Below we present an overview of the current state of circulator technology at Micro Harmonics. Our near term goal is to develop a line of circulators operating in sub-bands from 50 GHz to over 140 GHz with significantly improved performance over the current state-of-the-art. Our long term vision is to develop components operating up to 500 GHz. These components will find immediate use in a broad range of commercial and scientific systems. Please contact us to discuss how we can apply our capabilities to your requirements.
Introduction - Y-junction circulators are useful for directing signal flow in a wide variety of millimeter-wave transmit and receive systems. At the heart of the device is a ferrite core located at the junction of three waveguides. The magnetically biased ferrite is non-reciprocal which gives rise to the unique circulator behavior. Most of the commercially available components are based on designs that are more than forty years old and there has been little effort at modernization. Using modern electromagnetic simulation tools, we are able to design high-frequency circulators that exhibit significantly improved bandwidth making them useful for many transmit/receive systems such as millimeter-wave radar systems.
WR-5.1 Circulator - Micro Harmonics started developing millimeter-wave ferrite devices in 2012. We were convinced that the performance of the commercial isolators and circulators could be greatly enhanced through the use of modern electromagnetic simulation tools that we had been using for the development of state-of-the art millimeter-wave and terahertz varactor and varistor frequency multipliers. Our initial prototype circulator was designed to operate at 160 GHz in WR-5. The measured test data and the simulation data are shown in graph to the right. The data are in good agreement with only a 1.5 GHz shift in the center frequency. This result affirmed our ability to accurately simulate and build these ferrite components.
The measured insertion loss (solid red curve) is higher than the simulated data (dashed red curve) because the simulations did not include any waveguide losses. The bandwidth is about 2% of the center frequency when defined as the band over which the isolation and input return loss are both greater than 20 dB. At 161 GHz the isolation is greater than 36 dB and the input return loss is greater than 40 dB. The bandwidth at this frequency is close to the best reported on the commercial market.
WR-15 Circulator - Through further refinements in our impedance matching structures we were able to design a WR-15 circulator with a 20 dB bandwidth of more than 7 GHz using a simple cylindrical ferrite core. In WR-15 the industry standard is about 1-2 GHz, so this represents an increase in bandwidth of more than 300%. We recently built and tested our first prototype WR15 circulator. There is reasonably good agreement between the measured and simulated data shown in the graph to the right. The downward shift in the operating band can likely be corrected by a small adjustment of the ferrite dimensions.
The measured insertion loss is 0.5 dB. The data is being analyzed and the assembly will be adjusted to retune the performance. Representative isolation data for a commercially available circulator is also shown in the graph (brown curve). This result clearly demonstrates the viability of our approach.
WR-10 Circulator – The design of a WR-10 circulator has been completed. The simulation results are shown in the graph to the right. The isolation is 20 dB over a 6 GHz band from 89.5 GHz to 95.5 GHz and 25 dB over a 5 GHz bandwidth from 90-95 GHz. Circulators on the commercial market typically have 20 dB bandwidths specified at 2 GHz in this frequency range. The design is relatively simple, comprising a neodymium magnet, an H-plane split waveguide block and a cylindrical ferrite core. The significantly improved bandwidth is accomplished through impedance matching elements machined into the waveguide block. Most of the constituent parts have been fabricated. RF tests will likely occur in August or September 2017.
WR-3.4 Circulator - To demonstrate our approach at much higher frequencies we developed two models in WR-3.4. The first model is designed for maximum isolation but with narrower bandwidth. The second model is designed for broad bandwidth. The graph to the right shows the simulation results for the narrow band model at 272 GHz. The 20 dB bandwidth is 2 GHz from 271 GHz to 273 GHz. This is a preliminary result and little effort has been made to optimize this design. Based on our experience with these devices we are confident that significant improvements are possible.
The WR-3.4 design was reconfigured for 13 dB isolation over a 7 dB bandwidth. Two of the 13 dB designs were stacked in a single block to increase the isolation. The result of the simulation is shown in the graph to the right. The inherent symmetry between the circulator ports is lost in the cascade, so the isolation is different for the three sets of ports. The isolation of Port 1 (source/transmitter) from Port 2 (antenna) is greater than 20 dB over a 12 GHz band from 266-278 GHz. However, the isolation between Port 3 (receiver) and Port 1 (transmitter) is only 10 dB over the same 12 GHz band. A sketch showing the port definitions is superimposed in the lower left hand corner of the graph. Significant improvements in the performance may be possible through further refinement of the model. The limits of this approach are unknown at this time, but it is clear from our simulations and RF test data that our approach can yield significant improvements over the current state-of-the-art.
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