MATLAB: Cut Sinusoidal signal in a integer number of periods

algorithmcutfftfigurefitMATLABmeasurementplotsignalsimulinksinussinusoidal

Hey guys,
I have an measurement of three sinusoidal signals.
But it is enough to only work with one in this case.
The measurement doesn´t have an int number of periods and i need to cut for my further work.
To make it more clear here is an picutre:
In this case we have 4 periods.
2019-07-09_09-41-27.png
So i have to find the first negative to positive zero corssing and the last negative to positive zero crossing.
I found this code in the forum but it don´t work with my files but maby it can help u ?!
t = linspace(0,10*pi,200); 
x = sin(t); 
mcs = x .* circshift(x, [0 -1]); 
zxix = find(mcs <= 0); 
for k1 = 1:2:size(zxix,2)-1 
    zx(k1) = interp1(x(zxix(k1):zxix(k1)+1), t(zxix(k1):zxix(k1)+1), 0); 
end 
figure(1) 
plot(t, x) 
hold on 
plot(zx, zeros(size(zx)), '+r', 'MarkerSize',10) 
hold off 
grid
I hope u can help me.
Thanks in advance

Best Answer

Hi,
bear in mind that the sampling of your curve might play an important role in the game.
The code below might help you out:
t = linspace(0,10*pi,200); 
x = sin(t); 
zero_crossings_pos = [];
for kind = 1:length(t)-1
    if (x(kind) <= 0) && (x(kind+1) > 0)
    zero_crossings_pos = [zero_crossings_pos kind];
    end
end
figure,
plot(t,x,t,x,'o')
hold on
plot(t(zero_crossings_pos),x(zero_crossings_pos),'r*')
hold off
xlabel('x')
ylabel('sin(x)')
title('sin')
t_crop = t(zero_crossings_pos(2):zero_crossings_pos(5));
x_crop = x(zero_crossings_pos(2):zero_crossings_pos(5));
figure,
plot(t_crop,x_crop)
xlabel('x')
ylabel('sin(x)')
title('Cropped sin')
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