User manual

Math and Measure

FFT Function

For a large class of signals, you can gain greater insight by looking at spectral representation rather than

time description. Signals encountered in the frequency response of amplifiers, oscillator phase noise and

those in mechanical vibration analysis, for example, are easier to observe in the frequency domain.

If sampling is done at a rate fast enough to faithfully approximate the original waveform (usually five times

the highest frequency component in the signal), the resulting discrete data series will uniquely describe

the analog signal. This is of particular value when dealing with transient signals, which conventional swept

spectrum analyzers cannot handle.

While FFT has become a popular analysis tool, some care must be taken with it. In most instances,

incorrect positioning of the signal within the display grid will significantly alter the spectrum, producing

effects such as leakage and aliasing that distort the spectrum.

An effective way to reduce these effects is to maximize the acquisition record length. Record length

directly conditions the effective sampling rate and therefore determines the frequency resolution and

span at which spectral analysis can be carried out.

Setting Up FFT

1. Follow the usual steps to set up a math function, selecting FFT from the Frequency Analysis

submenu.

2. Open the FFT subdialog.

3. Choose an Output type.

4. If your Output Type is Power Spectrum, also enter Line Impedence. By default, the FFT function

assumes a termination of 50 Ohms. If an external terminator is being used, this setting can be

changed to properly calculate the FFT based on the new termination value.

5. Optionally, choose a weighting Window (see below).

6. Check the Suppress DC box to make the DC bin go to zero. Otherwise, leave it unchecked.