Microwave sources and filters having broadband characteristics and high spectral purity are generally required for modern satellite and ground communication systems.

Dielectric oscillators, not electronically tunable but characterized by high quality factors Q (about 10000), and varactor oscillators, tunable but having lower Q's (about 1000), have been extensively studied and are currently used depending on the preferred performance (tunability or quality factor).

Planar microwave oscillators and filters based on magnetostatic waves (MSWs) excitation in epitaxial garnet films propose themselves as a natural improvement of the existing devices, because they exhibit reasonable Qext values (oscillators: between 1000 and 3000, resonators: more than 3000) and broadband tunability. The operative frequency limits depend just on the active part (when oscillators are considered) and on the dc magnetic bias for the MSW resonator. Furthermore, the possibility to use a planar geometry favours the integrability in complex structures, overcoming in that way the problems introduced by the well established bulk yttrium iron garnet (YIG) sphere devices.

Since the 1970's, the optimization of the Liquid Phase Epitaxy (LPE) technique for the growth of magnetic garnet films has given the input for the design of MagnetoStatic Wave (MSW) planar devices for microwave applications.

The goal for such a kind of devices was to overcome the intrinsic limitations of already existing technologies based on waveguide systems, bulk materials or surface acoustic waves (SAW's). In fact, MSW technology proposed itself in terms of space reduction, integrability and broadband tunability directly at microwave frequencies.

The first MSW devices were, mainly, tunable delay lines designed to exploit the wideband characteristics of both, materials and devices, including in that research fundamental aspects and applicative purposes. Presently, the growing interest for planar, tunable microwave resonators and oscillators opens a new way for passive and active MSW components working from S-band (2-4 GHz) to Ku-band (12-18 GHz) and more.

Planar MSW resonators are among the MSW devices with potential applications in instruments and communication systems. The general characteristics of resonators obtained by means of different technologies and used at microwave frequencies are shown in the following table:

(*)bulk and planar

A further qualification factor for magnetic film based microwave devices is the low phase noise, down to -110 dBc/Hz @ 20 kHz at 10 GHz for the planar configuration, which is also 20 dBc better with respect to YIG sphere sources.

When compared to the well established YIG spheres technology, the main appealing features for the development of planar MSW based microwave devices can be summarized as it follows:

- a simple resonator realization is needed;

- lower dc magnetic fields are required for the bias at a given resonance frequency, then less power consumption has to be accounted;

- the microstrip transducer widths utilized in fabricating YIG film resonators are of the order of 10 - 100 micron, allowing straightforward photolithographic processing;

- the mechanical alignement in the microwave network is less critical.

MSWs are solutions of the Maxwell equations for a magnetic microwave field h when the displacement current related to the electromagnetic propagation (vanishing wavevector) and the exchange interaction effect (vanishing wavelength) can be considered negligible.

By properly imposing the boundary conditions on the h-components tangential and normal with respect to the magnetic film plane, the solutions for the Maxwell and for the Landau-Lifschitz equations will give a full description of the precession motion of the microwave magnetization m around the direction of a dc bias magnetic field H. h and m are oscillating fields inside the film if real solutions for the excited wavevector k are found, while exponentially decaying fields are originated by imaginary k-values. The above result depends on the orientation of the bias field with respect to the film plane and the direction of k. According to above picture, three types of MSWs can be excited:

(i) MagnetoStatic Forward Volume Waves (MSFVWs);

(ii) Backward Volume (MSBVWs);

(iii) Surface (MSSWs).

The schematic configuration of a magnetically biased yttrium iron garnet (YIG) film is chosen in Fig.1 to illustrate the three bias conditions.

Looking at the resonance properties, the resonating condition for a YIG resonator excited by means of a microstrip transducer can be written as follows:

kn,m l = np n,m = 1,2,3,...

where kn,m are the width mode wavenumbers of the MSW's.

A band-stop and a band-pass resonator configurations are shown in Fig.2 and in Fig.3, respectively.

The most interesting confrontation is between MSW and SAW technologies, because of the proximity of the operative frequencies. In fact, the two techniques have a frequency overlap between some hundreds of MHz and 1 GHz, but MSWs have not competitors when high frequency performances are required.

Furthermore, MSWs offer the possibility for a signal processing directly at microwave frequencies, without specific limitations on the highest possible working frequency for MSW-devices which only depends on the intensity of the bias field (except for the possible increase of the material magnetic linewidth and, consequently, of the device insertion loss vs frequency). The lowest frequency limit depends also on the sample biasing. In this case, the material losses increase because magnetic domains are formed inside the crystal to minimize the internal energy, thus introducing loss mechanisms due to the MSW passage through the domain wall. By accounting for theabove mentioned effects, the 1 - 20 GHz frequency interval can be considered the most reasonable range for MSW applications, even if work is in progress to test the validity of such a kind of technology for higher frequencies.

The MSWs bandwidth is much larger than that of SAW-devices. In fact, for MSWs the bias field guarantee up to 1 GHz ca. of bandwidth, while for the SAWs few tens of MHz is the maximum allowed. Furthermore, an evident improvement of MSWs with respect to SAWs in the loss level is measurable: 10 dB for MSWs vs 30-40 dB for SAWs.

Dielectric filters and oscillators have few appealing characteristics like a very high termal stability with respect to MSW, but they cannot be electronically tunable, and also the mechanical tunability is limited to few tens of MHz. Further to this, the microwave magnetic devices exhibit the lowest phase noise levels among those described above.