Using a threshold to detect frequency of oscillations

Hello Everyone,
I am looking for a way to analyze the frequency of oscillation using Excel.
I have the following data:
1. Data points - 50 data points per second, 30 seconds worth of data, representing approx. 3 oscillations per second (looks like a sine curve; each oscillation will use about 15-20 data points to represent increase to max, decrease to min, and increase to zero)
2. Threshold A (maximum)
3. Threshold B (minimum)
4. Start time
5. Stop time - Important: This is not a constant value, it needs to be calculated (see below)

I need the following data from the curve:
0. Stop time - Defined as the first time after oscillations begin that a maximum or minimum value does not reach the threshold. The first oscillation must reach the max and min or the test is thrown out (there are 0 valid oscillations).

1. Overall frequency - Number of oscillations from start time to stop time - how many times did the data go from zero, rise to a max above threshold A, decrease to a min below threshold B, and rise back to zero during the time from start to stop?

2. Local frequency - Number of oscillations second-by-second (a 30 second test would have 29 values for frequency; frequency for seconds 1-2, 2-3, etc)

3. Max/min value of each oscillation - If I have 500 oscillations, I will get 500 max values and 500 min values, regardless of whether or not it reaches the thresholds

Assume the test will start with an increase to max to start, so the first oscillation will increase, decrease, and increase.

Example:
In the attached picture image025.jpg, there are a total of 4 oscillations. All of them cross the thresholds and the rate is 3 per second. Of course, my data will be more complex than this as the frequency will vary, as well as the max/min values. Sample data that I will actually be using is attached as SampleData.xls. Setting a threshold of 150 max, -140 min should give 5 valid oscillations to calculate frequencies from.

Let me know if there is anything I can do to clear it up for you. Thanks for your help!

Sounds like you need some DSP. There's a good book, The Scientist and Engineer's Guide to Digital Signal Processing, that you can download free (by chapters) at http://www.dspguide.com/