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From: sulphox on 7 Dec 2005 04:16
Hi all,

I wanna plot a FFT graph.
Let's say i have
variable, t
y(t)=rect(t)
Y(f)=FFT of rect(t)

If i wanna plot the FFT graph.
What should be the x and the y axis?
is it 't' and 'absolute of Y(f)' respectively?

Please correct me if i'm wrong.

Thanks for any help in advance.

From: William C Bonner on 7 Dec 2005 11:42
I few years back I wrote an excel add in that allowed me to do FFTs
directly on a spreadsheet using array formulae. It still works with
the current version of Excel.

The package was called HRIXL but I do not believe that you can get it
any more.

The Demo has the first column being the position numbers, from 1 to
201.

The Second column is random data created with
=SIN(A:A*60*PI()/201)+RAND()-0.5

The third column is all zeros.

Column E and F are created with the formula =cplxFourier(B1:C201,FALSE)

Column H is created with the formula =SQRT(E1:E201^2+F1:F201^2)

So, Column H is the absolute value of the complex data, that is split
between columns E and F. I plot the data in Column B and Column H in
different graphs to show the original data and the FFT Data.

sulphox wrote:
> I wanna plot a FFT graph.
> Let's say i have
> variable, t
> y(t)=rect(t)
> Y(f)=FFT of rect(t)
>
> If i wanna plot the FFT graph.
> What should be the x and the y axis?
> is it 't' and 'absolute of Y(f)' respectively?

From: AntiSPAM_g9u5dd43 on 7 Dec 2005 11:54
On Wed, 07 Dec 2005 03:16:07 -0600,
sulphox77(a)hotmail-dot-com.no-spam.invalid (sulphox) wrote:
>I wanna plot a FFT graph.
>Let's say i have
>variable, t
>y(t)=rect(t)
>Y(f)=FFT of rect(t)
>If i wanna plot the FFT graph.
>What should be the x and the y axis?
>is it 't' and 'absolute of Y(f)' respectively?
>Please correct me if i'm wrong.
>Thanks for any help in advance.

1) You have to have the Analysis ToolPak enabled in Add-Ins to use the

menu Tools/Data Analysis;
Then you can select "Fourier Analysis"

2) The output from "Fourier Analysis" is a column complex data.
To plot othe absolute value, you have to use "IMABS"

3) if your independent variable is time: assuming a fixed interval DT
and a total number of data points N, then you can creat a NEW
column (Freq) where DF = 1/(N*DT) and
Freq(i) = i*DF from 0 to (N-1)

example, sampling period 0.001 seconds with 1000 samples for
a total time of 1 second from 0 to 0.999, then
N*DT = 1 and DF = 1/1=1.

Note that you don't have to plot ALL of the Fourier data since
it will be symmetric about the point( N/2*DF)



From: Fred Marshall on 7 Dec 2005 13:19

"sulphox" <sulphox77(a)hotmail-dot-com.no-spam.invalid> wrote in message
news:7tqdnWsu2fDKNQveRVn_vA(a)giganews.com...
> Hi all,
>
> I wanna plot a FFT graph.
> Let's say i have
> variable, t
> y(t)=rect(t)
> Y(f)=FFT of rect(t)
>
> If i wanna plot the FFT graph.
> What should be the x and the y axis?
> is it 't' and 'absolute of Y(f)' respectively?
>
> Please correct me if i'm wrong.
>
> Thanks for any help in advance.

There are various FFT outputs that are possible.
The most common goes from radian frequency=0 to radian frequency=2*pi or
frequency=1Hz for an assumed or normalized sample interval of "1" or 1
second.
For real functions being transformed, the magnitude repeats / mirrors at pi
radians/sec or 0.5Hz, so you may not want to plot the result beyond this.
x would be frequency in radians per second or Hz from 0>pi or 0>0.5 ... half
of the output points plus one.
y would be absY(f) where Y(f) is a complex sequence.

Since you're wanting to use Excel, check out:
http://www.fysik.uu.se/kurser/fy660/compendium/Signal/

Fred


From: sulphox on 8 Dec 2005 04:16
Hi,

First of all, thanks for all the help given.

I would like to ask guest regarding the
point no. 3.
I do not understand about the new frequency part.

If i have variable t=-1. -0.9, -08,...., 1 (64 sets of data in total)
I have computed:
The signal, y(t)=rect(t)
The Fourier analysis of y(t), Y(f)
The IMABS of Y(f)

How do I create the new column for the frequency from the data that i
have?

Sulphox

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