![]() Q: How are isolators and circulators fabricated?Ī: Though it may seem strange, they use ferromagnetic materials. (Note that there are other ways to implement the duplexer function, including using a more-expensive cavity duplexer.) Fig 3: A circulator is used to separate transmitter and receiver signal-energy paths and flow. (The general term for this function is a duplexer.) Ideally, the transmit signal energy output would go only to the antenna port and not be “seen” by the receiver. (Image: MECA Electronics, Inc.)Ī: The most common application is where a transmitter and receiver share an antenna, and the higher-power transit signal must not reach the sensitive receiver input as it would overload and possibly damage it (Figure 3). Fig 2: The isolator and circulator are very similar in structure, except that the isolator has its third port terminated in the characteristic impedance. Note that an isolator is a circulator with its third port terminated (Figure 2). (Image source: MECA Electronics, Inc.)Ī: As the larger “sibling” of the isolator, the circulator is a three-port device used to control the direction of signal flow (RF energy) in a circuit. Fig 1: One major use of the isolator is to prevent “backflow” of RF energy from a load back to the sensitive source, which may be due to impedance mismatch. This is done in the event that some of the energy at the DUT reflects back to the source (Figure 1). ![]() For example, isolators are used in test applications to provide electrical separation between a device under test (DUT) and a sensitive signal source. Note that the RF isolator has absolutely no relationship to galvanic (ohmic) isolation used in non-RF signal and power-supply designs, and it is not galvanically isolated.Ī: It is primarily used to protect other RF components from excessive signal reflection. ![]() It is analogous to a diode which only allows electrical current to flow in one direction, or a check valve in a fluid-flow system. Part 2 will look at couplers and splitters.Ī: An RF isolator is a two-port device which prevents RF energy from coming back to its source. Part 1 of this discussion looks at isolators and circulators, which are closely related devices. But these “strange” devices must be understood in order to develop systems for the ever-growing needs of microwave design. These operating principles, design, and construction are perhaps “alien” to engineers whose design experience is primarily at lower frequencies. Unlike R, L, and C devices and their relatively simple equations which define their basic voltage and current relationships, these microwave devices can only be formally analyzed using advanced mathematics and electromagnetic (EM) theory. This article will look at some passive devices which are used in the microwave world and are as commonplace as resistors, inductors, and capacitors are at lower, non-microwave frequencies. While all parts of the electromagnetic spectrum are ultimately governed by the same basic equations, the reality is that the appearance and disposition of those equations differ widely between regions.ĭespite the well-deserved attention that active devices such as PAs, LNAs, and ICs receive, passive devices are a large part of the microwave story. The world of microwaves is bounded on one side by lower frequencies with which more engineers are familiar, and on the other side by the very different world of terahertz waves and even optics. The objective of a typical microwave design is to guide, route, amplify and attenuate power, and translate frequencies (if needed) to meet project objectives. Instead of voltage, current, and resistance linked by Ohm’s Law, it’s a world of RF power and energy and defined by Maxwell’s equations and the interaction of electric fields with magnetic fields in many cases. Regardless of the definition you use, the microwave world is defined and characterized by parameters which differ significantly from those of lower frequencies. Others feel that the real world of microwaves begins at around 1 GHz. Although there is no single formal definition of which frequencies constitute microwaves, one widely used definition is that it concerns wavelengths ranging from about one meter to one millimeter with corresponding frequencies between 300 MHz and 300 GHz respectively. Despite the apparent simplicity of their functions, their performance is based on electromagnetic theory and Maxwell’s equations for insight into their operating principles and performance.ĭesign for the demands of microwave signals is a very different situation than working at lower frequencies. Basic passive microwave components have a vital role in RF/microwave system design.
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