Air Force

Tunable, high isolation RF/microwave circulator

Innovative RF circulator with tunable signal isolation of >70dB

Communications Sensors

Isolation comparison between the traditional circulator (red), and the AFRL invention (blue). Values experimentally determined.

The US Air Force Research Laboratory – Sensors Directorate (AFRL RY) has developed a high isolation RF circulator which provides enhanced detection ranges (radar) and improved reception (communications) by reduction of signal leakage and undesirable internal reflections between ports, at a tuned frequency (or frequency range) of interest. Isolation values exceeding 70dB have been demonstrated which far exceeds the 20dB typical to conventional 3-port circulators.

The AFRL circulator was invented to address shortcomings in monostatic radars, which have traditionally been restricted to pulse mode operation due to false target detection caused by internal leakage, and reflections in conventional circulators. The AFRL invention eliminates these internal artifacts and thus allows monostatic systems to operate in continuous mode, improving long-range detection and overall signal purity. The benefits offered by this system for radar can be extended to other applications that use RF circulators including communications, spectrum management, and e-warfare.

The tunable, high isolation circulator relies on destructive interference between two conventional circulators connected in a parallel configuration to cancel the internal reflections and signal leakage between ports. First, a power splitter is utilized to divide the incoming signal into two identical channels. By matching the load on each circulator, nearly identical internal reflections and leakage are generated in each circulator. Each circulator is further connected to a phase trimmer so that the phase offset between the two circulators may be adjusted to cancel the internal leakage and reflections, which occurs once the two channels are recombined in a power combiner. The resulting signal purification occurs solely in the analog regime, providing increased effectiveness relative to known isolation methods that require digital conversion and subsequent post-processing.

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