An Improved Colpitts VCO With Low Phase Noise Using a GaAs BiHEMT Process

In this letter, an improved balanced Colpitts voltage-controlled oscillator (VCO) with a collector-emitter cross-coupled capacitors is presented. The proposed Colpitts structure can introduce a more robust oscillation startup condition. The noise-shifting technique is used to enhance the loaded quality factor of the tank, which improves the phase noise in turn. In addition, three-bit switches composed of enhancement-mode high-electron mobility transistors (E-HEMTs) are utilized to expand the tuning range and reconfigurability of the proposed VCO. In proof of the concept, a VCO prototype is fabricated in a GaAs BiHEMT [enhancement- and depletion-mode high-electron mobility transistor (E/D-PHEMT)] process. The measured results demonstrate the best phase noise of -139.46 dBc/Hz at 1-MHz offset. To the best of our knowledge, this is the lowest phase noise among the reported ones fabricated on-chip with a similar frequency band, and it achieves a wide tuning frequency bandwidth of 21% (2.65-3.27 GHz) with peak figure of merit (FoM) of -192.26 dB and FOMT of -198.8 dB.