A Study On Equivalent Sampling Technologies Of Echo Signal In Microwave Liquid Detection Device

The microwave liquid detecting device is actually a detecting radar device using a carrierfree pulse system,which mainly identifies the safety of the liquid by analyzing the reflected signals of different liquids.Therefore,it is widely used in various security scenarios,such as subway security inspection.In the liquid detection device,the acquisition and analysis of the echo signal plays a crucial role.Thus,it is of great significance to develop a low-cost,highreliability echo signal acquisition module.This thesis mainly studies the signal acquisition circuit of liquid echo signal,which includes the pulse generator that generates sampling pulses and the sampling gate circuit.The main content of this paper consists of the following part:1.The basic principle of microwave liquid detecting device is introduced.The signal characteristics and signal sampling principle of liquid echo signal are analyzed.The advantages and disadvantages of real-time sampling method and equivalent time sampling method are dis cussed.Signal acquisition scheme based on equivalent time sampling method is determined.2.The common signal delay methods are introduced,including cascade delay method ramp wave generation method and programmable delay method.A signal delay circuit based on the triode switching principle is designed.3.Three common narrow pulse generating principles are discussed,including the principle of SRD pulse generation,the principle of avalanche pulse generation and the principle of emitter-coupled logic pulse generation.A narrow pulse generator based on the principle of avalanche pulse generation is designed.4.Four common sampling gate circuit structures are analyzed,including single tube sampling gate,symmetric sampling gate,single balanced sampling gate and double balanced sampling gate,and a sampling gate circuit based on double balanced sampling gate principle is designed.5.The delay sampling pulse circuit and the sampling gate circuit were simulated and optimized in ADS(Advanced Design System),and the PCB(Printed Circuit Board)layout design,physical production and actual circuit test were completed in the end.For the signal acquisition of the liquid echo signal,equivalent sampling method is adopted in this paper.The pulse repetition frequency of the proposed liquid echo signal is 1MHz,the pulse width of the liquid echo signal is around 10~20ns.The step-delay sampling pulse is converted to a pair of pulses of opposite polarity and equal amplitude,which are input to the sampling gate circuit.The number of sampling points of the receiving circuit on the echo pulse signal is around 10~20,between which the distance is equal.From the analysis of the actual test results of the circuit,it can be seen that through the joint test,the step-delay pulse circuit and the sampling gate circuit cooperate very well,the sampling pulse generating circuit can generate a sampling pulse sequence with a delay of 1 ns,a pulse width of 4 ns,and an amplitude of 1.5 V.After applying it to sample the echo signal with a pulse width of 20 ns and a repetition frequency of 1 MHz,the echo signal is stretched 1001 times on the time axis while its waveform is kept constant.The overall circuit basically realizes the collection of echo signals,which is in line with the design expectations.