1.2.2 Doppler signal demodulation method The signal received by the transducer in CW Doppler flow measurement can be considered as a superposition of multiple Doppler shift components, expressed as: Where ai is the frequency shift component amplitude, Ωi is the frequency of the frequency shift component, φi is the initial phase of the frequency shift component, and s1(t) is the signal coupled to the receiving probe via a non-moving medium such as a pipe wall or lining. The purpose of Doppler signal demodulation is to extract frequency shift information, and ideal demodulation should be proportional to Output. At present, the commonly used demodulation methods include amplitude detection, frequency discrimination, phase discrimination and multiplier detection. However, since s(t) is nonlinearly modulated by Doppler shift components, amplitude detection and phase detection are two solutions. The modulation mode will produce distortion, and the voltage amplitude of the frequency detection detection output will contain an amount proportional to the frequency shift Ωi, which is also undesirable. The multiplier detection multiplied by the reference signal cosω0t and the received signal s(t), and then the high-frequency component is filtered out by the low-pass filter to obtain an output: The output obtained by this method does not cause distortion, and thus has been widely used in the conventional industrial pipe ultrasonic Doppler flowmeter. 1.2.3 Flow direction direction information extraction The existing industrial pipe ultrasonic Doppler flowmeter can not judge the direction of the flow velocity. In the field of medical blood flow measurement, the flow field direction information is mainly extracted by the time domain method, the frequency domain method and the phase domain method, which are respectively introduced as follows: (1) Time domain method The basic principle is that the received ultrasonic Doppler signal s(t) is simultaneously sent to two multipliers, which are respectively multiplied by two co-frequency signals cosωot and cos(ω0t + β0) having a certain phase difference β0, and then low. Passing the filter removes the high frequency component, so the output of the first path is: The output of the second way is: If the blood flow direction is positive, then Ωi "0, the phase of the first output leads the phase of the second output; if the blood flow direction is negative, then Ωi "0, the phase of the second output leads the output of the first channel Phase. Therefore, comparing the two outputs in time can detect the direction information of the blood flow. Shareconn supplies different kinds of adapters,such as audio&video Adapter,HDMI adapter,RF adapter and so on. We stick to the principle of "quality first, service first, continuous improvement and innovation to satisfy the customers" for the management and "zero defect, zero complaints" as the quality objective. To perfect our service, we provide the products with good quality and reasonable price. Adapter Connectors,Stereo Audio Adapter,Multimode Adapter Connectors ,VGA to DVI Adapter Shareconn Development CO.,LTD , http://www.share-conn.com
