Let Fs = sampling frequency of data, xldata.xls be the file with time on first axis and data on second axis.

the following code should work

X = xlsread('xldata.xls');

x = X(:,2);

nfft = 2^nextpow2(length(x));

Pxx = abs(fft(x,nfft)).^2/length(x)/Fs;

% Create a single-sided spectrum

Hpsd = dspdata.psd(Pxx(1:length(Pxx)/2),'Fs',Fs);

plot(Hpsd);

Please, look at this: http://www.mathworks.com/help/signal/ref/dspdata.psd.html

Concerning the first part of the question: it may be better to convert the excel file to the "csv" (comma separated value) file format (this can be done in excel), and then to read it with the matlab command "csvread".

For reading the data from Excel file into Matlab it's better to use "xlsread" function.

Let Fs = sampling frequency of data, xldata.xls be the file with time on first axis and data on second axis.

the following code should work

X = xlsread('xldata.xls');

x = X(:,2);

nfft = 2^nextpow2(length(x));

Pxx = abs(fft(x,nfft)).^2/length(x)/Fs;

% Create a single-sided spectrum

Hpsd = dspdata.psd(Pxx(1:length(Pxx)/2),'Fs',Fs);

plot(Hpsd);

The data(voltage and time) values are in an excel file. From that I wanted to plot PSD, Y-axis db/dbm. X-axis frequency.......how can I do in this without MATLAB.

I want to plot a Power Spectral Density graph for my signal..