Harmonic Suppressed Tunable Bandpass Filter Using Varactor-Tuned Stub-Loaded Resonator

In this paper a harmonic suppressed tunable bandpass filter is proposed. Generally, tuning of filter not only tunes the passband but also shifts the harmonic bands that make the out-of-band suppression complicated. To eliminate this problem H-shaped defected ground structures are incorporated at the input/output feed line without affecting the passband performance. The proposed filter is designed and fabricated on a low-cost FR4 substrate. The measured results show that passband frequency can be tuned from a frequency of 0.92 to 1.2 GHz with more than 13.6 dB harmonic suppression up to 6 GHz. Measured and simulated results show a good agreement.