Spectral Doppler refers to the combination of either continuous wave Doppler or pulsed Doppler with a spectral display. Spectral Doppler provides a quantitative analysis of the velocity and direction of blood flow.
The Fourier spectrum analyzer performs a fast Fourier transformation on the Doppler signal. The amplitudes of the resulting spectra are encoded as brightness. In the 2D spectral display, the frequency shift is depicted in the vertical and the time in the horizontal axis. The range of blood velocities in the volume produces a corresponding range of frequency shifts.

See also Acceleration Index and Triplex Exam.

The autocorrelation is a mathematical procedure used to quantify periodicity in a Doppler signal and forms the basis of most color Doppler velocity estimators. The autocorrelation multiplies waveforms by successively time-shifted sections of itself.

The Doppler interrogation frequency is the frequency of the transmitted acoustic energy relative to the measured Doppler frequency shift. This frequency is most but not necessarily, the nominal transducer frequency.

See also Doppler Effect and Doppler Ultrasound.

The Doppler velocity signal refers to a signal whose voltage is proportional to the Doppler frequency shift, obtained by a frequency-to-voltage conversion of the Doppler signal.

See also Autocorrelation, Temporal Mean Velocity, Doppler Effect, Doppler Ultrasound and Maximum Venous Outflow.

The zero crossing detector displays Doppler frequency shifts. The circuit detects the zero crossing frequency and generates an output which is proportional to the average frequency at the point where the crossing occurred.

See also Motion Discrimination Detector.