This paper introduces a 2.5GHz low phase-noise cross-coupled LC-VCO realized in 0.35μm SiGe BiCMOS technology. The conventional definition of a VCO operating regime is revised from a new perspective. Analysis shows the importance of inductance and bias current selection for oscillator phase noise optimization. Differences between CMOS and BJT VCO design strategy are then analyzed and the conclusions are summarized. In this implementation, bonding wires form the resonator to improve the phase noise performance. The VCO is then integrated with other components to form a PLL frequency synthesizer with a loop bandwidth of 30kHz. Measurement shows a phase noise of - 95dBc/Hz at 100kHz offset and - 116dBc/Hz at 1MHz offset from a 2.5GHz carrier. At a supply voltage of 3V, the VCO core consumes 8mA. To our knowledge,this is the first differential cross-coupled VCO in SiGe BiCMOS technology in China.
An improved charge-averaging charge pump and the corresponding circuit implementation are presented. The charge-averaging charge pump proposed by Koo is analyzed and a new scheme is proposed. This new scheme decreases power by 1/3 and eliminates the practical defects in the original. Spectre Verilog behavioral simulation results show that the proposed scheme can strongly reduce the energy of spurs. Circuit implementation of this new charge pump for a frequency synthesizer with a fractional division ratio of 1/3 is then presented and multi-level simulation is performed to validate its feasibility at the circuit level. The simulation results show this new scheme outputs a flat voltage curve in a locked state and can thus effectively suppress fraction spurs.
An integer-N frequency synthesizer in 0.35μm SiGe BiCMOS is presented. By implementing different building blocks with different types of devices,a high purity frequency synthesizer with excellent spur and phase noise performance has been realized. All the building blocks are implemented with differential topology except for the off-chip loop filter. To further reduce the phase noise,bonding wires are used to form the resonator in the LC-VCO. The frequency synthesizer operates from 2.39 to 2.72GHz with output power of about 0dBm. The measured closed-loop phase noise is - 95dBc/Hz at 100kHz offset and - 116dBc/Hz at 1MHz offset from the carrier. The power level of the reference spur is less than - 72dBc. With a 3V power supply, the whole chip including the output buffers consumes 60mA.
