Interpolating for Zero-Crossing Detection in MATLAB

Today someone who read my post on sinc interpolation in MATLAB e-mailed me to ask about how to interpolate between data points in MATLAB to do accurate zero-crossing detection. Here, I summarize my response. Detecting zero crossings of (nearly) square waves that are sampled is difficult to do accurately because it pushes the limits of the "band-limited" signal approximation we use when sampling. That being said, if you’d like to use sinc interpolation , you should decide *HOW* close you need to be to the zero crossing. For example, your original signal might have 50 picoseconds between samples, and so without interpolation, you can detect zero crossings accurate within 50 picoseconds (ps). Maybe you need a closer result. How close do you need to be? 10ps? 5ps? 1ps? Less than that? Figure out how close you need to be and then generate the appropriate upsampled time vector: uptime = 0:step_size:final_time; where step_size is your tolerance (e.g., ...