Alternate way to find the frequencies using fft apart from the fft plot?

Question

rohail khan 2018-4-1

0
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https://ww2.mathworks.cn/matlabcentral/answers/392020-alternate-way-to-find-the-frequencies-using-fft-apart-from-the-fft-plot

评论： rohail khan 2018-4-2

hi, I have plotted the following three Sinusoidal wave forms of 70,80 and 90 MHz.

t=0:1/1000:0.2-1/1000;
w1=2*pi*70;
w2=2*pi*80;
w3=2*pi*90;
A1=1;
A2=1/2;
A3=1/3;
f1=A1*sin(w1*t);
f2=A2*sin(w2*t);
f3=A3*sin(w3*t)
f=f1+f2+f3;
plot(t,f)
title('3 combined curves')
xlabel('time')
hold off

Now I use fft to to find the frequencies from the combined frequency 'f'. I used the following code

%Now using FFT to find the frequencies 60Hz, 70 Hz, and 80 Hz
F=fft(f)
plot(abs(F))
xlable('frequency')

I am trying to recover back the frequency and amplitude of 60,70 and 80 MHz from the combined frquency f. I can find that it by magnifying the plot(abs(F)).

My first question is that is there any other way to find the frequencies and amplitudes?

My second question is that why the the amplitudes are now multiplied by a factor of 100 in the plot?