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.

Doppler Shift is the change in the perceived frequency relative to the transmitted frequency. The Doppler shift is dependent on the insonating frequency, the velocity of moving blood, and the angle between the sound beam and direction of moving blood.
Doppler equation:
Doppler shift frequency: fD = fr - f0 = 2f0v/c
Where fD is the Doppler shift frequency = the difference between transmitted and received frequencies.
Ultrasound system use the following equation:
Doppler shift frequency with incident angle: fD = 2f0v/c cosØ
Where f is the transmitted frequency, v is the blood velocity, c is the speed of sound in tissue, cosØ is the Cosine of the blood flow to beam angle.
The Doppler angle (theta) is the angle of incidence of the beam upon the object. If the beam is parallel to the flowing blood, the angle theta is zero, and the determination of flow is most accurate. If the angle of incidence is greater, the results are less reliable. Doppler shift results with an angle greater than 20° should not be used for the calculation.

See also Doppler Interrogation Frequency, Zero Crossing Detector, Doppler Effect, Doppler Ultrasound and Motion Discrimination Detector.

The motion discrimination detector is used for signal processing in color Doppler systems to distinguish between Doppler shifts from moving blood and moving tissue.

See also Zero Crossing Detector.