harmonic is a very poor predictor of essential frequency content and in fact has little relationship to critical components of a real data signal. A much better predictor of the necessary measurement bandwidth are the rise/fall times of the system.
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