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In this paper, a microwave oscillator using a high-Q spiral-shaped defected ground structure (DGS) resonator is proposed. The DGS resonator can be modeled by using the parallel LRC circuit of the transmission line to reject the RF signal at the resonant frequency. The high Q-factor of the spiral-shaped DGS provides high carrier output power and low phase noise. The output power is obtained as 14.2 dBm at 2.438 GHz with 2.0 V DC supply and 24 mA current consumption. It can be seen that the Q- factor of spiral-shaped DGS is 14, the phase noise is observed about -120 dBc/Hz at the 1 MHz offset and DC to RF conversion ratio is calculated 52.8%. The microwave oscillator using a DGS is fabricated on RT Duroid substrate using ATF13786 GaAs MESFET. A considerable enhanced output power was produced using the spiral-shaped DGS developed here.