Time Records and Frequency Data

The relationship between a time record and the frequency data is relatively straight forward if the minimum resolution bandwidth (and therefore the maximum time record length) is used. In this case the time record size is equal to the time record size required by the FFT (which is dictated by the number of frequency points and must be a power of 2). Therefore, the FFT converts the samples in the time record directly to display points.

If a higher resolution bandwidth is selected, the time length necessary to acquire the time samples is shorter and fewer samples are required. However, the FFT must still operate on the same time record length for display purposes, so the extra time record length is set to zero (zero padded) before the FFT process. This enables the FFT to properly convert the time record to the required number of display points without adversely affecting the frequency data.

The time record calculations

The length of the time record determines how long a given measurement will take and the maximum frequency resolution you can measure. As described above the FFT time record size may differ from the size dictated by the resolution bandwidth. The FFT record size must be a power of 2 equal to or greater than the number of frequency points.