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.
For doppler and pulse compression radar, which are frequency sensitive and noise limited devices, phase variations can severely limit the maximum range and minimum velocity of detection. Notably, frequency hopping and low-probability of intercept and low-probability of detection (LPI/LPD) radar may be compromised by these effects. Since, some forms of phase noise, such as vibration, may be intermittent or dynamic in the time-domain, this degradation factor may limit the operational reliability of a radar system if not properly diagnosed.
For example, the doppler shift of a 3 GHz S-band airport surveillance radar tracking a target moving at 100 mph in close proximity would need to be sensitive to 100 Hz to avoid roughly 10% range error. Another example, an X-band radar at 10 GHz tracking a jet plane traveling at Mach 1.5 would have to be accurate to roughly 1 kHz to avoid being off on the targets location by tens of miles. Aside from the direct range and velocity error that phase noise can lead to with doppler radar, phase noise can also add to the negative effects of clutter reducing radar accuracy and mask low-level echoes.
So in general, the higher the vibration environment and the slower the target, the more difficult tracking becomes due to masking by the vibration-induced phase noise.
Vibrations can be generated and described based upon the acceleration, velocity, and displacement of the vibration mechanism. These different aspects can induce phase noise either in narrowband or discrete frequencies, over a wide bandwidth, or even vary in the time-domain. Internal sources of vibration-induced phase noise in oscillator circuits could be, the resonator itself-- quartz, crystal, SAW, or BAW resonators--, vibrations effect on the oscillator loop circuitry, the discrete components of filters or diplexers, and air-core inductors.
External to the oscillator circuitry, the transmission line cables and connectors can channel vibration into the device through mechanical mounts, housing, and electrical contacts. Additionally, the mounting substrate can resonate and conduct vibrations into the signal source. Moreover, vibrations can be generated within an assembly by the housing/substrate vibrating under mechanical strain, intermittent contacts responding to, or creating, vibrations, and EM field fluctuations may generate vibrations or create noise by fluctuating in accordance to vibrations.
Vibrations can be reduced or mitigated in a variety of ways, depending upon the type and source of the vibration. Typically, vibrations for microwave assemblies containing oscillators/resonators can be handled at the assembly structural level, or within the oscillator/resonator circuitry itself.
As stiff and light structures tend to generate high natural frequencies, the more rigid and lighter a structure can be could lower the displacement and mitigate the strength of a vibration excitation. Contrary to popular belief, an assembly enclosure does not reduce vibrations to the internal components unless specifically designed for the purpose. Many enclosures even help conduct vibration energy toward internal components. In the case of acoustic vibration energy, certain assembly enclosures that are sealed may reduce this vibration source. However, such a sealed cavity may resonate with local vibration energy and worsen the problem.
Certain damping materials, such as elastic mounting hardware--namely rubber bumpers, spring suspensions, and air tables--will also reduce vibration into an enclosure at the cost of increased size and expense. Other methods for reducing vibrations rely on active damping approaches. One such method requires knowledge of the targeted vibration energy and its sources, which can be limited by tuned vibration absorbers with a higher level of vibration attenuation at specific frequencies. Also, certain actuators can be used to actively cancel vibration energy by producing out-of-phase vibrations matching the vibration energy profile.
Within an oscillator/resonator device, different circuit typologies can be used to reduce vibrations leading to phase noise. Several unmatched resonator components can be placed with opposing orientations to create out of phase signal responses that cancel each other out. Alternatively, and more expensively, matched resonators can be similarly arranged to reduce both the magnitude and direction component of the vibration-induced noise. Accelerometer circuits could also be included that could provide feedback to the oscillator tuning circuit, also cancelling out vibration-induced noise.
Knowledge and expertise around identifying the type and source of vibration-induced phase noise is key in choosing the most efficient vibration mitigation technique. Incorporating the vibration mitigation technique into a microwave assembly and system requires a coordinated team of engineers with extensive experience. TRAK’s experienced engineering team and long history of developing low phase noise radar sources for high vibration environments makes TRAK’s ability to handle this challenge both unique and increasingly critical.