Research On The Method Of Transmitted And Echo Signal Processing Of Guided Wave Radar Level Meter

Guided wave radar level meter(GWRLM) is a high performance instrument based on Time-Domain Reflectometer(TDR), widely used in chemical industry. But in Accuracy,resolution and other aspects, domestic products is far behind foreign products. So it’s necessary to develop high performance and low cost GWRLM.Firstly, the theis introduced the transmission line theory and reflection coefficient.Then the system measurement principle was discussed, and the design framework was presented.Secondly, design of pulse transmitter circuit was discussed. The avalanche transistor BFP420 was used to generate the high-speed pulse signal after comparing other methods of that. And the avalanche process and pulse generating circuit were presented. Then Advanced Design System(ADS) simulation was used to demonstrate the influence of circuit components on the amplitude and width of output pulse, and why the input signal should pass through RC differential circuit to trigger the avalanche transistor.Thirdly, design of echo signal processing module was discussed. Theis proposed the similar equivalent-time sampling(SE-TS) based on equivalent-sampling principle(E-TS),with reducing system design difficulty and cost. Using FPGA to control AD9500 was selected to realize 100 ps step delay, and the delay signal was transformed to sampling pulse sequences to open the four diodes sampling gate after passing through relevant circuits. Then the feasibility of SE-TS circuit by ADS simulation was proved, and the sampled signal has been passed through fast comparator AD9698 and monostable multivibrator SN74LS123 to get the square wave signal easily detected by microprocessor.Finally, Integrity Signal(SI) problems causing signal distortion on PCB were analysed and the corresponding improvement methods were proposed; Then the measurement of important signals on PCB and explanations were presented.Simulation and measurement results show that pulse transmitter circuit could generate pulse with 750 ps falling edge and 1V amplitude; 100 ps step sampling pulse sequences maked by AD9500 successfully implemented that echo signal extend 4000 times on time axis with retain the amplitude information after SE-TS, realizing accurate time difference measurement of high-speed pulse signal.