High-performance surface transverse wave based voltage controlled feedback oscillators in the 2.0 to 2.5 GHz range

This paper presents the current state-of-the-art in the development of microwave voltage controlled oscillators (VCO) in the 2.0 to 2.5 GHz range, using surface transverse wave (STW) based two-port resonators. Three different low-power feedback VCO concepts for phase-locked-loop (PLL) applications were designed,:built, tested and their phase noise performance was evaluated. One concept uses, as an active element in the loop, three cascaded silicon bipolar modular amplifiers and the other one, a highly efficient GaAs 2.5 GHz amplifier circuit. This makes the circuits simple, small and power efficient. However, only a moderate phase noise suppression of -95 dBc/Hz at 1 KHz offset with the Si bipolar design and a poor -75 dBc/Hz with the GaAs one were achieved. We found that in this high frequency range, in which the active circuit is the dominant source of 1/f noise, the close-to-carrier noise of the STWVCO strongly depends not only on the 1/f noise of the active components in the loop but also on their number. That is why, our third concept was realized with a minimum number of active components only two Si bipolar transistors and two varactors, compared to 6 integrated Si bipolar transistors in the first design and 5 integrated GaAs transistors in the second one. This approach in the third design allowed us not only to improve the VCO phase noise performance by 10 to 30 dB, compared to the other two designs, but also to keep the RF/d.c. efficiency close to 2% as with the GaAs amplifier.