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Highly linear analog frequency ramps are employed in various measurement techniques. Fractional phase locked loops offer the possibility of generating fast analog frequency ramps with linearity errors in the range of 10Â¿8. For the characterization of frequency ramps with such small linearity errors a heterodyne measurement principle is introduced which is able to measure linearity errors of less than 10Â¿8 thus allowing to check the actual linearity of fractional ramp generators. This heterodyne concept is not only useful for measuring the linearity of frequency ramps but is also applicable to all sorts of heterodyne measurement systems combining the high speed of analog frequency ramps with the absolute precision and stability of a fully synthesized signal generator.