High Power Tunable YIG Filters

Abstract

High power tunable microwave filters can be made with ferrimagnetic materials by using coupled spin wave -electromagnetic wave modes of propagation which are decoupled from the processes that limit the power handling capability of conventional YIG filters. These novel filters can be used for spectral containment of transmitter emissions or as feedback elements with microwave amplifiers, so as to create high power tunable oscillators. Measurements have been made at X and S bands on one implementation of this technique, namely a pair of waveguides coupled via a polycrystalline YIG disc. INTRODUCTIONHigh power filters are commonly used for the suppression of harmonics, image frequencies and other spurious outputs from microwave transmitters. The increasing use of frequency diversity in modern radars and of spectrum spreading techniques in secure communications links mean that spectral containment and intermodulation suppression now require the development of high power tunable filters. Such filters can also be operated as feedback elements with microwave amplifiers to create tunable oscillators, using the arrangement shown in fig 1. Such a system will oscillate at a frequency close to the centre of the filter's passband, at which the total phase shift round the loop is 2n Rt. This arrangement is especially useful for endurance tests of broadband microwave amplifiers since it avoids wasting the lifetime of high power variable frequency drivers. TWTs and FETs have already been tested in this way.These applications require tunable transmission filters that can handle high peak and CW powers without developing non-linear behaviour. Their absorption losses also need to be low to avoid the problems associated with the removal of heat created by-the dissipation of microwave energy.A study of such a filter is reported here. FILTER CONCEPTThis filter exploits the known characteristics of microwave propagation in magnetic materials [1] but in a way which decouples the transmission from most of the processes that produce losses and non-linear effects [2].The characteristics of magnetic materials are a consequence of unpaired electron spin dipoles that occur in certain elements in the transition series of the periodic table. A simple representation of a ferromagnetic material is that of dipoles distributed in a regular pattern over a * Royal Signals and Radar Establishment, Great Malvern, UK.405 crystal lattice. The ferrites and garnets are ferrimagnets, with two or more sub-lattices of dipoles, but the simple model can explain filter and non-reciprocal devices (eg isolators), though not non-linear devices (eg limiters).When a magnetic field H is applied to the spin dipoles, these dipoles tend to align themselves in its direction; if displaced, they precess about it with a rotation frequency (the gyromagnetic frequency) defined by f = yH, where the gyromatic ratio y is about 35 kHz (A/m)-l for most materials of microwave interest. A circularly polarised RF magnetic field, normal to the bias field H, couples to the spin dipoles;...