Design of Quadrature VCO for 2.4GHz

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Dr. Nagy Lajos
Department of Broadband Infocommunications and Electromagnetic Theory

This master thesis discusses the design of a Voltage-Controlled Oscillator (VCO) for a ZigBee transceiver working in the 2.4 GHz Band. One of the most important building block of a RF circuit is the frequency synthesizer. The quality of the synthesizer depends mostly on the oscillator. Resonator based LC VCO's have larger quality factor (Q), so they are less sensible to noise, then the RC variants. Therefore VCO's with LC tank are generally superior to phase shifting oscillators. In most of the cases, on the other hand, at least one off-chip inductor is needed, and the chip area is larger. The ring oscillator has poor phase noise performance compared to the LC oscillator, but it is cheaper and easier to fabricate. Serious efforts are taken to improve the phase noise performance of ring VCO's, designers try to analyse newer and newer delay cell structures. A ring VCO is designed and simulated in this thesis for specific design requirements.

Detailed calculations and simulations were made to find the optimal solution for the structure of the ring VCO. By the results, the final VCO consists of 5 inverter based delay cells. Frequency control is achieved by an additional PMOS device. The circuit is designed in a 0.13 micron CMOS technology with 1.2 V supply voltage. According to the simulations, the VCO operates from 4.68 GHz to 5.26 GHz with control voltages of 0 V to 1 V. The tuning range is about 11%. The peak VCO gain is -750 MHz/V, and the circuit power consumption is between 3.5 mW and 8.6 mW. Temperature and supply sensitivity were determined by the aid of simulation.


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