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A circulator is a passive non-reciprocal three- or four-port device, in which a microwave or radio frequency signal entering any port is transmitted to the next port in rotation (only). A ''port'' in this context is a point where an external waveguide or transmission line (such as a microstrip line or a coaxial cable), connects to the device. For a three-port circulator, a signal applied to port 1 only comes out of port 2; a signal applied to port 2 only comes out of port 3; a signal applied to port 3 only comes out of port 1, so to within a phase-factor, the scattering matrix for an ideal three-port circulator is : ==Types== Depending on the materials involved, circulators fall into two main categories: ferrite circulators and nonferrite circulators. Ferrite circulators refer to the radio frequency circulators which are composed of magnetised ferrite materials. They are the classical circulators commonly used even nowadays. They can fall into two main subcategories: 4-port waveguide circulators based on Faraday rotation of waves propagating in a magnetised material,〔 in which the four-port Faraday rotation circulator is proposed.〕 and 3-port "Y-junction" circulators based on cancellation of waves propagating over two different paths near a magnetised material. Waveguide circulators may be of either type, while more compact devices based on striplines are of the 3-port type. Sometimes two or more Y-junctions are combined in a single component to give four or more ports, but these differ in behaviour from a true 4-port circulator. A permanent magnet produces the magnetic flux through the waveguide. Ferrimagnetic garnet crystal is used in optical circulators. Though ferrite circulators can provide good 'forward' signal circulation while suppressing greatly the 'reverse' circulation, their major shortcomings, especially at the frequencies for civilian use, are the bulky sizes and the narrow bandwidths. Despite the popularity of the modern integrated circuit (IC) technology, the circulator is one of the few devices that can still not be integrated on chip. Thus, much effort has been made by researchers to develop nonferrite or solid-state circulators, which can realize the circulation functionality without using ferrite and be compatible with IC technology. Early work on nonferrite circulators includes active circulators using transistors that are non-reciprocal in nature. In contrast to ferrite circulators which are considered as passive devices, active circulators require a supply of DC power. Major issues associated with the transistor-based active circulator are the power limitation and the signal-to-noise degradation, which are critical when it is used as a duplexer for sustaining the strong transmit power and clean reception of the signal from the antenna. Recent work tends to employ varactors to resolve these issues. The representative work includes the independent researches at UCLA and at University of Texas at Austin. The former involves a structure similar to a time-varying transmission line with the effective nonreciprocity triggered by a one-direction propagating carrier pump. It is like an AC-powered active circulator which they claim can achieve positive gain and low noise for receiving path and broadband nonreciprocity. In contrast, the latter is based on resonance with nonreciprocity triggered by angular-momentum biasing, which more mimics the way that signals passively circulate in a ferrite circulator. Nevertheless, both work remains at early stages where IC versions, though possible, have not yet been developed. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「circulator」の詳細全文を読む スポンサード リンク
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