Why Is Low Phase Noise Critical For Radar Systems?

An ideal, mathematically calculated carrier signal is free of distortions with respect to time. In the frequency domain, it appears as a clean carrier signal at a specific power level. 

Real-world carrier signals have random fluctuations in amplitude and phase over time, which results in spectrum spreading. Instead of a narrow carrier frequency, unwanted sidebands develop on both sides of the carrier. Phase noise measurements are used to analyze these random frequency instabilities.