Abstract : This paper introduces the main structure of the AD6620 digital downconverter chip, clarifies the basic principle and method of the quadrature demodulation component in spectral Doppler imaging, discusses in detail the design method of implementing this function using AD6620, completes the hardware design and software programming of quadrature demodulation based on this method, and successfully debugs it. Keywords: Spectral Doppler imaging; Quadrature demodulation; Digital downconverter; AD6620 In recent years, a series of studies have been conducted on digital quadrature demodulation, and many methods have been proposed. Among them, the digital mixing quadrature transform method is consistent with the analog demodulation principle and is a relatively ideal demodulation method. Compared with other methods, it has higher accuracy and smaller error. AD6620 is a digital downconverter signal processor manufactured by Analog Devices (ADI). It uses digital mixing quadrature transform to complete the digital demodulation function and has been well applied in communication, radar and other electronic equipment. This paper successfully applies AD6620 to ultrasonic spectral Doppler imaging and completes its hardware design and software programming. Quadrature Demodulation Component in a Spectral Doppler System An ultrasonic spectral Doppler system consists of three components: quadrature demodulation, range gating, and spectrum analysis. All three are controlled by a single CPU and require communication with it. The quadrature demodulation component is implemented using a digital down-converter, AD6620, and its block diagram is shown in Figure 1. [b][align=center]For more details, please click: Design of a Quadrature Demodulator Based on AD6620[/align][/b]