A method of determining a filling level of a product contained in a tank using a radar level gauge system, said method comprising the steps of: generating a transmission signal (S T) using first pulse generating circuitry outputting a first signal (S 1) having a first oscillation frequency, said transmission signal being in the form of a first pulse train having a first pulse repetition frequency; generating, using second pulse generating circuitry comprising a resonator element having an input and an output, a reference signal (S REF) in the form of a second pulse train having a second pulse repetition frequency, said second pulse repetition frequency differing from said first pulse repetition frequency by a predetermined frequency difference; propagating said transmission signal (S T) towards a surface of said product contained in the tank; receiving a reflected signal (S R) resulting from reflection of said transmission signal at said surface of said product; forming a measurement signal comprising a sequence of values, each value representing a time correlation between a pulse of the reference signal and the reflected signal; and determining said filling level based on said measurement signal, characterized in that said step of generating said reference signal (S REF) comprises the steps of: providing said first signal (S 1) to said input of the resonator element comprised in said second pulse generating circuitry; monitoring an output signal from said resonator element, said output signal substantially corresponding to said first signal being delayed by a time delay; controlling at least one parameter of said second pulse generating circuitry to a parameter value resulting in said time delay corresponding to said predetermined frequency difference; and switching said radar level gauge system to disconnect said first signal (S 1) from said input of the resonator element and instead provide said output signal from said resonator element to said input of the resonator element while maintaining said parameter value of said at least one parameter, thereby generating said reference signal (S REF). generating a transmission signal (S T) using first pulse generating circuitry outputting a first signal (S 1) having a first oscillation frequency, said transmission signal being in the form of a first pulse train having a first pulse repetition frequency; generating, using second pulse generating circuitry comprising a resonator element having an input and an output, a reference signal (S REF) in the form of a second pulse train having a second pulse repetition frequency, said second pulse repetition frequency differing from said first pulse repetition frequency by a predetermined frequency difference; propagating said transmission signal (S T) towards a surface of said product contained in the tank; receiving a reflected signal (S R) resulting from reflection of said transmission signal at said surface of said product; forming a measurement signal comprising a sequence of values, each value representing a time correlation between a pulse of the reference signal and the reflected signal; and determining said filling level based on said measurement signal, characterized in that said step of generating said reference signal (S REF) comprises the steps of: providing said first signal (S 1) to said input of the resonator element comprised in said second pulse generating circuitry; monitoring an output signal from said resonator element, said output signal substantially corresponding to said first signal being delayed by a time delay; controlling at least one parameter of said second pulse generating circuitry to a parameter value resulting in said time delay corresponding to said predetermined frequency difference; and switching said radar level gauge system to disconnect said first signal (S 1) from said input of the resonator element and instead provide said output signal from said resonator element to said input of the resonator element while maintaining said parameter value of said at least one parameter, thereby generating said reference signal (S REF). providing said first signal (S 1) to said input of the resonator element comprised in said second pulse generating circuitry; monitoring an output signal from said resonator element, said output signal substantially corresponding to said first signal being delayed by a time delay; controlling at least one parameter of said second pulse generating circuitry to a parameter value resulting in said time delay corresponding to said predetermined frequency difference; and switching said radar level gauge system to disconnect said first signal (S 1) from said input of the resonator element and instead provide said output signal from said resonator element to said input of the resonator element while maintaining said parameter value of said at least one parameter, thereby generating said reference signal (S REF).