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Design a Diplexer for 275 to 500 GHz

A waveguide diplexer combines different-frequency LOs to cover a total frequency range of 275 to 500 GHz for astronomy applications.

Millimeter-wave frequencies and beyond are receiving a tremendous amount of attention for their expected application in 5G wireless communications systems. But heterodyne receivers at millimeter-wave and sub-millimeter-wave frequencies are already widely used for many scientific applications, including for remote sensing, security, and spectroscopy.

Signal frequencies as high as 500 GHz, for example, are used in many astronomical applications. In support of those uses, researchers from the National Astronomical Observatory of Japan (Tokyo) have developed a waveguide diplexer capable of combining local oscillator (LO) sources at different frequencies to achieve a single LO signal for the 275-to-500-GHz frequency range.

The 275-to-500-GHz diplexer uses two different hybrid couplers to divide and combine the LO signals coming from the input sources at two different frequency bands. Different waveguide sizes are used for the different frequencies to minimize loss, with waveguide configuration and size chosen to optimize impedance matching. The various lowpass-filter (LPF) and highpass-filter (HPF) sections of the diplexer were designed and simulated prior to fabrication to explore the feasibility of fabrication. Simulations were performed on the HFSS electromagnetic (EM) simulation software from Ansys with simulations performed with Microwave Office from National Instruments/Applied Wave Research (AWR).

Two wideband waveguide diplexers were fabricated each as two split blocks by direct machining in aluminum. Some rounding of corners was necessary to achieve the small wavelengths at such high frequencies. Machining was performed with tools having diameters as fine as 30 μm. Electrical measurements were performed by means of commercial millimeter-wave vector network analyzer (VNA) and accessory waveguide frequency extenders to reach the frequency range of interest.

As the researchers discovered, one of the challenges in working at such high frequencies is achieving the physical dimensions and tolerances required to achieve the electrical performance levels predicted by the software simulation tools. By exercising some creative machining steps, the researchers discovered how to fabricate the tight dimensions needed to achieve measured performance that is fairly close to the simulated performance levels.

See “275-500 GHz Waveguide Diplexer to Combine Local Oscillators for Different Frequency Bands,” IEEE Transactions on Terahertz Science and Technology, Vol. 7, No. 6, November 2017, p. 669.

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