Artificial surface plasmon polaritons phase shifter based on varactor series-parallel network

Abstract

The application discloses a kind of artificial surface plasmon phase shifters based on varactor tube series-parallel network.The phase shifter includes microstrip line transition structure and adjustable transmission structure formed by a plurality of suspended artificial surface plasmon periodic units in series,each periodic unit includes inverted T-shaped metal structure and varactor tube series-parallel network loaded with it.The varactor tube series-parallel network reduces equivalent parasitic series resistance by parallel connection of multiple varactor tubes,combines series compensation structure to maintain equivalent adjustable capacitance range,so as to reduce parasitic loss introduced by adjustable semiconductor device while maintaining artificial surface plasmon dispersion control capability.The application can significantly reduce the insertion loss of phase shifter while basically maintaining the original dispersion characteristics and continuous phase shifting capability,has the advantages of simple structure,feasible processing,low loss and continuous adjustment,etc.,and can be applied in phased array,reconfigurable microwave system,beam scanning system and other low-loss adjustable radio frequency front-end equipment.

Description

Technical Field

[0001] This invention belongs to the field of microwave and millimeter-wave tunable device technology, specifically relating to a low-loss continuously adjustable phase shifter based on a varactor series-parallel network and a suspended artificial surface plasmon structure. Background Technology

[0002] Artificial surface plasmon polaritons (SSPPs) are artificial electromagnetic wave modes that simulate the propagation characteristics of optical surface plasmons in the microwave, millimeter-wave, and terahertz frequency bands using periodic metallic structures. They possess characteristics such as strong field confinement and tunable dispersion, and have been widely used in microwave devices such as filters, power dividers, couplers, antennas, modulators, and phase shifters. With the development of phased arrays, SSPP phase shifters, which can achieve continuously tunable phase, demonstrate significant application value.

[0003] Existing tunable artificial surface plasmon phase shifters typically modulate dispersion characteristics and achieve phase tunability by loading tunable devices such as varactor diodes and PIN diodes into periodic metal cells to change the equivalent circuit parameters. However, the introduction of tunable devices leads to significant parasitic losses. Existing techniques propose reducing parasitic losses by connecting varactor diodes in parallel (PH He, Y. Fan, LY Niu, HC Zhang, CY Wei, J. Zhang, H. Gao, Y. Huang, J. Xu, W. Tang, and TJ Cui, “Analysis, Reduction, and Utilization of Lossin Reconfigurable Spoof Surface Plasmon Polaritons,” IEEE Transactions on Microwave Theory and Techniques, vol. 71, no. 3, pp. 945–955, Mar. 2023, doi:10.1109 / TMTT.2022.3228778.). While this approach can reduce losses caused by the parasitic resistance of varactor diodes to some extent, its loss reduction depends on increasing the number of varactor diodes connected in parallel. While reducing the equivalent resistance, the parallel connection of varactor diodes significantly increases the total equivalent capacitance, thereby altering the dispersion characteristics, cutoff frequency, and reconfigurable range of the original artificial surface plasmon resonance unit, affecting the operating frequency band, impedance matching, and continuous phase shift performance.

[0004] Meanwhile, a low-loss artificial surface plasmon waveguide structure based on substrate-free technology has been proposed in the literature (Y. Fan, PH He, HC Zhang, X. Chang, XD Wang, YJ Ge, LYNiu, DY Yao, WX Tang, and TJ Cui, “Ultra-low-loss spoof surface plasmon polariton waveguides using substrate-free technology,” OpticsLetters, vol. 50, no. 21, pp. 6871–6874, Nov. 2025, doi: 10.1364 / OL.577001.). However, this type of structure is mainly aimed at passive artificial surface plasmon waveguides, focusing on reducing dielectric loss. It does not address the continuous phase modulation problem after loading varactor diodes or PIN diodes, nor does it resolve the contradiction between parasitic parameters, equivalent capacitance changes, and dispersion characteristics of tunable devices. Therefore, this scheme is difficult to directly apply to low-loss, continuously tunable artificial surface plasmon phase shifters.

