R help - Aug 2007 - time series periodic data

Dear all

Please help me with analysis of some periodic data. 

I have an output from measurement each minute and this output is modulated 
by rotation of the equipment (approx 6.5 min/revolution). I can easily 
spot this frequency from

spectrum(mydata, some suitable span)

However from other analysis I suspect there is a longer term oscilation 
(about 70-80 min) I am not able to find it from mentioned data.

Plese give me some hint how I could prove that such long term modulation 
of my data exist in presence of quite strong modulation by this short term 
oscilations.

Thank you

Best regards

Petr Pikal
petr.pikal at precheza.cz

On 16/08/2007, at 12:26 AM, Petr PIKAL wrote:
> Dear all
>
> Please help me with analysis of some periodic data.
>
> I have an output from measurement each minute and this output is  
> modulated
> by rotation of the equipment (approx 6.5 min/revolution). I can easily
> spot this frequency from
>
> spectrum(mydata, some suitable span)
>
> However from other analysis I suspect there is a longer term  
> oscilation
> (about 70-80 min) I am not able to find it from mentioned data.
>
> Plese give me some hint how I could prove that such long term  
> modulation
> of my data exist in presence of quite strong modulation by this  
> short term
> oscilations.
	You may be able to get some mileage out of complex demodulation
	(at, say, frequencies of 1/70, 1/71, ..., 1/80 cycles per minute).

	See Peter Bloomfield's book (``Fourier Analysis of Time Series:  An  
Introduction'',
	2nd ed., Wiley, 2000) for a good introduction to complex demodulation.

			cheers,

				Rolf Turner

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