Electrically Tunable Ferrite Bandpass Filter in X-Band With Wide Tunability

Abstract

An electrically tunable bandpass filter is designed using ferrite inductor as a tuning element. The device is made electrically tunable by winding copper coils around ferrite substrate. Ferrite substrate is made of low-temperature co-fire ceramic compatible ferrite tape (ESL 40011). The $50~\Omega $ microstrip line is printed on the ferrite substrate in the form of a rectangular loop. The area of the loop determines the center frequency of the filter. This structure results in a closed-loop magnetic field in the ferrite substrate when bias is applied thereby reducing the losses and improving efficiency. Center frequency of bandpass filter is around 9.6 GHz. Tunability of the filter is around 11%. Bias current is varied from 50 to 350 mA. This corresponds to an effective magnetic field varying from 15 to 80 Oe. Center frequency of the bandpass filter shifted from 9.6 GHz to around 10.8 GHz due to the application of bias field. Due to the reduction in the slope of bias curve (hysteresis curve) of the ferrite material with an increase in the bias field, the tunability of the device saturates at high bias fields. The coils are wound around the ferrite material to provide a dc bias in the azimuthal direction (in the plane of substrate). High degree of tunability has been achieved from very low bias currents by this design. Insertion loss of the filter varied from 2 to 2.6 dB which is primarily due to the losses in the ferrite material. This filter can be used for high-power applications due to the ferrite material used for fabrication. High-power measurements show stability up to 2 W of input power. Beyond 2 W, this filter can be used for pulsed power applications. Low-field losses in the ferrite material can be avoided by choosing an operating frequency above $4\pi \text{M}_{\mathbf {s}}$ which is about 9 GHz for this material.