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Ixiasoft
4.1. Possible Exponent Values
Depending on the length of the FFT/IFFT, the number of passes through the radix engine is known and therefore the range of the exponent is known. The possible values of the exponent are determined by the following equations:
P = ceil{log4N}, where N is the transform length
R = 0 if log2N is even, otherwise R = 1
Single output range = (–3P+R, P+R–4)
Quad output range = (–3P+R+1, P+R–7)
These equations translate to the values in Table A–1.
N | P | Single Output Engine | Quad Output Engine | ||
---|---|---|---|---|---|
Max (2) | Min (2) | Max (2) | Min (2) | ||
64 | 3 | –9 | –1 | –8 | –4 |
128 | 4 | –11 | 1 | –10 | –2 |
256 | 4 | –12 | 0 | –11 | –3 |
512 | 5 | –14 | 2 | –13 | –1 |
1,024 | 5 | –15 | 1 | –14 | –2 |
2,048 | 6 | –17 | 3 | –16 | 0 |
4,096 | 6 | –18 | 2 | –17 | –1 |
8,192 | 7 | –20 | 4 | –19 | 1 |
16,384 | 7 | –21 | 3 | –20 | 0 |
Note to Table A–1 :
|