Time Record Length
Time record length in an FFT analyzer, as was discussed in the frequency analysis chapter, is the reciprocal of the frequency resolution of the spectrum. In other words, a spectrum with frequency resolution (or line spacing) of 10 Hz is generated from a time sample lasting 1/10th second. So, suppose in the example in the figure above the pinion is rotating at 1800 RPM and we want to look at one revolution. 1800 RPM is 30 Hz (30 revolutions per second), so one revolution will take 1/30th of a second. To set up an FFT analyzer to collect a 1/30th second time record requires that its frequency resolution, or line spacing, must be 30 Hz. In order to acquire a little more than one revolution, you might use 1/25th second as the time record length. There are many theoretical combinations of frequency range and resolution that could be used to do this - i.e., a span of 5000 Hz with 200 lines resolution, a span of 2500 Hz and 100 lines of resolution, or a span of 250 Hz at 10 lines resolution.
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