Low-loss magnetically tunable bandpass filters with YIG films

A magnetically tunable bandpass filter (BPF) with a yttrium iron garnet (YIG) film deposited on the gadolinium gallium garnet (GGG) substrate have been designed, fabricated, and characterized. Single-crystal YIG films at their ferromagnetic resonance were adopted as the resonator, which was coupled to a T-shaped microstrip coupling structure to form a bandpass filter design. The operating frequency of the BPF can be tuned via a bias magnetic field, and a large tunability of the operation frequency of 190-840 MHz with relatively small bias fields of 50-250 Oe, together with a low insertion loss of < 2 dB. This is equivalent to 12%-54% of the central frequency of the bandpass filter. Double-sided YIG films were used to enhance the filter bandwidth, and a maximum 3-dB bandwidth of 57 MHz was realized with the bias field of 155 Oe. These demonstrated magnetically tunable bandpass filters are very compact and energy efficient, which could have many applications in radio frequency (RF) front and other microwave circuits.