How to perform FFT on a signal with a 5 Hz frequency resolution?

I am trying to implement an algorithm to measure harmonics and interharmonics of a signal according to IEC 61000-4-7 standard. According to the standard I need to get the FFT of the signal with 5 Hz frequency resolution. Final output should look like the following figure.

Can any one help me with this....

[Dimitris Kalogiros]

I guess you have a time signal that is sampled with some sampling frequency. What is this sampling frequency?

[Jagannath Wijekoon]

The sampling frequency is 20000 Hz

Answer 1

[dpb]

The FFT resolution is based on the length of the data sample and the sample rate and the length of the computed FFT.

If

Fs=sample rate
L =FFT length,

then

df=Fs/L

To set df=5Hz, then solve for

L=Fs/5

or

L=20000/5

which would be a sample size of 4000 points or sampling time of 200 msec.

Or, of course, you can use a finer resolution and then sum over 5 Hz bins.

[Ahmed Hassan]

Hello,

Could you please help me, I have the same issue.

I have for example a recorded signal for 10 minutes.

I1 and I1_time are my varabile in the exported matlab file and the sampling frequency 20 kHz.

i need to calculate the harmonics of frequency in the range from 2 kHz up to 9 kHz with 5 Hz steps as well.

but I need to cut the 10 minutes into 3000 cut windows (0.2 seconds or 10 cycles for 50 hz system) so at the end I will get for example for 2KHz frequency 3000 values then take the max value out of these 3000 values.

my question is how to do FFT analysis for (I1 and I1_time) for each of the cut windows ?