fft application for signal processing to know frequency

I have some signal data as shown below. It is constructed with 624 sample points from text file. This signal generated from a 4MHz ultrasonic transducer. By applying FFT in Matlab, I want to know which frequency is dominating in the below signal.

For the above purpose I am able to read the data from text file and plot it as shown above. After that iam applying FFT by using abs(fft(‘textfile name’ , N)) Please suggest me where to define the frequency of signal in the code and what should be the N value in fft command line. With out giving N value and frequency parameters iam getting the following figure

How the X-axis values to be converted to frequency values, because the x-axis is showing only 624 values. Whether the above plot is correct or not. Please guide me

Answer 1

[Star Strider]

See the documentation for fft, specifically the section after the first figure that demonstrates how to calculate the frequency vector for your plot:

NFFT = 2^nextpow2(L); % Next power of 2 from length of y
Y = fft(y,NFFT)/L;
f = Fs/2*linspace(0,1,NFFT/2+1);

You need your sampling frequency (here ‘Fs’) to calculate the ‘f’ (frequency) vector.

[Ramesh]

in my case what is the Fs value. whether 624 or 4 MHz? and also what is 'L' value?

[Star Strider]

You have 624 sample points, and the carrier frequency of your transducer is 4 MHz. Neither of those are your sampling frequency.

Only you can determine what your sampling frequency is. If you have a time vector of the times your signal was sampled, as well as the values of the signal (that you obviously have), the sampling time ‘Ts’ is the difference between any two time elements in your time vector (that I call ‘tv’ here):

Ts = tv(2)-tv(1);      % Sampling Time
Fs = 1/Ts;             % Sampling Frequency
Fn = Fs/2;             % Nyquist Frequency

If you do not have or cannot calculate the sampling frequency, you cannot determine your frequency vector.

[Ramesh]

Dear Strider,

The thickness of my testing object is 25mm and the range of x-axis selected is 250 mm. so thats why i will be getting 10 echos, each located at 25mm on x-axis. with the above data available what is 'Fs' value? what is 'L' value?

[Star Strider]

You say your data are generated by a ‘4MHz ultrasonic transducer’. I assume the carrier frequency is 4MHz. You have to look through the documentation for your transducer and probably the file itself to find out what the sampling frequency ‘Fs’ is. I cannot tell it from your data.

If you have a time vector for your 624 data points, what is the difference (in seconds) between the consecutive sampling points? That is your sampling interval or sampling time ‘Ts’, and you can calculate the rest from the code I posted in my [24 Nov 2014, 18:42] Comment.

‘L’ is the length of your data vector, so here:

L = 624;