Beginner's question on DFT

[ebuddy]

Hi, I am learning DFT. When a continous signal gets sampled, we get as many sample points as we want. How many points do we need (N) to do DFT analysis? For my particular example, I have

t = 0:0.001:0.6;
x = sin(2*pi*50*t)+sin(2*pi*120*t);

So I get 601 samples. But what is the minimum N that is good enough to do DFT analysis?

Thanks.

 

[azerm]

It is better N to be in form of 2^k; so for example 512 is a good choise for your problem.

t = 0:0.001:0.6;
t=t[1:512];
x = sin(2*pi*50*t)+sin(2*pi*120*t);

then t and thus x has only 512 sample;

 

[ebuddy]

Thanks. How about 256 points? Will it be enough? Is the result different comparing to if I use 512 points? Thanks.

 

[ttdown]

the signal in analog continous domain is x = sin(2*pi*50*t)+sin(2*pi*120*t); the frequency of sin(2*pi*50*t) is 50Hz, while sin(2*pi*120*t) is 120Hz. so the highest frequency harmonic of x is 120Hz.

as the sampling theory: Fs > 2fmax, the digital sampling rate is at least Fs =240Hz, so 256 point is Okay.