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### Plot FFT for different DC Signals in 3D

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```Hi everyone,

I am new to matlab. My problem might sound bit simple :-) but i could not figure the exact solution for it. Hope some suggestions would help me to speed up my work.

I would like to plot a 3 dimensional graph which has Frequency (say X axis), Magnitude (Y axis) and input signal number on (z axis) for a better comparison.

As of now I can plot FFT of any given signal but plotting FFT for say 10 different signal in the same graph(3D) seems to be bit difficult.

```
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```"Vamsi Krishna" <nvamsikrishna@gmail.com> wrote in message <htqumn\$ksh\$1@fred.mathworks.com>...
> Hi everyone,
>
> I am new to matlab. My problem might sound bit simple :-) but i could not figure the exact solution for it. Hope some suggestions would help me to speed up my work.
>
> I would like to plot a 3 dimensional graph which has Frequency (say X axis), Magnitude (Y axis) and input signal number on (z axis) for a better comparison.
>
> As of now I can plot FFT of any given signal but plotting FFT for say 10 different signal in the same graph(3D) seems to be bit difficult.
>

Hi everyone ,

I figured out a function in matlab which will help plot such a graph with some modifications :-) ... one can use waterfall(x,y,z)

I have seen lot of members looking for one such function .. I hope this well help them to some extent ..

there is also a useful thread as well
http://www.mathworks.com/matlabcentral/fileexchange/2904-3d-waterfall-plot-demo

Now i am trying to plot a similar 3D plot for PSD in logarithmic scale :-) ..
```
 0

```In addition to being displayed as a waterfall plot, you could also
display it as an image, or you could use surf().

By the way, these are not DC signals like you said in your subject
line.  A DC signal has no frequencies at all, and it's Fourier
transform is a delta function (spike at zero).
```
 0

```ImageAnalyst <imageanalyst@mailinator.com> wrote in message <a5fc888f-6f94-49d8-ba08-9f7338cd3849@s1g2000prf.googlegroups.com>...
> In addition to being displayed as a waterfall plot, you could also
> display it as an image, or you could use surf().
>
> By the way, these are not DC signals like you said in your subject
> line.  A DC signal has no frequencies at all, and it's Fourier
> transform is a delta function (spike at zero).

Exactly as you said you are right , but in my case when I send such a DC signal through Sigma Delta Modulator of order 2 , and the output there by will have some small spikes (apart from spike at Zero) which are the result of 'Pattern Noise'.  And I am working on reducing this pattern noise.

I tried using surf as you suggested but problem with this is that it joins both column and row vectors which makes me hard to analyze the spikes ...

Do suggest me incase you have any idea how to plot a logarithmic scale (Power Spectrum Density) in a similar manner ..

I am happy for your suggestion .. Thank you
```
 0

```On May 31, 8:11=A0pm, "Vamsi Krishna" <nvamsikris...@gmail.com> wrote:
> ImageAnalyst <imageanal...@mailinator.com> wrote in message <a5fc888f-6f9=
> > In addition to being displayed as a waterfall plot, you could also
> > display it as an image, or you could use surf().
>
> > By the way, these are not DC signals like you said in your subject
> > line. =A0A DC signal has no frequencies at all, and it's Fourier
> > transform is a delta function (spike at zero).
>
> Exactly as you said you are right , but in my case when I send such a DC =
signal through Sigma Delta Modulator of order 2 , and the output there by w=
ill have some small spikes (apart from spike at Zero) which are the result =
of 'Pattern Noise'. =A0And I am working on reducing this pattern noise.
>
> I tried using surf as you suggested but problem with this is that it join=
s both column and row vectors which makes me hard to analyze the spikes ...
>
> Do suggest me incase you have any idea how to plot a logarithmic scale (P=
ower Spectrum Density) in a similar manner ..
>
> I am happy for your suggestion .. Thank you

Codswallop!!
I agree with IA.
A DC signal has no frequency content.
Otherwise it's not DC, by definition.
But of course you are free to redefine terms, just like Humpty Dumpty:
`When I use a word,' Humpty Dumpty said, in rather a scornful tone,
`it means just what I choose it to mean -- neither more nor less.'

But don't expect us to accept that.

On the matter of plotting PSD vs Frequency:
help loglog
```
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