MATLAB: Calculating Time Period of a Displacement Signal

Question

Hi,

I have signal data in the form of a sinusoidal wave, decreasing in amplitude and increasing in frequency.

Is there a way to calculate the frequency or time period of a signal in the time domain?

Signal Data

Below is the outcome I am hoping for

Thanks

Conor

Answer 1

The easiest way is to calculate the exact zero crossings and invert them to get the instantaneous frequency. I don’t have your data so I created some to test this. You will probably need to tweak it a bit to work correctly with your data.

The Code:

t = linspace(0, 60, 10000);                                                 % Create Data

y = sin(2*pi*(t/10).^3) .* exp(-t/10);                                      % Create Data
zci = @(v) find(v(:).*circshift(v(:), [-1 0]) <= 0);                        % Returns Approximate Zero-Crossing Indices Of Argument Vector
yidx = zci(y);                                                              % Approximate Zero Crossing Indices
for k1 = 2:length(yidx)-1
    tc(k1) = interp1(y(yidx(k1)+[-1:1]), t(yidx(k1)+[-1:1]), 0, 'linear');  % ‘Exact’ Zero Crossings
end
frq = 1./gradient(tc);                                                      % Approximate Frequency
figure(1)
subplot(2,1,1)
plot(t, y)
title('Original Signal')
grid
subplot(2,1,2)
plot(tc, frq)
title('Frequency')
grid