![]() This results in an increase in measurement performance time, especially for high-resolution FFTs. Modern high-resolution FFT analyzers offer the possibility to decouple the number of measurement results from the FFT block length. ![]() This exponential average is used when the spectrum is continuously monitored over a long period of time. The oldest of the measurements is taken the least into account, the most recent measurement contributes most effectively to the averaged result. However, the weighting is inversely proportional to the 'age' of the result. Here, too, a fixed number of results of the continuous measurements are considered. The exponential mean: FFTs are continuously measured.This method is suitable for measurements with a defined duration. Each result is considered in equal parts in the averaged final result. The classical mean: A number of FFTs are measured.noise or music, it is often advantageous to capture multiple FFT blocks and determine mean values therefrom. In the analysis of non-periodic signals, e.g. When selecting the time window, the following rule applies: Each window requires a compromise between frequency selectivity and amplitude accuracy. There are numerous types of windows, some of which differ only slightly. In the case of periodically-continuous signals, the time windowing serves to smooth the undesired transitional jumps at the end of the scanning (see part 1). The filter ensures that frequencies above the Nyquist frequency are suppressed. These unwanted mirror frequencies are counteracted with an analog low-pass filter (anti-aliasing filter) before the scanning. If, however, less than 2 samples are available, artifacts which do not occur in the sampled (original) signal are generated. With 2 samples or more it is still possible to reconstruct the signal without loss. At the Nyquist frequency, only 2 samples are available per cycle. With a 48 kHz sampling rate, for example, the 6 kHz frequency is sampled 8 times per cycle, while the 12 kHz frequency is only sampled 4 times per cycle. ![]() Half the sampling rate, in this example 24 kHz, is called the "Nyquist frequency".īut what happens if signals above the Nyquist frequency are fed in to the system? Aliasingįor the most, a signal is sampled with a more-than-sufficient number of samples. If, for example, a signal containing frequencies up to 24 kHz is to be sampled, a sampling rate of at least 48 kHz is required for this purpose. Harry Nyquist was the discoverer of a fundamental rule in the sampling of analog signals: the sampling frequency must be at least double the highest frequency of the signal. When recording wav files via a commercially-available PC sound card, for example, the audio signal is usually sampled 44,100 times per second. The sampling rate indicates how often the analog signal to be analyzed is scanned. This article provides valuable tips.Īs explained in the first part, the sampling rate fs of the measuring system and the block length BL are the two central parameters of an FFT. For accurate FFT measurements, there are some things to look out for. The results are usually presented as graphs and are easy to interpret. FFT measurements are used in numerous applications. This second part of this article deals with specific aspects that are helpful in the practical application of FFT measurements.
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