Alot of the main browser
’s operations are enabled only when the required number of time series (or
periodograms) are present in the main browser
.
Two (or more) time series can be combined using the Operations=>Combine Series item. Selecting this item creates a Combine Time Series popup window. Simply select the two series you wish to combine in the popup window and hit the Ok button.
Here (above) we see two series which have combined into a third series, this new series is independant of the originiating series and can be manipulated like any other series.
A time series can be detrended by using the Operations=>Detrend Data item in the series’ plot
window
. Selecting this item creates a Detrend Time Series popup window.
This popup allows you to subtract the D.C. bias from data, which if not removed gives rise to significatn power at 0 Hz of a periodogram. Detrending should therefore be carried out before creating a periodogram of your time series.
There are two options: If the data shows no long term trend, it is best to simply subtract the mean and divide by the standard deviation. This gives a dataset witha mean of zero and a standard deviation of one. Otherwise, it is best to subtract a low-order polynomial fit to the data, since if the long term variation is not removed a Fourier Transform will inject significant power at the frequency of the variations.
Here (above) we show a data set that has been detrended using a low order polynomial.
It should be noted that if two nights data show significantly different D.C. levels you may also wish to consider detrending them seperately before combining them using the Operations=>Combine Series item.
There are two main ways to aritmetically manipulate time series, both are available from the main
browser
Operations menu.
The Operations=>Constant Arithmetic item lets you add, subtract, multiply and divide a series by a constant. While the Operations=>sin( ) Arithmetic item lets you add, subtract, multiply and divide the series by a function