about fft ip core

Altera_Forum · ‎09-22-2007

I'v read the document about the fft ip core(ug_fft.pdf), the document says "For single engine output FFT/IFFTs, there is an additional factor of Q and the shift must be

2-exponent – Q, where Q = 1 for an even power of 2 and Q = 2 for an odd

power of 2."

I don't know how to choose the value of Q for the single engine output FFT

or what's the meaning of "where Q = 1 for an even power of 2 and Q = 2 for an odd

power of 2"

For example ,

1)if I use 1024 point single engine FFT and the output of exponent scaling values is -11.

Should I choose Q=1? and the shift will be 2^10?

2)if I use 512 point single engine FFT and the output of exponent scaling values is -11.

Should I choose Q=2? and the shift will be 2^9?

Altera_Forum · ‎09-24-2007

Sounds like it should be this way...

512 points = 2^9 exponent points, therefore:

=> exponents = 9

=> Q = 2

Therefore, shift amount = 2^(-exponent-Q) = 2^-11

1024 points = 2^10 points, thherefore:

=> exponents = 10

=> Q = 1

Therefore shift amount is also 2^-11

Altera_Forum · ‎09-24-2007

Also another good place to check out the scaling scheme is AN Application Note# 404. In fact I was able to use that Application note to create a unity gain example. (The design, built in 6.1, should be up somewhere on the forum.)

Altera_Forum · ‎11-12-2007

Can somebody provide a simple example of how to use Altera's FFT IP core?

Altera_Forum · ‎11-12-2007

Here is a FFT example (with DSP Builder) that I did a while back (back in 6.1), to test out the block floating point equation. Hope this helps...

Altera_Forum · ‎11-13-2007

Thanks for the quick reply. But what am I supposed to do with it?

Altera_Forum · ‎11-13-2007

Do u have DSP Builder (version 6.1)? If u do... then u simply need to bring up the model and reinstantiate all the megacores. If you don't have DSP Builder v6.1, then I suppose you can just look at the sample test bench. That should give you some hints regarding the FFT core...

Altera_Forum · ‎11-13-2007

Is the DSP builder part of Quartus II or Nios II IDE? I have the Quartus II and Nios II IDE.

Altera_Forum · ‎11-13-2007

It is really a separate platform that is built on top of Simulink/Matlab. DSP Builder calls Quartus from the back end... You can probably find more info on DSP Builder on Altera website: http://www.altera.com/products/software/products/dsp/dsp-builder.html