Compact tunable bandpass filter on YIG substrate

Compact planar hairpin bandpass filters on yttrium iron garnet (YIG) substrate with a low insertion loss and large tunable range are designed, fabricated and characterised. The magnetic field tunable permeability of the YIG substrate led to a large tunable frequency range of 26% from 3.72 to 2.74 GHz when the magnetic field was varied from 0 to 800 Oe, corresponding to a large magnetically tunable frequency range of 325% of the −3dB bandwidth. In the meantime, the high permeability of the YIG substrate led to a good impedance match and a low insertion loss between 0.01–1.4 dB for the tunable bandpass filter. The operating frequency of the bandpass filter showed a non-monotonic dependence on the bias magnetic fields, which was caused by a non-linear bias field dependence of the permeability spectra. The bandpass filter showed a high insertion loss of up to 7dB within the bias field range of 200–600 Oe, which was due to the negative permeability of the YIG substrate that prohibited wave propagation.