Theory and design of a phase-inverted balanced coupled-line DC-blocker

TL;DR

This paper introduces a new theoretical approach using wave scattering transfer matrices for designing a planar, wideband differential mode DC-blocker with improved fabrication tolerance and demonstrates its implementation along with a substrate integrated waveguide balun for measurement.

Contribution

It presents a novel transfer matrix interpretation for series components and a practical design of a wideband, less sensitive, high-voltage tolerant DC-blocker with a supporting measurement setup.

Findings

Achieved return loss better than 10 dB from 5.6 to 8.4 GHz.

Designed a structure with air-gap about 10 times larger than conventional.

Demonstrated a nearly perfect 5-9 GHz differential-to-coaxial mode conversion.

Abstract

A planar DC-blocker suitable for differential mode signaling applications is designed and fabricated. The theory of this component is explained in a new form which utilizes the wave scattering transfer matrix. The proposed interpretation of the transfer matrix is most suitable for series (cascade) elements like DC-blockers. In addition to the theoretical enhancement, design of a compressed balanced DC-blocker inserted through a shielded broadside coupled stripline (SBCSL) transmission line is presented. The return loss of better than 10 dB is obtained at 50-ohm differential-mode input ports of the fabricated DC-blocker in the entire frequency range of 5.6-8.4 GHz. The lowest air-gap width in the presented structure is about 10 times bigger than that of a conventional coupled-line structure. So, the structure is much less sensitive to fabrication tolerances. Moreover, the DC-blocker is likely to tolerate higher DC-voltage differences. Also, a demonstration for a millimeter-wave version of this DC-blocker suitable for integrated circuits (ICs) applications is proposed for future development. The final achievement of this paper is design and fabrication of a wideband substrate integrated waveguide (SIW)-mediated balun structure for single-ended measurement of a balanced SBCSL component. The fabricated balun exhibits a nearly perfect coaxial-mode to coupled-stripline differential-mode conversion in the full range of 5-9 GHz. The presented balun is successfully utilized to derive the scattering parameters (S-parameters) of the fabricated balanced SBCSL DC-blocker.