Assume your data looks like the one posted at
http://tinyurl.com/3c8o9t
Fourier Analysis will be used to find the period of the waveform.
Cut the 2 columns to 128 row.
Only lengths of 64, 128, 256 etc are allowed.
Name the 2 columns MyTime and MyVal.
Shown here are only 4 of 128 data rows:

MyTime MyVal FFT MyPower MyPowCu MyFreq
1.00E-11 0.19
-21.7
1.00E-07 0.00 -30.5-6.5i 972.9 972.9
9.922E-08
2.00E-07 -0.19 -38.2-3.1i 1473.3 2446.2
1.984E-07
3.00E-07 -1.24 -42.5+3.5i 1820.6 4266.9 2.977E-07

Half_Area
226338
Period
1.091E-06

Tools Data Analysis Fourier Analysis
Input Range: MyVal
Output Range: FFT
If the above matrix starts at A1, enter at D3, E3 and F3 respectively
=IMABS(C3)^2
=MyPower+E2
=(ROW()-2)*(MAX(MyTime)-MIN(MyTime))/COUNTA(MyTime)
Copy them down and name the ranges as shown in the above table.
Half_Area and Period have these formulas
=INDEX(MyPowCu,COUNTA(MyVal)/2+1,)/2
=LOOKUP(Half_Area,MyPowCu,MyFreq)