how to derive the frequency of a periodic noise

Hello all,

I was tasked to derive the frequency of periodic noise based on the magnitude spectrum I have obtained from fast Fourier transform of the signal. The signal contains noise and wanted signal. I have plotted the magnitude and phase spectrum of the signal.

I did try to pass the noisy signal to a low pass filter and the noise is still present in the signal.

Is there any formula of way to derive the frequency of periodic signal?

Thanks for all the help in advance.

Answer 1

[Dimitris Kalogiros]

Hi Felicia

First of all, you should plot magnitude response of your signal using dB scale on y-axis. It help at the visibility of all signals.

As far as it concerns your question, do you have any clue about what is the spectrum of your useful signal ? From your graph, it seams you have two signals: one at 2.5KHz and a second one (weaker) at 200hz. By means of filtering with an apropriate filter, you can even clear the noise and derive excactly your signal.

PS: I suppose, x-axis on your graph is measured in Hz, and not "fft bin-number". If the latter case is true the concept remains the same.

[Felicia Wong]

Hi,

Thanks for the response. I plotted the magnitude response of the fast Fourier transform (FFT) using the decibel scale on y axis.

From the magnitude(db) spectrum , I was able to observe that two frequency components 2500Hz and 8525Hz have higher magnitude as compared to the other frequencies. So, could i assume that the two frequency components are the signal I want.

I was given this signal with no information about the frequencies of the noise and speech.I am tasked to design a Band-stop fir filter to extract the speech from the noisy signal.

Thanks for the help and response!

Cheers, Felicia