Spectral Properties of Noise

Hello,

Please help me perform Power Spectral analysis of electrophysiological signal data recorded from an ion channel. I want to take fft of the Signal and plot it with frequency (log-log plot). and then I want linear fitting of the Power spectra. So that I can Calculate Slope or Noise Properties of PDS.

The Sampling frequency is 10 kHz Length of signal/Number of data point is 4096 Here is the code I am using

Fs = 10000; N = length(x); xdft = fft(x); xdft = xdft(1:N/2+1); psdx = (1/(Fs*N)).*abs(xdft).^2; psdx = 2*psdx; freq = (0:Fs/length(x):Fs/2)'; psdx = 10*log10(psdx); freq = (log10(freq)); plot(freq,psdx); grid on; title('Periodogram Using FFT'); xlabel('Frequency (10 kHz)'); ylabel('Power (dB/Hz)');

p = polyfit(freq,psdx,1) f = polyval(p,freq); hold on plot(freq,f,'r')

The Problem is that I am getting completly wrong Slope of my power spectra (I am comparing my results with the results of originlab software analysis.).

Please let me know what is wrong in my code.