[0005] Therefore, existing tunable artificial surface plasmon polariton (ASP) phase shifter designs still have significant limitations. Their low-loss designs often optimize only for a single loss source, making it difficult to simultaneously suppress parasitic losses and dielectric losses in the tunable device. Furthermore, reducing parasitic losses easily introduces changes in equivalent capacitance, leading to a deterioration in the dispersion characteristics of the original ASP cells, affecting the operating frequency band and transmission efficiency. Therefore, there is an urgent need for an ASP phase shifter that can reduce parasitic losses and dielectric losses in the tunable device while maintaining the strong field confinement and dispersion tunable characteristics of ASPs. Summary of the Invention

[0006] Technical Problem: To address the problems of high parasitic losses, high dielectric losses, and the difficulty in simultaneously achieving low-loss structure and continuously adjustable phase shifting functionality in existing adjustable artificial surface plasmon polariton (APT) phase shifters, this invention provides an APT phase shifter based on a series-parallel network of varactor diodes. This phase shifter reduces the equivalent parasitic resistance introduced by the adjustable devices through parallel varactor diodes, maintains unchanged dispersion characteristics through a series capacitor-inductor structure, and reduces the interaction between the electromagnetic field and the lossy dielectric material through a suspended APT structure, thereby reducing transmission losses while maintaining the control capability.

[0007] Technical solution: To achieve the above objectives, the present invention provides an artificial surface plasmon phase shifter based on a series-parallel network of varactor tubes, which adopts the following solution.

[0008] The present invention discloses an artificial surface plasmon phase shifter based on a series-parallel network of varactor tubes, comprising a matching structure, a suspended artificial surface plasmon transmission structure, and a ground plane; The matching structure is disposed at both ends of the suspended artificial surface plasmon transmission structure to realize the conversion and impedance matching between the external transmission line mode and the artificial surface plasmon transmission mode. The suspended artificial surface plasmon transmission structure includes multiple artificial surface plasmon periodic units arranged periodically along the electromagnetic wave propagation direction. Each artificial surface plasmon periodic unit includes an inverted T-shaped metal structure and a series-parallel network of varactor tubes loaded on the inverted T-shaped metal structure. The varactor series-parallel network includes two varactor tubes connected in parallel and a capacitor-inductor parallel structure connected in series with the two varactor tubes. The capacitor-inductor parallel structure is composed of a compensation capacitor and an inductor connected in parallel. The two parallel-connected varactor tubes are used to reduce the equivalent parasitic series resistance of the varactor tube loading structure. The compensation capacitor is used to compensate for the increase in equivalent capacitance caused by the parallel connection of the two varactor tubes. The bias inductor is used to provide a DC bias path. An air gap is formed between the suspended artificial surface plasmon transmission structure and the floor, so that the electromagnetic field is mainly distributed in the air region, thereby reducing the transmission loss caused by the dielectric substrate.

[0009] The number of artificial surface plasmon periodic units (21) is 60, and the period length is 3 mm.

[0010] The varactor series-parallel network includes two varactors, a compensation capacitor, and an inductor. The two varactors are connected in parallel to form a parallel varactor, and the compensation capacitor and inductor are connected in parallel to form a capacitor-inductor parallel structure. The parallel varactors are connected in series with the capacitor-inductor parallel structure.

[0011] The two varactor diodes are of the same type and are connected in parallel in the radio frequency equivalent circuit so that the equivalent parasitic resistance of the parallel varactor diodes is less than the equivalent parasitic resistance of a single varactor diode loading structure.

[0012] The compensation capacitor has a capacitance of 0.1pF, the inductor has an inductance of 20nH, and the adjustable capacitance range of the varactor is 0.03pF to 0.06pF. The parameters of the compensation capacitor and the inductor are configured such that the self-resonant frequency of the capacitor-inductor parallel structure avoids the operating frequency band of the phase shifter.

[0013] The suspended artificial surface plasmon transport structure is a metal integrated structure, which is prepared by metal three-dimensional printing process, circuit board printing process or a combination thereof.

[0014] An air gap is formed between the suspended artificial surface plasmon transmission structure and the floor. The height of the air gap is 2 mm, and no continuous dielectric substrate is placed in the air gap, so that the electromagnetic field is mainly distributed in the air region, thereby reducing the interaction between the electromagnetic field and the damaged dielectric material during the artificial surface plasmon transmission process.

