Radio frequency (RF) transceivers can be found in numerous applications, particularly in the field of wireless communications and radar sensors. In the automotive sector, there is an increasing demand for radar sensors used in so-called “adaptive cruise control” (ACC) or “radar cruise control” systems. Such systems may be used to automatically adjust the speed of an automobile so as to maintain a safe distance from other automobiles ahead.
Modern radar systems make use of highly integrated RF circuits, which may incorporate all core functions of an RF frontend of a radar transceiver in one single package (single chip transceiver). Such RF frontends usually include, inter alia, a local RF oscillator (LO), power amplifiers (PA), and low-noise amplifiers (LNA) mixers.
Frequency-modulated continuous-wave (FMCW) radar systems use radar signals whose frequency is modulated by ramping the signal frequency up and down. Such radar signals are often referred to as “chirp signals” or simply as chirps, wherein frequency is ramped up in an up-chirp and ramped down in a down-chirp. For generating such chirp signals the radar transmitter may include a local oscillator, which includes a voltage-controlled oscillator (VCO) connected in a phase-locked loop (PLL). The frequency of the VCO may be controlled by adjusting the frequency division ratio of a frequency divider arranged in the feedback loop of the PLL. To keep the phase noise of the local oscillator output signal low, the band-width of the PLL should be low. However, a low band-width contradicts the goal of generating chirp signals with steep frequency ramps.