A Millimeter-Wave Non-Magnetic Passive SOI CMOS Circulator Based on Spatio-Temporal Conductivity Modulation

Linear, time-invariant, passive circuits and systems constructed from conventional materials with symmetric permittivity and permeability tensors are reciprocal. Breaking Lorentz reciprocity enables the implementation of non-reciprocal components, such as gyrators, isolators, and circulators, which find application in numerous wireless communication systems. Non-reciprocal components are traditionally implemented using ferrite materials, which exhibit the Faraday effect under the application of an external magnetic field bias. However, ferrite materials cannot be integrated into CMOS fabrication processes and require an external biasing magnet, and hence are bulky and expensive. Recently, there has been significant research interest in the implementation of non-magnetic non-reciprocal components using temporal modulation, including a fully integrated 25-GHz circulator in a 45-nm SOI CMOS, demonstrating magneticfree passive non-reciprocity on silicon at millimeter waves for the first time. This paper presents a detailed analysis of the millimeter-wave circulator in both time and frequency domains. Millimeter-wave non-reciprocal operation is enabled by the concept of spatio-temporal conductivity modulation, which achieves broadband non-reciprocal gyrator functionality over theoretically infinite bandwidth (BW). When compared with prior approaches based on N-path filters, spatio-temporal conductivity modulation requires only four-phase 50% duty-cycle clocking at frequencies significantly lower than the operation frequency, enabling scaling to millimeter waves. The 25-GHz circulator achieves minimum transmitter (TX)-to-antenna (ANT)/ANT-to-receiver (RX) insertion losses of 3.3 dB/3.2 dB, respectively, with a 1-dB BW of 4.6 GHz. TX-to-RX isolation is 18.3-21.2 dB (limited by the measurement setup) over the same BW. The circulator IC occupies an area of 1.2 mm × 1.8 mm (λ/8 × λ/6). The spatiotemporal conductivity modulation concept is readily scalable across frequency and can be an enabler for higher millimeterwave (e.g., 77 GHz) circulators as well as optical isolators.