Millimeter-Wave Transmission-Line Reflectionless Filters

TL;DR

This work tackles the problem of achieving reflectionless, absorptive filtering at millimeter wavelengths by enforcing the condition $s_{ii}(f)=0$ in a transmission-line topology. It implements two high-frequency filters centered at $100\text{ GHz}$ and $230\text{ GHz}$ using Lange-interdigital coupled lines on an Alumina substrate with a thin-film CPW layout, designed around the parameter $z_x$ (e.g., $z_x=\sqrt{2}$). Fabrication proceeds on a $150\text{ mm}$ alumina wafer with multiple metal layers and TaN resistors, and wafer-probe measurements are performed up to $500\text{ GHz}$ with TRL calibration. The results push the operating frequency of reflectionless filters higher than previously reported, achieving compact footprints and good agreement with EM simulations, while highlighting practical challenges from substrate dispersion and fabrication tolerances.

Abstract

We report on the development of transmission-line reflectionless filters operating with passbands at 100 GHz and 230 GHz, and stopband absorption up to 500 GHz, the highest operating frequencies yet recorded for such filters. The designs are based on a previously reported mathematical solution to the reflectionless condition, now successfully implemented for the first time, using an advanced thin-film fabrication process on Alumina substrates. Sub-millimeter wave wafer probe measurements show good agreement with theory.

Figures (6)

• Figure 1: (a) Transmission-Line reflectionless filter topology. Lower-case parameters indicate normalized impedance values. (b) Simulated performance with ideal elements and $z_x=\sqrt{2}$. Return loss in dB is infinite.
• Figure 2: Layout plan for transmission-line reflectionless filters. Lange hybrids are used to implement the coupled line sections, while CPW is used for the remainder of the transmission lines and stubs.
• Figure 3: Microphotographs of transmission-line reflectionless filters at true relative scale. (a) $100\text{{-}GHz}$ bandpass filter. Chip dimensions are $850\text{ }\mu\text{m}\times900\text{ }\mu\text{m}$. (b) $230\text{{-}GHz}$ bandpass filter. Chip dimensions are $450\text{ }\mu\text{m}\times450\text{ }\mu\text{m}$.
• Figure 4: Plots of $s_{21}$ and $s_{11}$ for $100\text{{-}GHz}$ reflectionless filters. Measured curves are shown with solid black lines, while the EM-simulated results are shown with dashed gray lines.
• Figure 5: Plots of $s_{21}$ and $s_{11}$ for $230\text{{-}GHz}$ reflectionless filter. Measured curves are shown with solid black lines, while the EM-simulated results are shown with dashed gray lines.