A VCO with harmonic suppressed and output power improved using defected ground structure

In this paper, a novel defected ground structure (DGS) is proposed for suppressing harmonic and increasing the output power of a voltage-controlled oscillator (VCO) in microwave circuits. The DGS is formed with connecting in parallel two periodic structures which have different center frequencies (in a ratio of 2:3) of the stopband and provides the bandgap characteristic in certain frequency bands. Simulated and experimental results show that the microstrip line with DGS has a wide low-pass band for the fundamental frequency and a stopband for the second harmonic with good performance. To evaluate the effects of DGS on microwave VCOs, two GaAs field-effect transistor (FET) VCOs have been designed and fabricated. One of them has a 50Ω microstrip line with DGS at the output section, while the other has only 50Ω straight line. Measured results show that DGS suppresses the second harmonic more than -20dBm at the output and yields improved output power by 3-5%.