Because of its incredible stability under extreme environmental conditions, ferrite material has been a trusted core material for many components. But in particular, they’re used in RF and microwave circulators and isolators for their unique ferromagnetic interactions with electromagnetic energy. The opposing magnetic fields of two ferrite disks coupled with a stripline, or y-junction, can be biased in such a way as to force signal transmission in one direction or halt it. This material property has led to the development of circulators, isolators, and isolator/switches, which are uniquely capable amongst RF/microwave components, as there are no effective semiconductor solutions that improve or replace this legacy technology. However, application and market demands in the military, defense, aerospace, and critical communications arenas require innovation with these vital components to meet modern expectations and increasing cost pressures.
Circulators and isolators--which are simply circulators with a matched load on one of the ports--have offered unique solutions in RF circuitry since their inception, from legacy military components to state-of-the-art satellite communications. Circulators are a cornerstone component in the RF signal chain, for protective isolation, bidirectional communications, and high isolation switching. These devices effectively leverage the interaction between ferrite materials to direct the flow of EM signals in one direction with little reflection and low leakage into the other ports, which is why this category of components are commonly dubbed ferrites. Most circulators are three-port passive components with a low insertion loss (S21, S32, S13) and high return losses (S11, S22, S33) where the return losses are essentially the same as the isolation between all unused ports, a benefit of the non-reciprocal nature of this device.
Conventional circulators tend to be bulky due to the ferrite disks and length of transmission line used for impedance matching. These components are also hand assembled and tuned with internal matching networks or variable capacitors for minimum insertion and maximum isolation. This process is viable when volumes for microwave modules are low and packaging is geared for maximum performance as opposed to automation. Now, with volumes creeping up and RF/microwave circuitry getting smaller, the pressure to keep costs down are increasing leading to the necessity for a complete suite of lower cost, faster to install surface mount components.
Modern radar and communications manufacturers are under pressure to reduce weight and lower costs. Integrated microwave assemblies now offer smaller platforms to accomplish functions that used to necessitate large assembly and subassembly sizes. Phased array radar antennas, particularly actively scanning phased array radar, can require hundreds to thousands of transmit/receive (T/R ) modules to meet mission performance criteria. These T/R modules leverage duplexers, or circulators, to enable the antenna to be shared between the transmit. This is done by switching between the receive path and the transmit path during the transmitted pulse and back to the receive path during the echo pulse. Circulators offer this switching while still isolating transmit and receive paths. A reduction in size and cost of circulator components would also amount to desirable effects in a phased array overall cost and size.
With 5G being the next major phase of mobile telecommunications and massive MIMO already being applied improved coverage, current base stations will potentially be outfitted with a massive amount of antennas. These installations will require optimal efficiency in the size, weight, power, and cost (SWAP-C) microwave and millimeter wave circuitry, especially as phased-array antennas are considered the likely choice for implementing millimeter-wave systems. Circulators are also an enabling technology for full-duplex radios, low probability of intercept (LPI) radar, and many satellite communications. With radar entering the commercial and industrial space, there are more applications that require easily-integrated components. According to Research and Markets, the X-band radar market is estimated to grow at a compound annual growth rate (CAGR) of 4.1% from 2016 to 2021--making X-band radar one of the most rapid upwards trending application in the entire radar market for marine, air surveillance, space, and various defense and military applications.
Conclusion: The market is rocketing towards automated assembly and will drive component manufacturers towards surface mount tape-and-reel packaging of their devices.
Typically, incorporating circulators into a PCB design involves cut-outs prior to installation, to provide a better mechanical stability for the bulky component. During assembly, a circulator is normally screwed down and soldered. This level of hand assembly, and the additional assembly components and stages, comes with added risks and costs. For example, while soldering, the hot iron can induce excessive heat, ultimately damaging the magnetic circuit of the circulator. Rough handling of a circulator or isolator can also affect the device performance.
With hand assembly eliminated from the process, and with high precision automation replacing it, these added costs and failure points could be eliminated. In order to reduce the design considerations and failure rates with less human error, a true surface mount technology (SMT) circulator with comparable performance to prior technologies would be necessary. Hence, Smiths Interconnect's TRAK Microwave brand's efforts and investment in developing a true SMT X-band circulator with adequate isolation, insertion loss, and power handling to replace prior circulator technology.
Though there has been some headway in semiconductor-based active SMT circulators on chip, the performance of these devices has yet to approach what is necessary to meet mission critical performance expectations. TRAK’s ferrite-based true SMT circulator maintains a passive non-reciprocal behavior, as opposed to recent active SMT circulators. These active devices emulate ferrite EM radiation manipulation, but function without ferrites, so require external biasing or external matching network. Thus, these active devices add design restrictions and considerations, and offer limited bandwidths and isolation. With North America dominating the growth in the X-band radar market, X-band SMT circulators have more practical uses than ever with automation, accessibility, and a tune-free/hassle-free design.
More on the benefits of true surface mount packaging of circulators can be found here: The Evolution of Ferrite-based Microwave Circulators from Connectorized to True Surface Mount Devices
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