Plotting of FRF curves

I want to plot FRF vs Frequency graph.I have a matrix 'a' of acceleration & a force matrix 'f' both of 30000X1 size, collected in 3secs with a time step of 0.0001 sec.I am using the following steps to plot FRF. a=[ ]; f=[ ]; ffa=fft(a); fff = fft(f); frf, H=ffa./fff; aH =abs(H); I want to plot FRF vs Frequency graph using log(aH) on Y-axis. Pl tell me how to specify x-axis?

[Rick Rosson]

Please format your code:

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Answer 1

[bym]

see my answer here http://www.mathworks.com/matlabcentral/answers/10306-frf-plotting

[Shrinivas Gombi]

Dear Sir,

I had only asked this question in that link also.Thank u very much for ur answer.But I have tried many ways of setting x- axis but not getting correctly.I request u to kindly provide me the exact answer.

Prof.S L Gombi

Answer 2

[bym]

Frequency resolution is sampling frequency divided by FFT points. So for a one sided spectrum, it is (directly from the documentation)

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 can then plot as

semilogy(f,aH)

or

loglog(f,aH)

[Shrinivas Gombi]

Dear Sir, Thank u once again.

Kindly confirm my data wether is it correct wrt your answer or not.

For the case I have mentioned above:

L = 30 000, Fs = 10 000, What is 'y'?

Is y = accelration matrix 'a'(size:30000X1) or force matrix 'f'(size:30000X1) or the FRF matrix 'H'(size:30000X1),absolute value of H i.e 'aH'matrix(size:30000X1) or is it only a number 30000?

Actually 2^15 = 32,768 IS L = 15 ?. I am confused .Pl clarify my doubt

Prof.Shrinivas Gombi

[bym]

L = 30000; y = aH; NFFT = 32768; Fs = 10000

[Shrinivas Gombi]

Dear SIr,

Kindly check my steps:

a = [30000X1 readings];

ffa = fft(a);

F =[30000X1 readings];

fff = fft(F);

H = ffa./fff;

aH = abs(H);

L = 30000;

NFFT = 32768;

Fs = 10000;

y = aH;

Y = fft(y,NFFT)/L;

f = Fs/2*linspace(0,1,NFFT/2+1);

semilogy(f,aH)

After the Execution I am getting the following comment

??? Error using ==> semilogy

Vectors must be the same lengths.

Kindly let me know where is the mistake and how to correct it.As I required to submit my PhD thesis at the earliest, pl help me in solving this problem.Waiting for your reply

Prof> Shrinivas Gombi