Low-Cost Rectangular Coaxial Transversal Filters With Additive Manufacturing Insulator Support and Enhanced Out-of-Band Performance

In this article, several low-cost transversal filters are proposed by rectangular coaxial transmission line resonators with additive manufacturing insulator supports, and their filtering characteristics are synthesized. First, the working principle of the transmission zeros (TZs) of the transversal filter is investigated thoroughly. Next, the transversal filter is revealed with its unique ability in generating <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> TZs in an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i>th-order filter with no additional structure, which is competitive in the filter synthesis theory. Then, the configuration and unloaded <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> factor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q_{u}$</tex-math> </inline-formula> of the proposed resonator are investigated to show its enhanced <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q_{u}$</tex-math> </inline-formula>, and the proposed resonator is exhibited with good accuracy during assembly and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q_{u}$</tex-math> </inline-formula> is competitive. Hence, the characteristics of the resonator are studied with respect to the electrical length, the characteristic impedance <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Z_{0}$</tex-math> </inline-formula>, the feeding structure, and the extracted external coupling <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q_{e}$</tex-math> </inline-formula>. To verify the generality of our proposed synthesis of transversal filters with rectangular coaxial-line resonators, three bandpass filter prototypes are synthesized and simulated. As the order increases, all three filters achieve the same number of TZs as their filter order. Finally, the fourth-order transversal bandpass filter is fabricated and measured, the conductors are fabricated by CNC techniques, and the low-cost insulator supports are manufactured by 3-D printing techniques. The measured results are found in good agreement with the synthesized and simulated ones. The center frequency is at 2.00 GHz with 6.50% fractional bandwidth (FBW) and four TZs occur outside the passband. In addition, the measured insertion loss (IL) is around 0.70 dB, which is pretty competitive. Compared with other published transversal filters, the out-of-band performance of the proposed coaxial-line transversal filter is highly enhanced.