Linear broadband interference suppression circuit based on GaN monolithic microwave integrated circuits

This paper presents simulation and measurement results of a 2–4 GHz octave bandwidth interference suppression circuit. The circuit accomplishes the function of a tunable frequency notch through an interferometer architecture. The relative delay in the interferometer paths is varied with GaN monolithic microwave integrated circuit tunable delay lines. The delay is adjusted by varying the drain voltage of cold-FET connected high electron mobility transistors acting as varactors. Two types of periodically-loaded delay lines are compared: a uniform and a tapered design. A simple theoretical study, relating the delays and amplitudes in the interferometer circuit branches, is developed to inform the design. Two interference suppression hybrid circuits are implemented, and measurements demonstrate a 25–40 dB notch across the 2.24–4 GHz range for the uniform delay line, and 2.32–4.13 GHz for the tapered design. The return loss for both designs remains below 10 dB. Measurements with two tones spaced 0.5 and 1 GHz for varying tone power are performed to quantify suppression. The circuit can handle an input power of 37 dBm and maintains performance with two simultaneous 25 dBm tones spaced 0.5 GHz apart. Linearity is characterised with 10 MHz two-tone measurements, and the circuit demonstrates a 3rd-order intercept input power larger than 30 dBm for control biases above −12 V.