Problems with the FFT-IFFT v4.1

serjical · ‎01-24-2012

Hi to everybody. I'm here because I'm having several problems with the FFT v4_1 blocks in my design and they are a vital part of my thesis design. I will ilustrate the problem with some images.

1) First of all, I have a tone generator, identified as f1, attached to the FFT block. In this situation the output is the expected one. I mean, a delta in the corresponding frequency.

First architecture

First case architecture

First case FFT output (blue: abs(FFT) - green: done output)

2) Then, I switch the FFT input from f1 to f2. In this situation the output is also the expected one, another delta in other frequency bin.

Second case architecture

Second case FFT output (blue: abs(FFT) - green: done output)

3) Until this point everything is OK. The problem comes when I desire to do the FFT of a sum of tones (in this case f1 and f2). In this situation I expect to see two pair of deltas corresponding to f1 and f2 but the output is quite different. I have to say that I'm making the FFT of 256 points and I have selected f1 and f2 in order to make them coincide with a frequency bin.

Third case architecture

Third case FFT output (blue: abs(FFT) - green: done output)

Thanks a lot and I hope someone could help me, because it's very important to me to have this design working in order to finally be an engineer.

PD: I also attach the .mdl file.

vlavruhin · ‎12-08-2010

Hi.

Can't run your model because of missing 'Generador_control' block. But I know what is the problem.

Look carefully at input port of your FPGA model. The 'Gateeway In' block has following charactestics of output type:

- arithmetic type = signed,

- number of bits = 14,

- binary point = 13,

- overflow = saturate.

It means that you can drive input port with floating point signal in range [-1, 1). All your tone generators have amplitude = 1. So when only one tone generator is connected to input, then it's ok. But when you feed sum of outputs of tone generators ( and this sum exceeds range [-1,1) ), then your signal will be saturated. That's why you can see additional harmonics (which appears after saturation and aliasing).

In order to fix that problem, try to set amplitudes of tone generators, for example, to 0.25.

Best Regards,
Vitaly.