The analysis library contains two VIs that compute the FFT of a signal. They are the Real FFT VI and Complex FFT VI.
The difference between the two VIs is that the Real FFT VI computes the FFT of a real-valued signal, whereas the Complex FFT VI computes the FFT of a complex-valued signal. However, keep in mind that the outputs of both VIs are complex.
Most real-world signals are real valued, and hence you can use the Real FFT VI for most applications. Of course, you could also use the Complex FFT VI by setting the imaginary part of the signal to zero. An example of an application where you could use the Complex FFT VI is when the signal consists of both a real and imaginary component. Such a type of signal occurs frequently in the field of telecommunications, where you modulate a waveform by a complex exponential. The process of modulation by a complex exponential results in a complex signal, as shown below.
The block diagram below shows a simplified version of how you can generate 10 cycles of a complex signal.