A compact low phase-noise reconfigurable voltage-controlled oscillator using tunable phase shifter

Abstract

This article presents a reconfigurable voltage-controlled oscillator (VCO) featuring low phase noise. The design incorporates a highly selective tunable bandpass filter employing two identical stub-loaded open-loop resonators. The filter provides a fixed lower frequency band and a tunable higher band, achieving a total tuning bandwidth of 1.4 GHz. This filter, integrated into the amplifier’s feedback loop as a stabilizing element, ensures that the oscillator operates at a specific frequency within the filter’s frequency range. Reconfigurability is achieved by incorporating a tunable phase-shifter, which introduces additional phase shift to the oscillator circuit. By varying the bias voltage of the phase-shifter’s varactor diodes, the oscillator generates discrete oscillation frequencies (2.74 GHz, 3 GHz, 3.22 GHz, 3.68 GHz, 3.82 GHz and 4.22 GHz) corresponding to the tunable filter’s frequency bands without altering the physical structure of the oscillator. To validate, the proposed design is fabricated. The prototype demonstrates robust performance, achieving measured output powers of −3.97 dBm at 2.74 GHz and −4.37 dBm at 3.68 GHz. Furthermore, the measured phase noise remains low at −129.5 dBc/Hz for 2.74 GHz and −126.9 dBc/Hz for 3.68 GHz at a 1 MHz offset, demonstrating its suitability for RF applications.