RF and Microwave Circuit Design

6Microwave Ferrites

6.1 Introduction

Ferrite materials are important constituents in a number of microwave circuits, primarily because of their potential to provide passive non-reciprocal transmission when magnetized with an external DC magnetic field. Non-reciprocal transmission is fundamental to the performance of two very important passive microwave components, namely isolators and circulators. The traditional method of fabricating ferrite-based microwave devices was to mount the ferrite material in a metallic waveguide, and to use a permanent magnet to provide the required DC magnetic field. Ferrite pucks have also been used in planar microstrip and stripline circuits, again with a permanent magnet to provide the magnetizing field. However, the use of a permanent magnet makes planar circuits bulky and costly. More recently, the need for permanent magnets has been overcome by incorporating magnetizing windings within multilayer packages. This makes the integration of ferrite devices within planar circuits more practical, and is an important aspect of the development of system-in-package (SiP) devices. Also, the development of ferrite materials that can be screen printed or deposited as thin films has made the inclusion of non-reciprocal components within hybrid microwave integrated circuits more feasible. In related work, ferrite LTCC tapes have been developed that permit non-reciprocal components to be included within low-cost, highly integrated multilayer packages. Research ...