Below we present an overview of the current state of isolator technology at Micro Harmonics. Our near term goal is to develop a full line of components operating from 60 GHz to 325 GHz with significantly improved performance over the current state-of-the-art. Our long term vision is to extend coverage to 400 GHz and beyond. 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.
Isolators are useful for controlling standing waves in a wide variety of millimeter-wave and terahertz systems. For example, in submillimeter-wave multiplied local oscillator systems standing waves arise due to impedance mismatches between the highly tuned frequency multipliers. This in turn gives rise to dips or even nulls in the output of the multiplier chain. Because of the lack of availability of suitable isolators, the standing waves are typically mitigated through complicated impedance matching techniques that are performed on a case-by-case basis at a great cost in time and money.
At frequencies above 75 GHz, there are relatively few vendors and the available components are unsuitable for many systems due to high insertion loss. Most of the commercial components were developed more than forty years ago and there has been little effort at modernization. Using modern electromagnetic simulation tools, we have demonstrated isolators that exhibit significantly reduced loss and improved bandwidth. Our simulations and supporting test data indicate that the isolators can be designed to work over significantly broad bands in excess of the standard waveguide bands.
WR-10 Isolators - Micro Harmonics has been pursuing a two pronged approach to millimeter-wave isolator development. The first approach utilizes three stacked plates as described by Erickson [1, 2]. The graphic to the right shows an exploded view of the three-plate isolator on the left and a photograph on the right. These isolators employ a diamond substrate between the input cone and ferrite. Power incident on the isolator output port is absorbed in the lossy film bisecting the input cone. The subsequent heat energy is efficiently channeled to the block through the diamond substrate. Most, if not all commercial isolators do not use diamond supports but rather use supports that are in the class of thermal insulators.
 N. R. Erickson, “Very Low Loss Wideband Isolators for mm-Wavelengths,” 2001 IEEE MTT-S Dig., pp. 1175-1178.
 N. R. Erickson and Ronald M. Grosslein, “A Low-Loss 74–110-GHz Faraday Polarization Rotator,” IEEE Trans. Microwave Theory Tech., vol. 55, no. 12, Dec. 2007.
The graph to the right shows measured data from a recent prototype, SN26. In the standard WR-10 band (75 -110 GHz) the insertion loss is less than 1.3 dB, the isolation is greater than 29 dB and the input and output return loss is less than 17 dB. The isolator performs well over the extended band from 67-115 GHz.
We refer to our second approach as a “drop-in” isolator. The drop-in isolator comprises a center plate that houses the core assembly and an E-plane split waveguide block that houses the center plate and contains a stepped waveguide twist on both the input and output ports.
An exploded view diagram of the center plate is shown in the top part of the graphic. The gold rectangular piece is the machined center plate. The two silver discs are neodymium bias magnets. The magnetic armatures are at the far left and far right. The alumina cones, diamond support disc, and ferrite core are shown together to the left of the center plate.
The sketch in the lower left corner shows how the centerplate fits into the E-plane split waveguide block. The photograph in the lower right shows the centerplate mounted in the bottom half of the split-plane block.
The drop-in isolator topology offers some unique advantages. First, the waveguides on the flanges are perfectly aligned in the drop-in isolator whereas in stacked plate isolators the waveguides are typically aligned at 11.25 degrees rotation from the normal. Although this difference is mostly cosmetic, it is important to some users. Second, the drop-in topology makes it possible to include interior access to two of the four waveguide flange screws on each flange which is an important feature for many users. And third, the drop-in topology makes it possible to integrate the isolator with other waveguide components in an E-plane split waveguide block.
If you are interested in integrating the drop-in isolator into your system, please contact Micro Harmonics for more information.
The graph to the right shows measured data from one of our WR-10 drop-in isolator prototypes. In the standard WR‑10 band from 75-110 GHz, the insertion loss is less than 1.3 dB, the isolation is greater than 25 dB and the input and output return loss is less than 17 dB. Insertion loss is less than 1.5 dB over the extended band 66-115 GHz and isolation is greater than 25 dB over the band 64-117 GHz. Again, the extended band performance and low insertion loss sets this unit apart from others on the commercial market.
Both the three-plate and drop-in isolators exhibit very good performance over extended waveguide bands. We will continue to pursue both approaches going forward. The three plate version is less costly to produce and has a smaller footprint. The drop-in isolator offers interior waveguide screws access and potential integration with other components.
WR-12 Isolator - Our first prototype WR-12 isolator was assembled and tested in April 2017. The results are shown in the graph to the right. In the WR-12 band 60-90 GHz, insertion loss is less than 2 dB, isolation is greater than 18 dB and reflection is less than -15 dB. There is a significant drop off in performance at the low end of the band near 62 GHz where the isolation drops from 30 dB to 18 dB. Machining errors were discovered in the stepped waveguide section and new parts are being fabricated.
WR-8 Isolator – Our first WR-8 isolator prototypes were assembled and tested in July 2017. The graph to the right shows measured data for prototype SN04. The measured insertion loss is near 1 dB across the WR-8 band from 90-140 GHz. A second prototype, SN02, had similar performance. Simulations indicate good performance over a much broader band in excess of 80-150 GHz. However, the network analyzer for the tests was not calibrated in the extended band. Our WR-8 isolator is a cube with dimensions (0.75 x 0.75 x 0.53) inches where the flange to flange distance is 0.53 inches.
WR-6.5 Isolator – Our first WR-6.5 isolator prototypes were assembled and tested in January 2018. The graph to the right shows measured data for prototype SN003. The measured insertion loss is near 1 dB across the WR-6.5 band from 110-170 GHz. The isolation is greater than 30 dB over most of the band but does roll off on the low end where it reaches 20 dB at 110 GHz. The data is being analyzed and a tweak in the design will be implemented to improve the performance at 110 GHz. These units will become available for sale in the first quarter of 2018.
Isolators at Higher Frequencies - Designs are complete for isolators at WR-5.1, WR-4.3 and WR-3.4. We are currently assembling and testing prototypes at WR-6.5 and WR-5.1 (see the initial test data for the WR-6.5 units above). Some of the constituent parts for the WR-4.3 and WR-3.4 isolators are still in fabrication and we hope to begin prototyping during the first quarter of 2018. The possibility of extending this work to even higher bands is being evaluated.
The data shown to the right for the WR-5.1 isolator are from HFSS simulations. The HFSS simulation data for the WR-4.3 and WR-3.4 isolators are shown below.
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