How do I plot a phase plot for FFT and code time history for down-sampled signal?

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omar ali 2023-5-6

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1959154-how-do-i-plot-a-phase-plot-for-fft-and-code-time-history-for-down-sampled-signal

评论： Star Strider 2023-5-8

Hello,

Thus far, i have completed this code which gave me a frequency vs amplitude graph. I'm ussure how to get a phase graph from this or time history of down sampled signal by a given rate etc I nwould be grateful for assistance for this.

data = [0	0.829668000000000
    0.00833300000000000	0.829801000000000
    0.0166670000000000	0.828810000000000
    0.0250000000000000	0.829008000000000
    0.0333330000000000	0.828352000000000
    0.0416670000000000	0.827073000000000
    0.0500000000000000	0.826807000000000
    0.0583330000000000	0.826828000000000
    0.0666670000000000	0.826857000000000
    0.0750000000000000	0.827056000000000
    0.0833330000000000	0.827139000000000
    0.0916670000000000	0.828689000000000]
data = 12×2
         0    0.8297
    0.0083    0.8298
    0.0167    0.8288
    0.0250    0.8290
    0.0333    0.8284
    0.0417    0.8271
    0.0500    0.8268
    0.0583    0.8268
    0.0667    0.8269
    0.0750    0.8271
t = data(:,1);
s = data(:,2);
Ts= 0.0083;
Fs=1/Ts;
Fn= Fs/2;
L=4000;
smean=mean(s);
Fts=fft(s-smean)/L;
Fv= linspace(0,1,fix(L/2)+1)*Fn;
Iv=1:numel(Fv);
figure
plot(Fv,abs(Fts(Iv))*2)
Index exceeds the number of array elements. Index must not exceed 12.
grid
xlabel('frequency')
ylabel('Amplitude')

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Answer 1

Star Strider 2023-5-7

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1959154-how-do-i-plot-a-phase-plot-for-fft-and-code-time-history-for-down-sampled-signal#answer_1230409

在 MATLAB Online 中打开

Perhaps this —

data = [0 0.829668000000000

0.00833300000000000 0.829801000000000

0.0166670000000000 0.828810000000000

0.0250000000000000 0.829008000000000

0.0333330000000000 0.828352000000000

0.0416670000000000 0.827073000000000

0.0500000000000000 0.826807000000000

0.0583330000000000 0.826828000000000

0.0666670000000000 0.826857000000000

0.0750000000000000 0.827056000000000

0.0833330000000000 0.827139000000000

0.0916670000000000 0.828689000000000]

data = 12×2

0 0.8297 0.0083 0.8298 0.0167 0.8288 0.0250 0.8290 0.0333 0.8284 0.0417 0.8271 0.0500 0.8268 0.0583 0.8268 0.0667 0.8269 0.0750 0.8271

t = data(:,1);

s = data(:,2);

Ts= 0.0083;

Fs=1/Ts;

Fn= Fs/2;

L=size(data,1);

smean=mean(s);

NFFT = 2^nextpow2(L);

FTs=fft((s-smean).*hann(L))/L;

Fv= linspace(0,1,NFFT/2+1)*Fn;

Iv=1:numel(Fv);

figure

subplot(2,1,1)

plot(Fv,abs(FTs(Iv))*2)

grid

ylabel('Amplitude')

subplot(2,1,2)

plot(Fv,rad2deg(angle(FTs(Iv))))

grid

xlabel('Frequency')

ylabel('Phase (°)')

Also consider using unwrap for the radian phase, as: ‘rad2deg(unwrap(angle(FTs(Iv))))’. (I made a few improvements in my original code.)

.

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omar ali 2023-5-7

Picture3.png

Hello,

Thanks, I have got the phase plot . For ' showing time history of the signal downsampled for example to fs/5' I changed the fs value in above code but I am unsure what else this requires/means

I've included some example codes for this.

time history of a signal down-sampled under the rates of 25Hz & 40Hz and find its DFT analysis using Matlab

Fs = 1000; % sampling rate

T=1/Fs; % sample time

L= 20000; % lenght of signal

t=(0:L-1)*T; % time vector

figure(1)

plot(t,u)

grid;

title('Time History of the Signal')

% dft analysis

yf=fft(u);

w=[0:2*pi/L:(L-1)*2*pi/L];

figure(2)

plot(w/(2*pi*T),abs(yf))

grid

title('DFT analysis of the signal sampled at 1000Hz')

Example 2 for a function , but not for my data set as desired.

x(t) = cos ( 10 t) + e (t) , t <10 sec fs= 512 hz

fs=512; dt=1/fs;

t=0:dt:10;

N=length(t);

e=0.5*(rand(1,N)-0.5)

x=cos(10*t)+e;

lpFilt = designfilt('lowpassiir','FilterOrder',20, ...

'PassbandFrequency',15/(2*pi),'PassbandRipple',0.01, ...

'SampleRate',fs);

y=filter(lpFilt,x);

figure(1)

w=[0:2*pi/N:2*pi*(N-1)/N]

X=abs(fft(x)/(N/2));

plot(w,X)

hold on

xlim([0,0.1*pi])

figure(2)

Y=abs(fft(y)/(N/2));

plot(w,Y)

hold on

xlim([0,0.1*pi])

figure(3)

plot(t,x)

hold on

plot(t,y)

omar ali 2023-5-8

Thanks for assisting with your obvious expertise!

This is my first time using matlab and for filtering I came across a simple code for our case above but it displayed error message

inputSignal = (t, s);

↑

Invalid expression. When calling a function or indexing a variable, use parentheses. Otherwise, check for mismatched delimiters

F = 8

Fs = 30

[y, x] = butter(5, F/(Fs/2), 'low’)

inputSignal = (t, s);

outSignal = filter(y, x, inputSignal);

plot(outSignal)

Star Strider 2023-5-8

As always, my pleasure!

I would do this instead —

F = 8

Fs = 30

[y, x] = butter(5, F/(Fs/2), 'low’)

inputSignal = s;

outSignal = filtfilt(y, x, inputSignal);

figure

plot(t, outSignal)

There is no reason to include the time vector in the signal being filtered, so I eliminated it here.

However if you want to concatenate two column vectors, use square brackets as [a, b] (although the comma is optional) and if you want to concatenate two row vectors [a; b] using the semicolon to create a second row (to vertically concatenate them). In all instances, the vectos must have the same row and column dimensions.

Actually, I would design the filter differently (I prefer elliptic filters because they are more computationally efficient), however to design a Butterworth filter I would begin with the buttord function, then butter, however producing zero-pole-gain output, and then use the zp2sos function to produce a second-order-section representation, and call filtfilt as:

outSignal = filtfilt(sos, g, inputSignal);

The filtering will be more effective and the filter will always be stable with these changes.

.

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How do I plot a phase plot for FFT and code time history for down-sampled signal?

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How do I plot a phase plot for FFT and code time history for down-sampled signal?

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