FFT Analyzer Setup for Waveform Collection
When setting up an analyzer to store waveforms, an important point should be born in mind, and that is that the frequency range normally convenient for looking at a spectrum is usually not suited to looking at the waveform. Most FFT analyzers, with a few notable exceptions, do not allow you to set up specific sampling rates or time domain record lengths – you must set them up in terms of frequency span and frequency resolution. Remember from the FFT Analysis chapter that the time record length used by the analyzer to calculate the spectrum is the reciprocal of the line spacing, or resolution, of the spectrum.
Spectra are generally scaled so relatively wide frequency ranges can be examined, and the FFT analyzer of necessity acquires a short time record. For instance, a 400-line spectrum extending from DC to 1000 Hz will have a line spacing of 1000/400, or 2.5 Hz. The time record length used to calculate this spectrum is 1/2.5, or 0.4 seconds. This time record, which is the actual waveform, will show details that happen in that 0.4-second time span, but in practice, when looking at a machine vibration waveform, we are often looking for events that occur over a much longer time than that. If we are looking for beats in the vibration signature of an electric motor, or of the combined vibration of two machines running at slightly different speeds, we need to see a waveform that is at least several seconds long.
To acquire a waveform lasting five seconds, we need to set up a line spacing of 1/5 Hz, and this can be done by adjusting the number of lines of resolution and the frequency span to suitable values.
To find out the sampling rate of the waveform, and thus it time resolution, again we need to get the information from the spectrum characteristics. The sampling frequency of the time record for most analyzers is 2.56 times the highest frequency in the spectrum. Thus a frequency span of 100 Hz implies a sampling frequency of 256 samples per second, and a span of 1000 Hz requires a sampling rate of 2560 samples per second.
Remember that a meaningful time record contains many more data points that the usual spectrum, and therefore you need to take care that you have enough memory available in your data collector to store the waveform data. For this reason, it is best to use the lowest sampling rate and the shortest time record length that will provide the needed data. For example if you just want to resolve beats in waveform that only occur once in several seconds, the sampling rate need not be very high – 50 samples per second is probably fast enough. This corresponds to a frequency span of 50/2.56, or 19.53 Hz. So you can select 20 Hz in the frequency span set up.
On the other hand, if you want to examine a waveform that might have interesting glitches at 50 times per second, then you need to sample fast enough to resolve each glitch. You might sample at 1000 samples per second, and this requires a frequency span of 1000/2.56, or about 390 Hz.
A good rule of thumb to memorize is that the time record length depends only on the line spacing of the FFT spectrum and the sampling rate depends only on the frequency range of the FFT spectrum, and they are independently adjustable. We will return to this subject of time resolution versus frequency resolution soon, when we look into Synchronous Averaging.
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