Monday, April 3, 2017

Mitigating Phase Noise from Vibration in Air Traffic Control and EW Radar Systems

Trak Microwave, April 3, 2017

Radio Detection and Ranging (RADAR or radar) systems used for air traffic control, targeting, military surveillance and electronic warfare (EW) rely on extremely precise time-domain to frequency-domain conversions. This conversion is either performed with high quality dedicated RF/Microwave hardware, or a blend of digital and RF hardware. This is especially true in the case of modern and complex pulse compression and digitally modulated radar. All radar systems are sensitive to phase noise, but the sensitivity to phase noise is a limiting factor in doppler and pulse compression radar.

One view of phase noise is that of a measure of the spectral purity of a signal, and can be produced by internal effects and external effects. Internal effects are generally in the form of impurities or non-idealities in oscillator circuits and resonators. The most common external effect is that of phase noise due to vibration by certain components and circuits which convert mechanical vibration to phase noise. These components and circuits are considered piezoelectric, and ironically they are usually the resonators, oscillators, and filters which most define the source signal’s frequency and spectral purity in a non-vibrational environment. For radar systems operating in very high vibration environments, the vibration-induced phase noise can be orders of magnitude greater than the static phase noise. Therefore, it is vital to understand the impact that the radar system’s operating environment plays in generating phase noise, and how to mitigate its effects.

There really is no such thing as a radar system operating in a static environment. Even benign environments, such as an office building or a radar installation in a secure facility with mild outdoor weather can experience mechanical and acoustic vibrations responsible for phase noise. Moreover, depending upon the severity, type, and frequency of vibrations, different phase noise producing effects can occur, within an assembly, and around its interconnect.

Essentially, any type of disturbance or perturbation that induces frequency or phase fluctuations produces phase noise. It is important to note, that like the various forms of amplitude noise, phase noise can have distinct narrowband, harmonic, or broadband components. From the highly sensitive crystal oscillator components, to even ruggedized external transmission lines, cables, and connectors, vibrations can produce several forms of noise and phase noise simultaneously.