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3rdClassToaster
Visitor
Visitor
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Registered: ‎08-25-2020

Some questions to IFFT-Configuration of XFFT IP V9.1

Hi,

I just read the documentation of XFFT IP V9.1, read some threads and also did some testing, but still had some questions especially as far as the IFFT Mode of the IP is concerned.

Perhaps somebody did similar experience and can refer to some of them.

1)

Is it correct that IFFT mode of IP adds an intrinsic scaling of 1/N to its input which can't be prevented and which is independent from the configuration of the IP and the eventually configured scaling schedule on axi_stream_config bus ?

With relation to previous question, is it correct that if i do a scaled FFT (scaling schedule or auto scaling) and put the result vector into IFFT i can't get back original amplitude of time signal because [scaled FFT + intrinsic scaled IFFT] resulting in kinda double scaling ?

So with relation to previous question, if approach would be to get back to original time signal after FFT + IFFT an unscaled FFT had to be chosen ?

2)

If i choosed the option "natural order" for FFT output and then put the output vector back into IFFT again (after switching from FFT mode to IFFT mode on axi_stream_config bus), do i have to reorder the indices of the vector first to get original time signal back, or can i leave the order as it is (if configuration of IP remains unchanged except switching it to IFFT) ?

3)

Is it normal that IFFT doubles the frequency of original signal in its output vector (so that if original FFT input was 1 Period of time signel, you will receive 2 Periods in IFFT output) or is this a mistake in my test application ?

4)

Does the "scaling schedule" on the axi_stream_config bus (if the IP is configured for manual scaling) have to be sent only once before starting to read in a frame, or does it have to be sent repeatedly for each sample during the read process?

5)

Is it necessary to reset the core via the optional "aresetn" port before or after sending a new configuration via the axi_stream_config bus ?

Ty and Greetings

1 Reply
nathanx
Moderator
Moderator
158 Views
Registered: ‎08-01-2007

Q1: 

s it correct that IFFT mode of IP adds an intrinsic scaling of 1/N to its input which can't be prevented and which is independent from the configuration of the IP and the eventually configured scaling schedule on axi_stream_config bus ?

With relation to previous question, is it correct that if i do a scaled FFT (scaling schedule or auto scaling) and put the result vector into IFFT i can't get back original amplitude of time signal because [scaled FFT + intrinsic scaled IFFT] resulting in kinda double scaling ?

So with relation to previous question, if approach would be to get back to original time signal after FFT + IFFT an unscaled FFT had to be chosen ?

 

A1 -> The answer is No, FFT IP does provide unscaled option in the IP GUI. 

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