Power Spectrum Plotting of a Signal

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?

Answer 1

[Surya Talluri]

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.