Patent ID: 6087978
Filing Date: 2000-07-11
Classification: G01F,G01S

Abstract:
A level measuring device operating with microwaves comprising:a transmission pulse generator which generates microwave transmission pulses at a transmission repetition rate, the transmission pulses being transmitted during operation by means of an antenna (7) in the direction of a filling material whose level is to be measured, the transmission pulses are reflected at a surface of the filling material causing echo signals which are received by the antenna,a reference pulse generator which generates microwave reference pulses similar to the transmission pulses generated by the transmission pulse generator, the reference pulses being generated at a reference repetition rate which is different from the transmission repetition rate by a small frequency difference,a first mixer (17) having a first input and a second input,the first input of the first mixer receiving the echo signals,the second input of the first mixer receiving the reference pulses, andthe first mixer generating an intermediate frequency signal that, with respect to the echo signals, is slowed down by a time dilation factor which is equal to a quotient of the transmission repetition rate and the frequency difference,a first subcircuit (27) for sampling the intermediate frequency signal,a time interval between two successive samplings being equal to a time unit,means for generating a differential signal which changes with the frequency difference between the reference repetition rate and the transmission repetition rate,a second subcircuit (47) for determining a period duration of the differential signal on the basis of the time unit, andan evaluation unit (41)which determines a time dilation factor in the time unit for the period duration, anddetermines an actual time interval between two points of the echo signal that correspond to the two sampling points, from the time dilation factor in the time unit (.tau.), and a time interval present in the time unit between two sampling points of the intermediate frequency signal.