Novel miniaturized circulator opens way to doubling wireless capacity

Researchers developed a microelectronic substitute for larger-scale magnetic components and open a pathway to more efficient communications and more capable radar systems

A DARPA-funded team has drastically miniaturized highly specialized electronic components called circulators and for the first time integrated them into standard silicon-based circuitry. The feat could lead to a doubling of radiofrequency (RF) capacity for wireless communications—meaning even faster web-searching and downloads, for example—as well as the development of smaller, less expensive and more readily upgraded antenna arrays for radar, signals intelligence, and other applications.

The defining feature of circulators is that RF signals, in the form of electronic waves in the circuitry, travel only in a forward direction with reverse propagation of the wave forbidden by the physics of the circuit. That’s what you need for minimizing on-chip interference and for keeping signals separated. Most materials can’t play this role because RF traffic can flow both ways through them; these materials exhibit what engineers refer to as reciprocal behavior. Nonreciprocal components like the new circulator, on the other hand, act like one-way highways for RF signals. Traditionally, circulators have relied on external, ferrite-based magnets to force RF signals into a one-way course through downstream circuitry. Those magnets and ferrite materials have rendered the circulators bulky, expensive, and incompatible with the workhorse microcircuit technology, known by insiders as CMOS, which stands for complementary metal-oxide semiconductor. So it has been hard to miniaturize circulators for CMOS integrated circuits.

The Columbia researchers got around this roadblock to miniaturization by coming up with a path-breaking design that does away with the need for bulky ferrites and magnets. Their design achieves the one-way RF flow with a series of capacitors coordinated with a minuscule and precise clock, electronically emulating the direction-dictating magnetic “twist” that in conventional ferrite circulators is imposed on RF signals by an external magnetic field. That novel design makes possible an unprecedented microelectronic assemblage: A receiver connected to one “on-ramp” (or port) of the new circulator structure; a transmitter connected to another port of that same circulator; and an antenna shared by those two tiny devices, itself coupled to the circulator via a third port situated between the other two. Since the RF propagation is one way (non-reciprocal) in the circulator, the transmitted and received signals smoothly traverse their respective paths without getting mixed up with one another.

That clean segregation of received and transmitted signals opens a powerful new capability. In most two-way RF systems, transmission and reception at a given frequency have to be staggered in time with a switching process, slowing communication speeds. The way around this bottleneck has been to transmit and receive at two different frequencies, which requires twice as much spectrum—a limited resource. By contrast, the new pinky-nail-sized circulator opens the door to communications and radar systems operating in full duplex mode—that is, transmitting and receiving at the same frequency at the same time with a single shared antenna.

“This new circulator component could enable full-duplex systems that let you speak and listen all at once,” said William Chappell, director of DARPA’s Microsystems Technology Office. In radar applications, this capability could put an end to brief but potentially deadly blind moments since the system would not have to toggle between separate transmission and reception modes. And by halving the frequency needs, Krishnaswamy said, “full-duplex communication has the potential to double a network’s capacity” for voice, data, and other forms of information. In powerful radar and other RF systems that require large arrays of transmitters and receivers, he continued, “a compact, efficient, high-performance circulator” makes it easier for RF engineers to make their systems smaller. Finally, noted Chappell, the new circulator’s CMOS-compatibility feature is critical because it should ease integration into existing chip-manufacturing methods, potentially making all the difference between a laboratory achievement that stays in the lab and one that transforms a raft of RF technologies.

The work is under the Arrays at Commercial Timescales (ACT) project.

Today’s electromagnetic (EM) systems use antenna arrays to provide unique capabilities, such as multiple beam forming and electronic steering, which are important for a wide variety of applications such as communications, signal intelligence (SIGINT), radar, and electronic warfare. However, wider use of arrays has been limited by lengthy system development times and the inability to upgrade already- fielded capabilities—problems exacerbated by the fact that military electronics have evolved at a slower cadence than in the commercial sector. In particular, the performance gap is widening between the radio frequency (RF) capabilities of fielded military systems and the continuously improving digital electronics surrounding those systems. The Arrays at Commercial Timescales (ACT) aims to shorten design cycles and in-field updates and push past the traditional barriers that lead to 10-year array development cycles, 20- to 30-year static life cycles and costly service-life extension programs.

Specifically, as an alternative to large undertakings focused on traditional monolithic array systems, ACT seeks to develop a digitally-interconnected building block from which larger systems can be formed. The desired building block, composed of a common module and a reconfigurable EM interface, would be scalable and customizable for each application, without requiring a full redesign for each application space.

The ACT program has two thrusts, each focused on a specific enabling technology for rapidly upgradable and widely deployable array architectures:

  • A digitally-influenced common module comprising 80 to 90 percent of an array’s core functionality for insertion into a wide range of applications
  • Reconfigurable and tunable RF apertures for spanning S-band to X-band frequencies (and points between) for a wide variety of characteristics

Nature Communications – Magnetic-free non-reciprocity based on staggered commutation

14 pages of supplemental information

Abstract Magnetic-free non-reciprocity based on staggered commutation

Lorentz reciprocity is a fundamental characteristic of the vast majority of electronic and photonic structures. However, non-reciprocal components such as isolators, circulators and gyrators enable new applications ranging from radio frequencies to optical frequencies, including full-duplex wireless communication and on-chip all-optical information processing. Such components today dominantly rely on the phenomenon of Faraday rotation in magneto-optic materials. However, they are typically bulky, expensive and not suitable for insertion in a conventional integrated circuit. Here we demonstrate magnetic-free linear passive non-reciprocity based on the concept of staggered commutation. Commutation is a form of parametric modulation with very high modulation ratio. We observe that staggered commutation enables time-reversal symmetry breaking within very small dimensions (λ/1,250 × λ/1,250 in our device), resulting in a miniature radio-frequency circulator that exhibits reduced implementation complexity, very low loss, strong non-reciprocity, significantly enhanced linearity and real-time reconfigurability, and is integrated in a conventional complementary metal–oxide–semiconductor integrated circuit for the first time.

SOURCES – DARPA, nature communications,