FFT simulation setup

[Junus2012]

Dear all

I am trying to use the FFT for the first time in my Eldo simulator, I have used a test signal of rectangular signal with Duty cycle=50% and f= 1KHz, after running the FFT I got the attached signal. Also please help me to find the optimum setup of the FFT.

One more question, in signal processing we apply the FFT only on the discrete signal, but here I am using it on a contionous signal as I am thinking that FFT function will sample it. please correct me if I am not right

Thank and looking for your participation

 

[keith1200rs]

As you have a known waveform and so know what the FFT should be you should easily be able to experiment with the parameters to see what works best. I don't use Eldo so don't have the same options as you are seeing.

In general, more points is better - it just takes longer to calculate.

Rectangular is good if the waveform you are looking at is exactly periodic over the simulation time so a 1kHz waveform over 10ms for example. I think it needs the reference frequency to be specified. In general you would use one of the windowed functions (Hanning for example).

If you know the fundamental frequency, include that - I think that is the "reference frequency" in your window. It will give better results.

In general you would not "use datapoints" for the FFT - it needs resampling. So, use Cubic or Uniform. Again, Cubic is better but takes longer.

I am not sure what your "periodic" does but as your waveform is periodic it would seem a better option than "symmetric".

Have a look at your "advanced" tab as well (and read the Eldo manual!).

As I say, with a known waveform you have the perfect method to experiment and see the effect of the parameters because you know what the results should be - i.e. not what you have.

Keith