Conventional circulators are based on the Faraday effect, whereby some magnetically biased materials respond differently to waves propagating in opposite directions. Application of this phenomenon in circulators requires a strong magnetic bias and suitable magnetic materials, presenting physical and cost limitations to existing circulator technology. Using time modulation, Silicon Audio has created a smaller, more efficient, magnetic-free radio-wave circulator that may be integrated in radars, cellphones, and other wireless devices.
Cockrell School of Engineering, University of Texas at Austin
Graphically depicted here, Silicon Audio relies on a suitable form of time modulation in a resonant circuit that creates an effective angular momentum bias to the system, effectively replacing the need for magnetic bias and magnetic materials. By eliminating the magnet, the new radio-frequency circulator has a much smaller footprint, lower weight, and larger isolation levels. In addition, it can be fully realized with components already available on an integrated circuit board. The functionality is fully passive, meaning that the signal network does not draw power from the modulation network, thus ensuring applicability in low-power systems.
Silicon Audio’s first prototypes of radio-frequency on-chip circulators have been realized based on time-modulated varactors connected in a lumped circuit design to form a ring resonator. This ring resonator effectively rotates in time as the signals enter and exit the device. While the footprint in these prototypes is about two orders of magnitude smaller than the wavelength of operation, work is underway to translate these concepts into integrated circuits with significantly smaller footprints. The concept lends itself to other realizations using MEMS, mechanical or optical modulation for different applications, including high-power and low-noise needs.
As depicted in the Isolation v. Frequency chart, a unique feature of the on-chip circulator is its tunability in real-time over a broad range of frequencies, a major advantage over conventional magnetic circulators. This may be ideal for communication systems in which the need for instantaneous bandwidth is not necessarily very large, yet it is necessary to tune the operation in real-time as a function of the assigned communication channel. Additionally, the bandwidth of operation can be tuned by design to make it suitable for different applications.
Silicon Audio’s circuit design expertise enables the realization of on-chip circulators meeting the specific metrics for various applications of interest, such as telecommunications, RFID, wi-fi, and radar systems.
As development of the on-chip circulator technology advances, Silicon Audio is applying its MEMS expertise to fabrication of the circulator for targeted applications where size and linearity are of particular concern.
The functionality of Silicon Audio’s circulators is inspired by our research on metamaterials and specifically the magnetic bias on a ferromagnetic molecule, which is mimicked by time modulation in a resonant circuit in the on-chip circulator.