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A phase-locked-loop (PLL) concept for the generation of two extremely stable RF-signals with a small frequency difference is presented. The system is based on only one PLL and comprises two crystal oscillators. One is fixed frequency acting as the reference while the other one is tuneable being controlled by the PLL-system. A special PLL-concept allows us to achieve a very small frequency difference of only some 100 Hz between the two oscillators while maintaining a short settling time below 2 ms as well as a very good phase-noise performance. These features mainly rely on low division factors in the feedback of the PLL and a high reference frequency at the phase-frequency-detector.