[0015] Beneficial effects: Compared with the prior art, the present invention has the following advantages: First, the present invention uses two varactor tubes connected in parallel to reduce the equivalent parasitic series resistance, thereby reducing the parasitic losses caused by the adjustable device. Second, the present invention compensates for the change in equivalent capacitance by using a capacitor-inductor parallel structure connected in series with a parallel varactor tube, thereby reducing parasitic losses while maintaining the dispersion control capability required for artificial surface plasmon units. Third, the present invention employs a suspended artificial surface plasmon structure, which makes the electromagnetic field mainly distributed in the air region, thereby reducing dielectric loss caused by the dielectric substrate. Fourth, this invention can achieve low-loss transmission while maintaining continuous phase shift capability and good impedance matching characteristics, and is suitable for application scenarios such as phased array, beam scanning and low-loss adjustable microwave systems. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of a low-loss suspended artificial surface plasmon phase shifter. Figure 2 This is a side view schematic diagram of a low-loss suspended artificial surface plasmon phase shifter. Figure 3 This is a schematic diagram of a low-loss suspended artificial surface plasmon phase shifter unit structure; Figure 4 These are the dispersion curves of a low-loss suspended artificial surface plasmon phase shifter at 0° and 360° phase shift. Figure 5 This is a graph of the S-parameters of a low-loss suspended artificial surface plasmon phase shifter at 0° phase shift. Figure 6 This is a graph of the S-parameters of a low-loss suspended artificial surface plasmon phase shifter during a 360° phase shift. Figure 7 This is a diagram showing the phase shift results of a low-loss suspended artificial surface plasmon phase shifter. The figure includes: matching structure 1, suspended artificial surface plasmon transport structure 2, ground plane 3, artificial surface plasmon periodic unit 21, inverted T-shaped metal structure 22, varactor series-parallel network 23, varactor V, compensation capacitor C, inductor L, capacitor-inductor parallel structure CL; where a is the center-to-center distance between adjacent square pads, h is the distance from the top of the square pad to the center metal strip, d is the side length of the square pad, r is the radius of the metal via, p is the period of the suspended artificial surface plasmon transport structure, g is the distance between adjacent metal pads or the distance between the metal pad and the center metal strip, t1 is the thickness of the dielectric substrate, and t2 is the thickness of the metal ground plane. Detailed Implementation

[0017] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0018] This invention proposes an artificial surface plasmon phase shifter based on a series-parallel network of varactor tubes, including a matching structure 1, a suspended artificial surface plasmon transmission structure 2, and a ground plane 3; The matching structure 1 is disposed at both ends of the suspended artificial surface plasmon transmission structure 2, and is used to realize the conversion and impedance matching between the external transmission line mode and the artificial surface plasmon transmission mode. The suspended artificial surface plasmon transmission structure 2 includes multiple artificial surface plasmon periodic units 21 arranged periodically along the electromagnetic wave propagation direction. Each artificial surface plasmon periodic unit includes an inverted T-shaped metal structure 22 and a series-parallel network of varactor tubes 23 loaded on the inverted T-shaped metal structure 22. The varactor series-parallel network 23 includes two varactors V connected in parallel and a capacitor-inductor parallel structure CL connected in series with the two varactors V connected in parallel. The capacitor-inductor parallel structure is composed of a compensation capacitor C and an inductor L connected in parallel. The two parallel-connected varactor tubes V are used to reduce the equivalent parasitic series resistance of the varactor tube loading structure. The compensation capacitor is used to compensate for the increase in equivalent capacitance caused by the parallel connection of the two varactor tubes. The bias inductor is used to provide a DC bias path. An air gap is formed between the suspended artificial surface plasmon transmission structure 2 and the floor 3, so that the electromagnetic field is mainly distributed in the air region, thereby reducing the transmission loss caused by the dielectric substrate.

[0019] The varactor series-parallel network 23 includes two varactors V, a compensation capacitor C, and an inductor L. The two varactors V are connected in parallel to form a parallel varactor, and the compensation capacitor C and the inductor L are connected in parallel to form a capacitor-inductor parallel structure CL. The parallel varactor is connected in series with the capacitor-inductor parallel structure CL.

[0020] The two varactor diodes V are of the same type and are connected in parallel in the radio frequency equivalent circuit so that the equivalent parasitic resistance of the parallel varactor diodes is less than the equivalent parasitic resistance of a single varactor diode loading structure.

[0021] The compensation capacitor C has a capacitance of 0.1pF, the inductor L has an inductance of 20nH, and the adjustable capacitance of the varactor V ranges from 0.03pF to 0.06pF. The parameters of the compensation capacitor C and the inductor L are configured such that the self-resonant frequency of the capacitor-inductor parallel structure CL avoids the operating frequency band of the phase shifter.

