FMCW ramp non-linearity effects and measurement technique for cooperative radar

In FMCW radars, the linearity of the generated ramp has a determining influence on the range resolution and accuracy. This is especially true for cooperative radar systems, where the ramps for modulation and down-conversion are generated separately, strongly increasing the impact of ramp nonlinearities. In this paper, the causes for ramp non-linearities in a PLL-based synthesizer are discussed and modeled as phase deviations from an ideal ramp. A measurement technique to determine these deviations is introduced, where both the hardware setup and the necessary digital processing steps are presented. It is exemplarily applied to an FMCW synthesizer with an integrated 122-GHz transceiver, evaluating the influences of PLL parameters on ramp linearity. Reference measurements show that phase deviations in the low mrad range can be determined with this technique.