Precision level measurement based on time-domain reflection (TDR) measurements

Abstract. A system for a high precision multi target level measurement based on guided microwave pulses is presented. A wide-band technique based on time-domain reflectometry (TDR) in combination with a TEM-waveguide as the probe fulfils the requirements of mm-precision level measurements in tanks. The coaxial waveguide provides very low dispersion for wide-band signals. Inside the coaxial waveguide the different fluids with their specific dielectric constants influence the waveguide’s characteristic impedance, so that reflections take place at each discontinuity and separating layer respectively. A second very important requirement of the system is a high resolution. Thin layers (< 10 mm) should be measured reliably. For that reason the pulse width must be sufficiently small. In this case a pulse width about 100 ps is suitable. It is obvious, that a high bandwidth of the whole system is necessary to provide the precision and the resolution. One further requirement is a nearly jitter free generation of two pulse trains with slightly different pulse repetition rates. These pulse trains are used for sequential sampling. The following analog to digital conversion of the received signal occurs at a relatively slow rate, in order to allow an A/D conversion with a high resolution.