The standard FFT function in SIGVIEW operates on real-valued sequences, i.e. real signal. If your signal is complex, i.e. contains complex samples, you can still calculate its FFT by using the complex-FFT function in Signal calculator. Good examples of such signals are complex signals used for signal analysis in radio communication.
Before applying this function, you need to load both the real and complex part of your signal as separate signal windows in SIGVIEW. Alternatively, you can perform 2-channel data acquisition whereby the first channel contains real values and the second one complex values. Afterwards, simply open the “Signal calculator” window, choose real signal component, then press the “Complex FFT” button, and then finally choose your imaginary signal component. The result should look like this:
Press the OK button in the “Signal calculator” dialog and a new window containing complex FFT will appear. As expected, its frequency axis will contain both negative and positive frequencies.
If your real and complex signals have different lengths, SIGVIEW will use only a part of the longer signal (starting from its first sample) which has the length of the smaller signal.