A back-to-back series varactor configuration minimizing the amplitude-to-phase noise conversion in Si-Ge HBT technology VCOs

Amplitude-to-phase-noise conversion due to varactors for tuning the oscillation frequency can limit the close-in phase noise performance in LC voltage-controlled oscillators (VCOs). Highly nonlinear varactors make the output frequency dependent on oscillation amplitude and consequently amplitude noise translates into phase noise. The conversion coefficient KAM−FM quantifies this effect, and reveals a useful parameter to evaluate a varactors performance. The single-ended varactor configuration is the most used configuration in VCOs, but exhibits a higher conversion coefficient with respect to the back-to-back series configuration, that is demonstrated efficient only in CMOS technology and only with reverse-biased diode varactors. This work compares the two varactor configurations side-by-side, by designing two Differential Hartley VCOs: VCO1 is designed with the single-ended varactor configuration, VCO2 with the back-to-back series varactor configuration. For both VCOs, accumulation-mode MOS varactors are used, and both are designed in 180 nm SiGe technology. It is shown that the back-to-back series varactor configuration provides a 3dB phase noise improvement, and an increase in the achievable tuning range, without consuming significantly more power, confirming that the back-to-back series topology is very efficent in SiGe technology with MOS varactors. This results in a 4dB advantage in terms of Tuning Range based Figure of Merit (FoM). The designed Hartley VCO with the back-to-back series topology, as a low-phase noise (–113.5 dBc/Hz at 100 KHz offset), high tuning range (25%) and low-power (755 uW) configuration, classifies itself as a challenging VCO, and would suggest the opportunity to be considered for further investigations and implementations in SiGe HBT technology.