1. An apparatus for interpolating a discrete time series that represents voltages of a waveform, to determine a point at which the waveform crosses a threshold voltage, the apparatus comprising: means for upsampling the discrete time series to create an upsampled discrete time series; means, coupled to the means for upsampling, for determining a first digital word and a second digital word of the upsampled series of digital words that surround the threshold voltage; and means, coupled to the means for determining, for performing linear interpolation between the first digital word and the second digital word to determine a time tag corresponding to the point at which the waveform crosses the threshold voltage; wherein the means for low-pass filtering includes: a low-pass filter that has a plurality of coefficients; means for inserting a digital word with a zero value between each adjacent pair of digital words in the series of digital words, to create a zero-valued series of digital words; and means for low-pass filtering the zero-valued series of digital words with the low-pass filter, to create the upsampled series of digital words.