A digital signal processing program has been developed to measure the phases of coherent VLF signals from analog tape recordings made in the field. The program uses a constant frequency pilot tone recorded with the VLF data to correct tape speed errors and reconstruct the signal phases. We analyze several examples of whistler mode signals from VLF transmitter at simple Station, Antarctica, as received at Roberval, Quebec. Pulses with temporal growth show a relative phase advance with time, and thus a postive frequency offset from the transmitted signal, often from the beginning of the pulse. Amplitude beating is often seen toward the end of a pulse, sometimes with phase cycle-skipping as the emission becomes unlocked from the input signal. Current theories of wave-particle interaction are reviewed and found to explain some of the observed signal features, though no theory predicts the initial frequency offset of a growing pulse.