[0022] Two varactor diodes are connected in parallel to reduce their equivalent parasitic resistance. A compensation capacitor is used to compensate for the increase in equivalent capacitance caused by the parallel connection of the varactor diodes. An inductor is used to provide a DC bias path. By adjusting the reverse bias voltage applied to the varactor diodes, the equivalent capacitance of the artificial surface plasmon periodic cell can be changed, thereby controlling its dispersion characteristics and achieving continuous phase adjustment.

[0023] The suspended artificial surface plasmon transport structure 2 is an integrated metal structure, which is prepared by metal three-dimensional printing process, circuit board printing process or a combination thereof.

[0024] An air gap, 2mm high, is formed between the suspended artificial surface plasmon resonance (ASPR) transmission structure 2 and the ground plane 3. No continuous dielectric substrate is placed within this air gap, ensuring that the electromagnetic field is primarily distributed in the air region. This reduces the interaction between the electromagnetic field and the lossy dielectric material during ASPR transmission. The air gap between the suspended ASPR transmission structure and the grounded reference structure further concentrates the main electromagnetic field of the ASPR wave within the air region, reducing or eliminating the loading of the transmission field by the continuous dielectric substrate and thus lowering dielectric loss. The suspended ASPR transmission structure can be fabricated using metal 3D printing, circuit board printing, or a combination thereof.

[0025] like Figure 1 As shown. The suspended artificial surface plasmon transmission structure 2 is composed of artificial surface plasmon periodic units arranged in series. Each artificial surface plasmon periodic unit consists of an inverted T-shaped metal structure 22 and a series-parallel network of varactor tubes loaded on the inverted T-shaped metal structure. The overall structural dimensions are 200 mm × 50 mm × 2 mm.

[0026] Side view of the overall phase shifter structure as shown Figure 2 As shown, the unit period p = 3 mm, the substrate thickness t1 = 1.5 mm, and the metal ground thickness t2 = 0.5 mm.

[0027] The artificial surface plasmon periodic unit 21 comprises 60 units with a period length of 3 mm. The specific structure of the artificial surface plasmon periodic unit is as follows: Figure 3 As shown, the unit period p = 3mm, pad spacing a = 1mm, via radius r = 0.35mm, pad side length d = 2mm, and pad spacing g = 1mm. The varactor series-parallel network includes two varactors connected in parallel and a capacitor-inductor parallel structure connected in series with the branches of the two parallel varactors. The two varactors are connected in parallel to reduce the equivalent parasitic series resistance. The capacitor-inductor parallel structure consists of a compensation capacitor and an inductor connected in parallel. The compensation capacitor compensates for the increase in equivalent capacitance caused by the parallel connection of the two varactors, and the inductor provides a DC bias path. The varactor is model MAVR-000120-1411, with an adjustable capacitance range of 0.03pF to 0.06pF, a compensation capacitor value of 0.1pF, and a bias inductance value of 20 nH.

[0028] The dispersion curves of the relevant structures are as follows Figure 4 As shown in the figure, by adjusting the reverse bias voltage of the varactor diodes, the equivalent capacitance of the artificial surface plasmon periodic unit can be changed, causing a continuous shift in the dispersion curve and thus achieving continuous adjustment of the propagation phase. Simultaneously, because the two varactor diodes are connected in parallel to reduce the equivalent parasitic resistance, and the required equivalent adjustable capacitance range is maintained through a compensation capacitor, this structure can reduce parasitic losses introduced by the adjustable devices while maintaining the original dispersion control capability.

[0029] Simulation results of S-parameters under zero phase shift state are as follows Figure 5 As shown in the figure. Simulation results show that, under zero-phase-shift operation, the phase shifter has good impedance matching and transmission performance at 7GHz, with a reflection coefficient below -15 dB and a transmission coefficient better than -2 dB, enabling low-loss signal transmission.

[0030] The S-parameter simulation results under the 360° maximum phase shift state are as follows: Figure 6 As shown in the figure. Simulation results show that, under the maximum phase shift operating condition, the phase shifter can still maintain good impedance matching characteristics, with a reflection coefficient below -15dB and a transmission coefficient better than -5dB, and good transmission performance stability across the entire phase shift adjustment range.

