A radar level gauge system for determining a filling level of a product contained in a tank, comprising: a transceiver for generating, transmitting and receiving frequency-modulated electromagnetic signals; a transmitting propagating device electrically connected to the transceiver and arranged to propagate transmitted electromagnetic signals towards a surface of the product contained in the tank; and a receiving propagating device electrically connected to the transceiver and arranged to return echo signals resulting from reflections at impedance transitions encountered by the transmitted electromagnetic signals, including a surface echo signal resulting from reflection at the surface, back to the transceiver. A signal isolation between the transmitting propagating device and the receiving propagating device is at least 30dB, and the transceiver comprises: a signal generator for generating a frequency-modulated signal having a predetermined center frequency; a transmitter branch connecting an output of the signal generator with the transmitting propagating device via a first frequency converter for converting the frequency of the frequency-modulated signal to provide the transmitted electromagnetic signal; a receiver branch connected to the receiving propagating device for receiving the surface echo signal; and a mixer having a first input connected to the signal generator via a second frequency converter for converting the frequency of the frequency-modulated signal and a second input connected to the receiver branch, for forming an intermediate frequency signal indicative of a phase difference between the transmitted electromagnetic signal and the surface echo signal. The radar level gauge system further comprising processing circuitry connected to the transceiver for determining the filling level based on the intermediate frequency signal.