Nonlinear Transmission Line GaN MMIC Frequency Comb Generator

This article presents the design and characterization of a uniform nonlinear transmission line (NLTL) monolithic microwave integrated circuit (MMIC) implemented in a 150-nm GaN process on a 50- $\mu $ m-thick SiC substrate. The implementation in GaN allows for high power handling, and this article presents measured phase noise at fundamental and harmonics generated with a 1-W GaN NLTL. The NLTL unit cell is designed with two series lumped inductors and a diode-connected transistor in reverse bias. After varying the number of unit cells in nonlinear simulations, eight elements are chosen for the final implementation at a fundamental input frequency range from 2 to 3 GHz, with maximized conversion efficiency for harmonics below 8.5 GHz. For input powers between 20 and 30 dBm at 2 GHz, the generated frequency comb is measured and has a minimum conversion loss of 10.7, 17.3, and 24.9 dB and a maximum output power of 17.8, 11.6, and 4.1 dBm at 4, 6, and 8GHz, respectively. Similarly, at 3 GHz, conversion loss of 9.9 and 22.7 dB and output power of 16.0 and 4.3 dBm at 6 and 9 GHz are measured, respectively. In all cases, the input return loss is less than 10 dB. In the time domain, the NLTL produces pulses with widths of 80 and 67 ps when excited with 30 dBm input power at 2 and 3 GHz, respectively. The phase noise measured at the first three harmonics ( ${N} =2$ , 3, 4) increases as 20 log₁₀(N) with very low additive phase noise, where N is the frequency multiplication factor.