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I am designing 8 point FFT by radix 2 using verilog. m using radix 2 butterfly unit with 8 bits input and so output i expect to be 8 bit so that i can use this structure again and again for further sta. i am applying DIT. if i consider my twiddle factors are of 8 bits stored somewhere then i multiply to one of butterfly inputs result is 16 bits,and when added/subtracted to other input output is for sure going to be min 16 bit/ 17 bit. but i expect my output to be of 8 bit so that same butterfly with 8 bit input can be used. how do i scale this without loss of accuracy. please reply if you exactly dont understand my problem.
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Pravesh R. wrote: > how do i scale this without loss of accuracy. I would design the filter with a higher level language like python or matlab. There it's more easy to see which internal bitwidth are required to avoid overflows. Duke
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Obviously if you multiply two 8bit numbers and then want the result to fit in 8 bits you will loose some precision. You need to make a decision regarding how much error you are willing to accumulate in each stage. If you are not willing to have any error then you will need to increase the number of bits in each stage. When doing fixed point arithmetic like this its often useful to interpret the vectors as scaled integers. For signed numbers you could use the following scale... 1000_0000... = 1.00... 0000_0000... = 0.00... 0111_1111... = 0.99... The multiplication of two Nbit signed vectors results in a vector that is 2*N  1 bits long. When using scaled integers you would then just take the upper N bits of the result to get back to your original scale.