TUNABLE MILLIMETER WAVE FILTER

In the past microwave/millimeter wave (MMW) ferrite filters have been fabricated utilizing insulating ferrite materials. We propose for the first time a degisn in which metallic magnetic films are used. Whereas conventional filter designs which include insulating ferrite materials utilize the phenomenon of ferromagnetic resonance (FMR), we propose the use of magnetic metal films in filter designs and utilize the phenomenon of ferromagnetic anti-resonance (FMAR). Preliminary studies indicate that in comparing the characteristics of the two thypes of filters the filter utilizing magnetic metal films are superior in terms of insertion loss and integrability with other planer MMW devices. Our design includes a thin magnetic metal film in the substrate layer of a microstrip line parallel to the ground plane. The characteristic impedance of the microstrip line in the absence of the metal film is 50 ohm. A dc magnetic field is applied normal to the film plane. When the magnetic film is biased away from FMAR the magnetic metal film interferes strongly with wave propagation. The characteristic impedance of the line is much smaller than 50 ohm. When the magnetic metal film is biased at FMAR the skin depth within the magnetic metal film becomes substantially greater than the film thickness. At FMAR the impedance of the line changes back to 50 ohm which matches the input signal feeder line. Therefore, design of band-pass filter can be realized in which the transmission frequency occurs at FMAR frequency with a frequency bandwidth equal to the FMAR linewidth.