FFT function gives strange phase

[stchalon]

Hello

I try to use FFT and then try to rebuild a function using some of the coefficients like formula below
f(t) for I2 = _c0/2 + _c1*cos( 2*PI()*_f1*A2-_p1 ) + _c2*cos( 2*PI()*_f2*A2-_p2 ) + _c3*cos( 2*PI()*_c3*A2-_c3 )

where _ci are module of complexe coefficients, _fi are multiple of main frequency, _pi are phases =-arctan(bn/an)

I have done few tests:
Input function : f(t)=sin(2*PI()*A2/86400) where A2 is time. For this function I can generate FFT and rebuild g(t)~f(t) using few coefficients

*but* with Input function : f(t)=t, I need to change the sign of the phases to retrieve g(t)~f(t)

I have tried for a while checking all formulas w/o success; There is something I don't understand...

Thanks for your help
Stchalon

[djapigo]

Hi Stchalon,

I've never done this before, so I looked it up...

Have you tried look at sites like this?

http://www.ehow.com/how_4670778_four...oft-excel.html

Let us know if this still doesn't help.

[shg]

Post a workbook?

[stchalon]

Thanks djapigo and Shg.
I know the site suggested by djapigo. The good news is that when trying to build a workbook to share my issue the problem has gone. So, I think thread can be closed. As I cannot see how to attach a workbook, I cannot share the results with you.

Thanks again, stchalon

[shg]

As I cannot see how to attach a workbook, ...

It's explained in the forum rules.