A radar level gauge system controllable between a measurement state and a signal processing state. In the measurement state a first timing signal circuit is enabled, a microwave signal source generates a transmit signal with a time-varying frequency being related to first timing signals from the first timing signal generating circuitry, and a sampler samples a mixer signal at sampling times related to the first timing signals. in the signal processing state, the first timing signal circuit is disabled, and a signal processor determines the filling level based on the sampled values of the mixer signal using second timing signals from a second timing signal generating circuit.