# How to convert fft plot into PSD(Normalized) vs Freqeuncy (Hz)

Status
Not open for further replies.

#### moeenhappy

##### Newbie level 4
I am trying to convert my fft signal into PSD(Normalized) vs Freqeuncy (Hz) but can't get it right. I want to get some thing like shown in the figure. I found these figures in the literature. How to plot this in matlab? PSD from 0-1 and frequency in log scale.

If some one can explain with a simple example, it will be highly appreciated

#### KlausST

##### Super Moderator
Staff member
Hi,

X-axis:
You see it is a logarithmic scale. You need to start with a frequency larger than 0Hz (0Hz = DC is not possible)
In the top picture this is 1000Hz, in all other pictures it is 100Hz.
But this is somehow untypicial for an FFT output. An FFT output always starts with 0Hz (=DC) and has 2^n frequency taps with equal frequency distance. It is linear, not logarithmic.

Example: An FFT result of 1024 values sampled at 10.24kHz will give 512 results stating with 0Hz and an equal frequency distance of 10Hz = 10.240 Hz / (2 x 512)

****
Y-Axis.
Often this is in a dB scale.
In your top picture you see the referece signal. It is "normalized" to be in scale of "1" wich is 0dB.

I can´t see how the other diagrams are normalized. I expect either a reference signal (value = 1 or 0dB) or a fixed power like "1mW" which is 0dBm. But both is missing. Maybe they refer to the first diagram.

***

Klaus

#### moeenhappy

##### Newbie level 4
Hi,

X-axis:
You see it is a logarithmic scale. You need to start with a frequency larger than 0Hz (0Hz = DC is not possible)
In the top picture this is 1000Hz, in all other pictures it is 100Hz.
But this is somehow untypicial for an FFT output. An FFT output always starts with 0Hz (=DC) and has 2^n frequency taps with equal frequency distance. It is linear, not logarithmic.

Example: An FFT result of 1024 values sampled at 10.24kHz will give 512 results stating with 0Hz and an equal frequency distance of 10Hz = 10.240 Hz / (2 x 512)

****
Y-Axis.
Often this is in a dB scale.
In your top picture you see the referece signal. It is "normalized" to be in scale of "1" wich is 0dB.

I can´t see how the other diagrams are normalized. I expect either a reference signal (value = 1 or 0dB) or a fixed power like "1mW" which is 0dBm. But both is missing. Maybe they refer to the first diagram.

***

Klaus
How to plot this in matlab? can you give me a simple example? I am struggling in calculating a normalized PSD which range from 0-1 on y-axis?

Status
Not open for further replies.