Propagation time measuring method for determining a distance

An apparatus for ascertaining and monitoring the fill level of a fill substance in a container, comprising:

• a transmitting/receiving unit, which emits high-frequency, transmission signals directed at a surface of the fill substance, and which receives reflected signals,
• a delay circuit, which transforms the high-frequency, reflected signals and the high-frequency, transmission signals into an intermediate frequency signal, said delay circuit comprises at least a sampling clocking oscillator, which produces a sampling signal having a sampling frequency, a transmission clocking oscillator, which produces a pulse repetition signal having a pulse repetition frequency, and a frequency converter, which, by means of sequential sampling, produces from the pulse repetition signal and the sampling signal a difference signal having a frequency difference;
• and
• a control/evaluation unit, which, on the basis of travel time of the emitted, high-frequency, transmission signals and the received, high-frequency, reflected signals, determines the fill level of the fill substance in the container, wherein:
• provided on said control/evaluation unit is an external clocking signal input, which is connected via a clocking line with a first clocking signal output of the sampling clocking oscillators or with a second clocking signal output of said transmission clocking oscillator, so that the clocking of said control/evaluation unit is done with the sampling signal at the sampling frequency or with the pulse repetition signal with the pulse repetition frequency.