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Visitor serjical
Visitor
4,108 Views
Registered: ‎01-24-2012

Problems with the FFT-IFFT v4.1

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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

 

out_2.jpg

 

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.

 

conf_2.JPG

Second case architecture

 

out_1.jpg

 

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.

 

conf_3.JPG

Third case architecture

 

out_3.jpg

 

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.

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Explorer
Explorer
4,688 Views
Registered: ‎12-08-2010

Re: Problems with the FFT-IFFT v4.1

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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.
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2 Replies
Explorer
Explorer
4,689 Views
Registered: ‎12-08-2010

Re: Problems with the FFT-IFFT v4.1

Jump to solution

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.
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Visitor serjical
Visitor
4,063 Views
Registered: ‎01-24-2012

Re: Problems with the FFT-IFFT v4.1

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Oh God, I'm such an idiot! Thanks a lot :D

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