Remembering that this is not the actual waveform, but a frequency vs time plot, a delay $\tau_0$ in the waveform also introduces a difference in frequency $f_b$, known as the beat frequency or Intermediate Frequency (IF). \text{Change in frequency} &= \frac{\text{Change in frequency}}{\text{Time}}\times \text{Time} \\ This beat frequency $f_b$ arises from a cross term during the mixing process between Tx and Rx chirps because $\mu \tau_0$ is the change in frequency per unit time multiplied with time, and hence similar to Doppler frequency.