Ultralow PM and AM Noise Generation With an Ensemble of Phase-Coherent Oscillators

This article investigates the performance of an array of multiple phase-coherent power-combined oscillators (PPOs) in terms of phase modulation (PM) noise and amplitude modulation (AM) noise. The array consists of six individual oscillator modules that generate three distinct frequencies: 10, 100 MHz, and 1 GHz. By meticulously aligning the phases, we observed a notable improvement of approximately 7.8 dB in the white frequency region for the power-combined signal's AM and PM noise. This closely matches the theoretical value of 10log₁₀(k) dB, where k is the total number of oscillators. The enhancement arises from the fact that when multiple sources are combined, the power of each source adds coherently, while the random noise adds noncoherently. Our experiments resulted in single-sideband (SSB) white phase noise levels of -182, -191, and -168 dBc/Hz for 10, 100 MHz, and 1 GHz, respectively. The corresponding white AM noise levels are approximately -191, -194, and -182 dBc/Hz. Notably, these noise levels represent some of the lowest ever reported at these frequencies. However, the AM noise results for frequencies close to the carrier do not achieve the theoretical 7.8-dB improvement due to PM-to-AM conversion caused by imperfect phase alignment of the individual summed signals. Furthermore, we discuss the use of carrier-suppressed noise measurement and propose a novel, straightforward technique for optimizing phase alignment to minimize PM-to-AM and AM-to-PM conversion in phase-coherent oscillator arrays.