RF isolators and circulators are passive components essential to modern communication systems. They act as traffic routers for RF signals, moving them to any place in the circuit. They are manufactured using anisotropic ferrite materials, often biased by a permanent magnet, making them non-reciprocal devices.
Circulators are built using the anisotropic magnetic material called ferrite, which has special properties. Anisotropic materials exhibit different electrical characteristics that depend on the direction an electrical signal moves. To build a circulator, a designer needs ferrite and a permanent magnet to generate a rotating magnetic field — the RF signal is driven by the flow of the field. Once the signal is inside the circulator, it only follows the direction of the rotating field, much like a boat floating down a river.
A major problem with circulators is the fact that, because of the need for a magnet for their operation, they are too big to fit in today's very small electronic devices, like wearables, or in systems like the coming 5G networks, self-driving cars or virtual reality systems. The ideal circulator would be contained in a semiconductor chip, but this development has not been successful until now.
A team of Columbia University researchers has demonstrated the very first RF circulator on a silicon chip. At the IEEE International Solid-State Circuits Conference, the team, led by Harish Krishnaswamy, associate professor of electrical engineering, in collaboration with Professor Andrea Alu's group from UT-Austin, showed the development of the first magnet-free non-reciprocal circulator on a chip that operates at millimeter-wave frequencies (frequencies near and above 30 gigahertz).
To develop the circulators, the team used synchronized high-speed transistors that route forward and reverse signals differently, to achieve non-reciprocity. Most common devices are reciprocal: forward and reverse signals on a circuit travel in the same manner. For instance, current on a cable changes direction if the polarity of the voltage source changes, but in both directions, the value of the current is the same. Other devices, such as circulators of RF amplifiers, are called non-reciprocal because the systems allow forward and reverse signals to use different paths, so they are separated.
The main achievement of this research is that it creates circulators operating at millimeter frequencies built into semiconductor chips. This allows two-way (duplex) wireless communication in which a transceiver's transmitter and a receiver operate simultaneously on the same channel and the same frequency. This doubles the data rate within existing bandwidth.
"This gives us a lot more real estate," notes Krishnaswamy. "This mm-wave circulator enables mm-wave wireless full-duplex communications, and this could revolutionize emerging 5G cellular networks, wireless links for virtual reality and automotive radar."
The applications of the Columbia circulator are many. Self-driving cars, for instance, use mm-wave radars that must be duplex. Virtual reality (VR) headsets need mm-wave full duplex links, but current headsets rely on a bulky wired connection. “For a smooth wireless VR experience, a huge amount of data has to be sent back and forth between the computer and the headset requiring low-latency bi-directional communication,” says Krishnaswamy. “A mm-wave full-duplex transceiver enabled by our CMOS circulator could be a promising solution as it has the potential to deliver high-speed data with low latency, in a small size with low cost.”
The result of the research was published in Nature Communication on October 7, 2017. An abstract can be found on their site.