# Using the first two values of the time variable t to calculate the sampling frequency.

Download a data file called ‘data.mat’ from the folder ‘Lab 5 – Signal processing’ on Blackboard. Save it to your Matlab work directory. Then load this data file into Matlab using the Matlab command:

This data file contains two variables: the time variable t (in seconds) and the data variable y (in milli-volt) which contains measurements from an electrophysiological experiment.

 _Plot the measurement y in time. You will see a sharp negative deflection around t=0. Paste this figure to your answer sheet. Now in order to reveal the dynamic characteristics of this deflection, zoom into the figure so that the variable y is displayed approximately in the time range [0 0.05]. Paste this figure to your answer sheet. What is the approximate time at which the negative peak occurred?
⦁ Using the first two values of the time variable t to calculate the sampling frequency.

⦁ Estimate the PSD of the variable y using ‘fft’ and plot the PSD vs frequency. (Hint: The frequency range should be calculated from freq=sample_freq*(0:L/2)/L, where sample_freq is the sampling frequency you have calculated previously, and L is the data length.) You should see a spike near the zero frequency but nothing at higher frequencies. Paste this figure to your answer sheet. Zoom into the figure so that the PSD is displayed approximately in the frequency range [0 250] Hz. Paste this figure to your answer sheet.

⦁ A spike should be observed in the PSD at 50Hz. Explain what the spike might represent.

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