Abstract
This paper presents a highly miniaturized tuneable microstrip line phase shifter for 5 GHz to 67 GHz. The design takes advantage of the microstrip topology by substituting the ground plane by a metallic-nanowire-filled porous alumina membrane (NaM). This leads to a slow-wave (SW) effect of the transmission line; thus, the transmission line can be physically compact while maintaining its electric length. By applying a liquid crystal (LC) with its anisotropic permittivity as substrate between the transmission line and the NaM, a tuneable microstrip line phase shifter is realized. Three demonstrators are identically fabricated filled with different types of high-performance microwave LCs from three generations (GT3-23001, GT5-26001 and GT7-29001). The measurement results show good matching in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$50\ \Omega$</tex-math></inline-formula> system with reflection less than −10 dB over a wide frequency range. These demonstrators are able to reach a maximum figure of merit (FoM) of 41 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> /dB, 48 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> /dB, and 70 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> /dB for different LCs (GT3-23001, GT5-26001 and GT7-29001, respectively). In addition, experiments show that all three LCs should be biased with square wave voltage at approximately 1 kHz to achieve maximum tuneability and response speed. The achieved response times with GT3-23001, GT5-26001 and GT7-29001 are 116 ms, 613 ms, and 125 ms, respectively, which are much faster than other reported LC phase shifter implementations. Large-signal analysis shows that these implementations have high linearity with third-order interception (IP3) points of approximately 60 dBm and a power handling capability of 25 dBm.
Highlights
As demand for higher data rates in wireless communication systems is increasing rapidly, 20, 40 and even 100 Gbps rates are expected for wireless technologies
The reason is that the transmission lines are designed to work with liquid crystal (LC) ( r ≈ 2.4 to 3.5), which leads to a distinct impedance mismatch when filled with air ( r = 1)
The LC-based dielectric waveguide (LC-DG) phase shifter and dielectric image line (LC-DIL) phase shifter in [13] and [15] yield the highest figure of merit (FoM), mainly due to their low loss as a result of fully and partially eliminating the use of metal electrodes, respectively; they suffer from a bulky structure and long response time of 17 s and 5 s, respectively
Summary
As demand for higher data rates in wireless communication systems is increasing rapidly, 20, 40 and even 100 Gbps rates are expected for wireless technologies. In addition to BST, liquid crystals (LCs) represent another promising tuneable dielectric, but generally above 5 GHz, even up to at least 8 THz [8]–[10] These materials provide continuous tuneability, high linearity and low loss at low cost by using standard technologies. The dimensions are given as follows: radial pad radius rpad = 470 μm and phase shifter line length lPS = 3600 μm To solve this issue, considerable efforts have been invested to combine LCs with high-performance slow-wave (SW) transmission lines (TLs). Later investigation shows that NaM melts during laser cutting and the coagulum forms edge with several μm thickness This results in much higher hLC which deteriorates the impedance matching and response. These dimensions provide a microstrip line Zc of 50 when filled with LCs
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