Generation of precise frequency ramps for FSCW applications with a fractional-N-PLL at 24 GHz

For industrial microwave sensors operating in continuous wave (CW) mode in general an adequate frequency modulation is required. A common modulation scheme for distance sensors is frequency stepping (FS). In this paper we present a fast and highly accurate stepped ramp generator based on an integrated 24-GHz Silicon Germanium (SiGe) oscillator with fractional-N-PLL stabilization and modulation. Fast and accurate frequency steps are a prerequisite for achieving a good sensor performance. The dynamic behavior is verified by a phase and frequency evaluation of the ramp signal. Frequency overshoot typical for large frequency steps is reduced by a compensation technique in the PLL control signal of the ramp generator and is demonstrated by 100-MHz steps at 24.5 GHz.