[0031] Simulation results of phase shift characteristics of the phase shifter are as follows: Figure 7As shown in the figure, this phase shifter can achieve continuous phase adjustment within the operating frequency band, with a continuous phase shift range of 360° at 7 GHz. Compared with typical artificial surface plasmon phase shifters, transmission loss is reduced by 64% for the same phase shift capability. This invention removes the continuous dielectric substrate in the main transmission region through a suspended structure, so that the electromagnetic field is mainly distributed in the air region, thereby reducing the transmission loss caused by the dielectric substrate; at the same time, it reduces the parasitic loss of the tunable device through a series-parallel network of varactor diodes, achieving low-loss continuous phase shifting without significantly changing the dispersion characteristics of artificial surface plasmons. Therefore, this structure is suitable for applications such as phased arrays, beam scanning, and low-loss tunable microwave systems.

Claims

1. An artificial surface plasmon phase shifter based on a series-parallel network of varactor diodes, characterized in that, It includes a matching structure (1), a suspended artificial surface plasmon transport structure (2), and a floor (3); The matching structure (1) is disposed at both ends of the suspended artificial surface plasmon transmission structure (2) to realize the conversion and impedance matching between the external transmission line mode and the artificial surface plasmon transmission mode. The suspended artificial surface plasmon transmission structure (2) includes multiple artificial surface plasmon periodic units (21) arranged periodically along the electromagnetic wave propagation direction. Each artificial surface plasmon periodic unit includes an inverted T-shaped metal structure (22) and a series-parallel network of varactor tubes (23) loaded on the inverted T-shaped metal structure (22). The series-parallel network of varactor tubes (23) includes two varactor tubes (V) connected in parallel and a capacitor-inductor parallel structure (CL) connected in series with the two varactor tubes. The capacitor-inductor parallel structure is composed of a compensation capacitor (C) and an inductor (L) connected in parallel. The two parallel-connected varactor tubes (V) are used to reduce the equivalent parasitic series resistance of the varactor tube loading structure. The compensation capacitor is used to compensate for the increase in equivalent capacitance caused by the parallel connection of the two varactor tubes. The bias inductor is used to provide a DC bias path. An air gap is formed between the suspended artificial surface plasmon transmission structure (2) and the floor (3), so that the electromagnetic field is mainly distributed in the air region, thereby reducing the transmission loss caused by the dielectric substrate.

2. The artificial surface plasmon phase shifter based on a series-parallel network of varactor diodes according to claim 1, characterized in that, The number of artificial surface plasmon periodic units (21) is 60, and the period length is 3 mm.

3. The artificial surface plasmon phase shifter based on a series-parallel network of varactor diodes according to claim 1, characterized in that, The varactor series-parallel network (23) includes two varactors (V), a compensation capacitor (C), and an inductor (L). The two varactors (V) are connected in parallel to form a parallel varactor, and the compensation capacitor (C) and the inductor (L) are connected in parallel to form a capacitor-inductor parallel structure (CL). The parallel varactor is connected in series with the capacitor-inductor parallel structure (CL).

4. The artificial surface plasmon phase shifter based on a series-parallel network of varactor diodes according to claim 3, characterized in that, The two varactor diodes (V) are of the same type and are connected in parallel in the radio frequency equivalent circuit so that the equivalent parasitic resistance of the parallel varactor diodes is less than the equivalent parasitic resistance of a single varactor diode loading structure.

5. An artificial surface plasmon phase shifter based on a series-parallel network of varactor diodes according to claim 3, characterized in that, The compensation capacitor (C) has a capacitance of 0.1pF, the inductor (L) has an inductance of 20nH, and the adjustable capacitance of the varactor (V) ranges from 0.03pF to 0.06pF. The parameters of the compensation capacitor (C) and the inductor (L) are configured such that the self-resonant frequency of the capacitor-inductor parallel structure (CL) avoids the operating frequency band of the phase shifter.

6. The artificial surface plasmon phase shifter based on a series-parallel network of varactor diodes according to claim 1, characterized in that, The suspended artificial surface plasmon transport structure (2) is a metal integrated structure, which is prepared by metal three-dimensional printing process, circuit board printing process or a combination thereof.

7. The artificial surface plasmon phase shifter based on a series-parallel network of varactor diodes according to claim 1, characterized in that, An air gap is formed between the suspended artificial surface plasmon transmission structure (2) and the floor (3). The height of the air gap is 2 mm, and no continuous dielectric substrate is set in the air gap, so that the electromagnetic field is mainly distributed in the air region, thereby reducing the interaction between the electromagnetic field and the damaged dielectric material during the artificial surface plasmon transmission process.

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