Driver assistance systems are increasingly employed in motor vehicles, in which a radar sensor is used for the purpose of locating objects in the surroundings of the vehicle. Depending on the functional principle and intended usage of the radar sensor, it may be necessary to analyze not only the amplitude, but rather also the phase of the received radar signals. For example, a precise knowledge of the phase of the received signal is necessary for a precise distance measurement in the immediate surroundings with the aid of a CW (continuous wave) radar. In the case of an angular resolving radar sensor, for example, an FMCW (frequency modulated continuous wave) radar, having multiple transmitting and receiving channels, an analysis of the phase differences between the various channels allows a more precise angle determination. Using a radar sensor of the above-mentioned type, the phase may be determined from the ratio of the I and Q signals delivered by the two mixers of the receiving part.
For example, this sensor may be a heterodyne sensor, in which one oscillator is used to generate the transmission signal and the other oscillator is used to generate the comparison signals. If an oscillator having a fixed frequency is used to generate the comparison signals, a simple and precise generation of the 90° phase shift may be achieved. However, a disadvantage in the case of this sensor type is that the phase noise of the two oscillators is not correlated, so that relatively complex oscillators having a low phase noise are required.
In contrast thereto, in a homodyne sensor concept, the transmission signal and the two comparison signals are generated by the same oscillator, so that the transmission and comparison signals have a correlated phase noise, which makes it easier to suppress the effects caused by the phase noise during the analysis of the received signals. However, the disadvantage exists here that whether and to what extent the phase shift generated by the phase shifter deviates from the ideal value of 90° may be monitored only with great effort or not at all. Such deviations result in phase errors in the received signal, which may be ascertained and corrected only with great effort or not at all.