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I measured turbulent velocity in 1Hz sampling rate. To remove fluctuations with T>900 sec I decompose the time series by wavelet packets, problem is that I don't know which levels should be kept and which levels should be removed.

In other words, if the wavelet packet decomposition is imagined as a tree what would be the frequency range at each node?

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In each level, frequency range will be divided by two, I mean, in node $()$ fs is 0-1Khz, in node $(0)$ fs is 0Hz-500Hz(1KHz/2) and in node $(1)$ fs is 500Hz-1000Hz. in node $(0,0)$ fs is 0-250 and in node $(0,1)$ fs is 250-500; in node $(1,0)$ fs is 500-750 and in node $(1,1)$ fs is 750-1000, and so on.

You can follow this rule to reach to proper level with adequate frequency resolution. To know what is the resolution in particular decomposition level, you can use the following equation: $0.5 * fs/2^L$ where $fs$ is frequency sampling which is 1KHz in your case and $L$ is desired decomposition level.

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