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Design of Low Phase Noise Cross-coupled VCO in 90nm CMOS Technology
Keywords: Cross-coupled VCO, Phase noise, Power Consumption
Voltage Controlled Oscillator (VCO) is widely used in modern wireless communication system. Oscillators can be divided into two categories, ring oscillators and LC oscillators. Ring Oscillators have the advantage of easy tuning and easy implementation. However, the ring oscillator exhibits poor phase noise. Although LC Oscillators are difficult to implement because of integration challenges with passive components such as capacitors and inductors, they are still widely used in current communication systems for their lower phase noise. In this presentation we will discuss about a 2.5GHz low phase noise cross-coupled LC VCO realized in 90nm CMOS technology. The design and circuit simulations of VCOs were carried out with high Q and low Q inductors. Results indicate that the phase noise of the VCO with low Q inductor is -98.2dBc/Hz at 100 KHz offset frequency and -122.7dBc/Hz at 1MHz offset frequency respectively when the frequency of oscillation is 2.5GHz. Whereas, the phase noise of the VCO with high Q inductor is -97.7dBc/Hz at 100KHz offset frequency and -122.0dBc/Hz at 1MHz offset frequency respectively at same 2.5 GHz frequency of oscillation. Both designs show same 13.2% tuning range. As for the power consumption of the VCO we find that it is 25mW with the low Q inductor and 8.52mW with the high Q inductor. Therefore, the higher Q inductor leads to a 66% reduction of power consumption of the VCO when all the other parameters are kept the same. Simulation also indicates good linear behavior for both designs. In summary, comparing results one can conclude that the value of Q has significant effect on the performance of the VCO and higher value of inductor is preferred for VCOs performance improvement.
Lu Wang, Research Associate
Syracuse University
Syracuse, NY

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