| IN silicon-based radio-frequency (RF) integrated circuits, on-chip spiral inductors are widely used due to their low cost and ease of integration. As a necessary element for circuit design, equivalent circuit models of spiral inductors should efficiently represent their electrical performance for circuit simulation, but model inaccuracy, which stems from the complexity of on-chip inductor structures and high-frequency phenomena, presents one of the most challenging problems for RFIC designers. So we focus our discussion on the improvement of spiral inductors'quality factor and the building of precise models.In order to make an accurate inductor model, the loss mechanisms of spiral inductors are analyzed and some main kinds of methods for improving Quality factor of spiral inductor are summarized at the same time. Then a simplified 2-PI model of planar spiral inductor is presented, which include the effects of Skin effect, Proximity effect, line-line parasitic capacitance, capacitive substrate coupling, inductive substrate coupling, and so on. The formula is given for estimating the components in the model, and the estimated value can be used as the initial value for fitting process in the SPICE, which can make the fitting process efficiently. According to the simulation, the simplified modeling methodology shows excellent agreement with the results of the electromagnetic field solver (ASITIC) over a wide frequency range, and a method of synthesis and optimization for spiral inductor is presented.At last, on the basis of the method above and the requirements of frequency synthesizer, a series of spiral inductors are synthesized and optimized. The simulation result demonstrates that the phase-noise of VCO is -121.25dBc/Hz@600KHz on UMC.18μm process, which is 5dB better than the inductors supplied by UMC.
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