Hi How to find out the amplitude and duration of a sine wave of different amplitudes?

Consider a voltage sag ie, a 50hz signal of amplitude less than the normal one.I have to find out the amplitude and duration at wich this disturbance occurs.

I have read some articles which gives some information that we can extract time and frequency information using wavelets db4 using multi resolution analysis .As i am a beginner i have just a rough idea about the work.Can anyone please direct me to accomplish my works and obtain output in matlab?

Thanks

 

You can do a least-squares fit to a sine wave to determine the amplitude, frequency and phase.