What An "FFT" Is Actually Made Up Of ?

One of the "parameters" that must be programmed into the database or the analyzer is known as the "Number of Lines" (of resolution). This parameter determines how many individual amplitude values will make up the final FFT plot. That is what a spectrum is made up of - a certain number of amplitude values (e.g. 800) that each measure the vibration found in a relatively small frequency range. This parameter - number of lines - works in conjunction with your Maximum Frequency, or "Fmax", to establish your "Spectrum Resolution" - a critically important subject. The Fmax divided by the # of lines equals the spectrum resolution. The units are: "CPM per Line of Resolution"

Spectrum Resolution =	Fmax  Number of Lines
The spectrum shown here shows 15 of the amplitude ranges that make up this spectrum. Each frequency range in this spectrum is 30 cpm wide. Each red circle is labeled as an exact multiple of 30 except the peaks. The frequency values shown at the tops of the peaks are able to be calculated more accurately.	Close-up Look At FFT Plot Shows Individual, Frequency-Based Amplitudes With Straight Lines Connecting The Dots

The y-axis value of each red circle is the amplitude for that frequency range. For instance, the y-axis value for the red circle with the number 2130 above it is the amplitude of vibration detected by the FFT process between frequencies of 2101 - 2130 cpm. The number of lines of resolution can be 100, 200, 800, 1600, 3200 (and others) but typically is 400, 800 or 1600. The maximum frequency shown on the plot is called the Fmax. If you select an Fmax of 60,000 cpm (60kcpm, 1000 Hz, 1kHz - all mean the same thing), your spectrum will cover a frequency range of 0-60kcpm. With 400 lines, for instance, each line of resolution will be 150 cpm wide (60k/400=150). The width of each frequency range is called the Spectrum Resolution. It is of CRUCIAL importance to understand spectrum resolution.