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# Power Spectrum Plotting of a Signal

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Parth T.A. on 27 Jul 2020
Answered: Surya Talluri on 13 Aug 2020
I need to plot a power (y axis) vs frequency (x axis) graph to represent the power spectrum of a signal. Something looking like this: Here is my code:
LFP = doFilter(Signal);
sampling_rate = 1000;
length_LFP = length(LFP);
time = [0:1:length_LFP-1] * (1/sampling_rate);
figure;
plot(time, LFP);
for i = 1:length(LFP) %filtering out certain values
if abs(LFP(i))>5900
LFP(i) = 0;
end
end
figure;
plot(time, LFP);
ylim([-30000 20000]);
figure;
pwelch(LFP, [], [], [], sampling_rate);
function y = doFilter(x)
%DOFILTER Filters input x and returns output y.
% MATLAB Code
% Generated by MATLAB(R) 9.8 and DSP System Toolbox 9.10.
% Generated on: 27-Jul-2020 14:11:04
%#codegen
% To generate C/C++ code from this function use the codegen command.
% Type 'help codegen' for more information.
persistent Hd;
if isempty(Hd)
% The following code was used to design the filter coefficients:
%
% N = 2; % Order
% F3dB1 = 0.1; % First
% F3dB2 = 100; % Second
% Fs = 1000; % Sampling Frequency
%
% h = fdesign.bandpass('n,f3db1,f3db2', N, F3dB1, F3dB2, Fs);
%
% Hd = design(h, 'butter', ...
% 'SystemObject', true);
Hd = dsp.BiquadFilter( ...
'Structure', 'Direct form II', ...
'SOSMatrix', [1 0 -1 1 -1.50961300935918 0.509921232956256], ...
'ScaleValues', [0.245039383521872; 1]);
end
s = double(x);
y = step(Hd,s);
end
I am unable to get power (in V) on the y axis. How can I rectify this?
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### Accepted Answer

Surya Talluri on 13 Aug 2020
I understand that you want to plot the power spectrum using pwelch. It can be achieved by mentioning spectrumtype input of "pwelch” function to ‘power’.
pwelch(LFP, [], [], [], sampling_rate, 'power');
Default units of power spectrum is dB. You can save the spectrum to a variable and then change the units.
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