Hello,

I am ploting a fft periodogram of my data using a script (found in

internet: http://linuxgazette.net/115/andreasen.html ) that gave me

good results before. There should be a periodicity of >160 in the data

as can be seen by eye. However, this script now says that there is

periodicity of ~9. Can somebody explain me what's wrong? Thank you in

advance.

I present first the python script and second my data.

###### usage ############

python <script_name<data_file>

######### PYTHON SCRIPT ###########

from scipy import *

import scipy.io.array_import

from scipy import gplt

from scipy import fftpack

# Read file

f=open(sys.argv[1], 'r')

tempdata = scipy.io.array_import.read_array(f)

minutes=tempdata[:,0]

wolfer=tempdata[:,1]

# plot data

gplt.plot(minutes,wolfer,'title "Meas" with linespoints')

gplt.xtitle('Minutes')

gplt.ytitle('Wolfer number')

gplt.grid("off")

Y=fft(wolfer)

n=len(Y)

power = abs(Y[1:(n/2)])**2

nyquist=1./2

print nyquist

freq=array(range(n/2))/(n/2.0)*nyquist

period=1./freq

# plot period

gplt.plot(period[1:len(period)], power,'title "Simul" with

linespoints')

gplt.xaxis((50,400))

gplt.xtitle('Period [minutes]')

gplt.ytitle('|FFT|**2')

##########################

######## DATA ############

0 48

20 49

40 54

60 49

80 69

100 92

120 98

140 58

160 50

180 66

200 76

220 82

240 119

260 141

280 128

300 107

320 93

340 78

360 74

380 74

400 93

420 109

440 135

460 118

480 90

500 64

520 58

540 56

560 70

580 90

600 115

620 158

640 150

660 96

680 73

700 61

720 55

740 66

760 81

780 116

800 142

820 111

840 102

860 84

880 64

#################