Electrically tunable low-pass filter using permalloy films near resonance

A low-pass filter with electrically controllable cut-off frequency using Permalloy films near resonance has been investigated both analytically and experimentally. In a Permalloy film loaded transmission line, the wave propagation constant is strongly affected by the film\´s permeability. For a fixed bias field HM, the attenuation constant α and the phase constant β are large and nonlinear at the resonance frequency f0, but for the frequency band far below is much larger than α and is almost linear with respect to frequency. Hence the impedance Z0, proportional to β/ω is constant for the previous frequency band. In a film loaded transmission line section terminated by Z0, the insertion loss is very small in the passband and large near f0, the cut-off frequency. The cut-off frequency f0can be varied by varying the applied field HM. Attenuation in the stop band is provided by many sections in tandem biased at fields higher than HMor biased by a tapered field. Such a low-pass filter has been built and tested successfully over a frequency range from 500 MHz/s to 2.8 GHz/s. The experimental data agree very well with the analytical result.