1. Field of the Invention
This invention relates generally to the design and fabrication of frequency tunable millimeter wave (MMW) filters. More particularly, this invention relates to the design and fabrication processes of the frequency tunable microwave/millimeter wavelength (MMW) filters which utilize metallic magnetic thin films biased near ferromagnetic anti-resonance (FMAR) to achieve wide frequency-tuning range, low insertion loss, high isolation, fast response time and relative high power handling capability.
2. Description of the Prior Art
Conventional techniques of system design for radar transmission and reception are limited by the difficulty that frequency tunable filters are not commonly available. In order to eliminate the receiver images and to increase the amplifier efficiency, it is desirable to incorporate the frequency tunable filters in the radar transmission and reception systems. However, due to the conventional design approaches generally employed by those skilled in designing the microwave and millimeter wavelength (MMW) filters, the range achievable for those filters in frequency tuning is very limited.
In a conventional approach, the MMW filters are typically designed based on varying the capacitive or inductive loading of the resonators. When the design is based on the capacitive loading of the resonator, varactors are commonly used and the range of the frequency tuning is only a few percent of the transmission frequency. On the other hand, when the filter design is based on the inductive loading of the resonator, ferrite insulators are used which are generally in the form of polished spheres of single crystal yitrium iron garnet (YIG). The ferrite spheres are biased by a magnetic field and the transmission frequency is designed at ferromagnetic resonance (FMR). At FMR the insertion loss of the device is relatively high (&gt;1 dB) and the frequency tuning range is normally limited by the spurious transmission due to the coupling of the high order magnetostatic modes. In either case, the range allowable for frequency tuning by implementing these MMW filters in a radar system are quite restrictive. Due to this limitation, higher quality of the transmitted images and greater efficiency of amplification for the radar systems thus become more difficult to achieve.
Due to the use of varactors and ferrite insulators, the conventional filter designs are subject to another limitation that the filters are only capable of being operated in low power applications. Due to the small amount of charge carriers available in the junctions, the varactors fabricated on semiconductor junctions which incorporate depletion layers are limited by low power levels generally below a few watts. Meanwhile, the spin-wave instabilities caused by the excitation of higher order magnetic waves in the ferrite insulators also limits the achievable power level in a frequency tunable filters. Application of the conventional frequency tunable filters to radar transmission is limited due to this intrinsic lower power characteristic.
Furthermore, with the varactors or ferrite insulators, the filters can not be conveniently fabricated and be compatible with the microwave planar technology. Due to this limit, the filters which employ varactors and ferrite insulator cannot take advantage of the mass-production capability of current microwave monolithic integrated circuit (MMIC) technology to produce frequency tuning filters in large quantity at low cost. Broad and economical applications of the filters are thus prevented due to these difficulties.
Therefore, there is still a demand in the art of MMW filter design and fabrication to provide a new technique in designing and fabricating an MMW filter which is able to achieve wide frequency-tuning range, low insertion loss, high isolation, fast response time and relative high power handling capability.