A frequency-modulated continuous-wave (FMCW) radar system for level or distance measurement in which a frequency modulated signal to be transmitted to a target is mixed with an echo signal from the target to produce a beat signal that passes through an analog filter before being digitized and processed in a digital processor to determine the level or distance to be measured, where a test signal is generated by a signal generator, and a switch is controlled to connect the beat signal or the test signal to the analog filter, the signal generator generates the test signal as a broadband signal having a periodic waveform, e.g., a square wave, and the digital processor calculates a spectrum of the digitized filtered test signal by Fast Fourier Transform and generates a fault signal if the spectrum differs from a reference spectrum by a predetermined amount to allow for failure detection.