Which Doppler analysis technique displays velocity information as a plot of frequency content versus time?

• A. Zero-crossing detection
• B. Autocorrelation
• C. Phase quadrature detection
• D. Spectral analysis

Answer: (D)

The idea being tested is that velocity information from a Doppler signal is carried in the frequency shifts of the received wave, and the way to visualize how those shifts change over time is through spectral analysis. By applying a Fourier transform to short time segments of the signal, you obtain the frequency content at each moment and can stack these spectra over time to create a plot of frequency versus time. This time–frequency visualization, often called a spectrogram, directly shows how the Doppler frequency moves as velocity changes, giving a dynamic picture of motion.

Other methods focus on time-domain or phase-based measures rather than a full time–frequency map. Zero-crossing detection estimates instantaneous frequency from when the waveform crosses zero, but it doesn’t reveal how the spectrum evolves over time. Autocorrelation looks at similarity across delays to infer periodicity or velocity trends, not a time-varying spectrum. Phase quadrature detection uses the relationship between the in-phase and quadrature components to track Doppler frequency, offering velocity information without producing a time–frequency plot. In contrast, spectral analysis inherently provides the frequency content as it evolves over time, making it the best choice for this visualization.