[R] time series periodic data

[Petr PIKAL]

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