how to decide num of points in fft?

[newbie_1]

hi,
i wanna do the fft analysis of n matlab, assume fs is the sample frequency, f0 is the fundamental signal freq,and i have got N samples loaded in matlab, then how can i decide whether to use window or not? how to select the number of fft points? will the number of points affect the fft results?
and if possible, can anyone give me some matlab code for testing dynamic characteristics of a adc?

thanks a lot
newbie

 

[Renjith]

fft points will affect the results of the display.
as u increase the fft points the Bin resolution will be more.
i.e assume u have an Fs = 1Mhz, and then if u r FFt pts = 1000,
then ur Bin = 1 KHz(1 Mhz/1000).

that means in ur plot u will b able to visualize 1k,2k,3k....upto fs.

now if u increase ur nots = 2000, then ur Bin resolution = 0.5 Khz. so the plot can display frequencies like 0.5k, 1k,1.5k,2k,2.5k etc...
so higher the no: pts, higher will be ur plot resolution.

Cheers !!!
Renjith

 

[newbie_1]

hi Renjith,
you mean so long as the resolution is enough then increasing of the points will not be necessary, am i correct?
thanks

 

[claudiocamera]

The problem you raised is called spectrum estimation, when trying to using fft to visualise the signal frequency you are performing non parametrical estimation. When you increase the number of points in the fft you increase the resolution in this way the plot appear to be more dense, the drawback is when you have frequencies to close, when you dont know the frequency content of the signal it is hard to decide if what are you seeing is frequency content or spectrum leakage, in order to minimize the efect of spectrum leakage you can use windows techniques, using for example , barlet, blackman or kaiser windows, usualy you will perform trial and error untill you got a reasonable result, nom parametrical spectrum estimation is sometimes refered as an art since there are no defined steps to follow...

 

[Renjith]

hi Renjith,
you mean so long as the resolution is enough then increasing of the points will not be necessary, am i correct?
thanks

yes... if u find ur plot has got enough expected resolution, then u need not increase the no: pts.

Cheers !!!
Renjith