A Fractional Ramp Generator with Improved Linearity and Phase-Noise Performance for the Use in Heterodyne Measurement Systems

Concepts for the generation and the measurement of highly linear frequency ramps are presented. The fractional ramp synthesiser concept shown here is able to generate frequency ramps with a very low phase noise level, a very good frequency linearity and reproducibility. Related to the bandwidth of the generated frequency ramps of 4.5 GHz a relative linearity error below 4·10–10 is demonstrated in a prototype system. This linearity error is close to the limit set by the phase noise floor of the ramp generators and marks a significant improvement over existing aproaches (Musch and Schiek, 2000). The basic measurement sensitivity due to the phase noise is 1.8·10–10 without averaging. As the phase noise is important for the linearity of the frequency ramp the set-up has to be optimised for a good phase noise behaviour, too. In order to achieve this good phase noise a special phase-frequency detector is introduced that is especially designed for the use in a fractional phase locked loop.