T-shaped double-strip spoof surface plasmon polariton transmission lines and application to microwave resonators

Abstract

A microwave double-strip spoof surface plasmon polariton (DS SSPP) is proposed for high-speed interconnects and high-performance microwave circuits. Based on the dispersion analysis, a T-shaped double-strip structure is designed to provide strong surface- and slow-wave properties from very low to very high frequencies (~ 40 GHz). It allows the tight field confinement and greatly reduces the electromagnetic wave leakage. It exhibits broadband performance with reduced ripples in the insertion loss. It also shows more constant group delay and impedance than counterpart single-strip SSPP. The compact coaxial-to-microstrip-to-DS SSPP transition are designed using gradient grooves. The measurement shows that the DS SSPP lines can exhibit the lower coupling and lower insertion loss than the microstrip lines, so that the former is well-suited for the densely packed high-speed interconnects. The designed DS SSPP is utilized for high quality (Q)-factor microwave ring resonator. The measured unloaded Q-factor is 107.9 at the resonant frequency of 8.7 GHz, which is 1.3 times higher than the microstrip ring resonator. It is found to be caused by the reduction of the radiation loss, according to the loss analysis. The size is also reduced due to the short wavelength, occupying 56.8% of that of the microstrip ring resonator. Therefore, the proposed T-shaped DS SSPP can be also applied for high-performance miniaturized microwave circuits.

Figure 1

Unit cell of designed DS SSPP. (a) DS SSPP with T-shaped metal films on both sides of dielectric substrate. (b) Metal strip in which $D_T$ = 0.4 mm, $W$ = 1.0 mm, $G_T$ = 0.3 mm, $P$ = 0.5 mm, $H_T$ = 0.2 mm, $L_T$ = 0.4 mm, $L_{T0}$ = 0.2 mm.

Figure 2

Dispersion curve and group velocity of lightline, microstrip, SS SSPP, and DS SSPP in the same dielectric substrate. (a) Dispersion curve. (b) Group velocity.

Figure 3

Single straight transmission line. (a) DS SSPP line. (b) Simulated $|S_{11}|$. (c) Simulated $|S_{21}|$. (d) Simulated group velocity.

Figure 4

Loss analysis of straight transmission lines at 35.0 GHz. (a) Calculated insertion loss per unit length and per a wavelength. (b) Loss components.

Figure 5

Coupled transmission lines and simulated S-parameters in microstrip (slotted) and DS SSPP (solid). (a) DS SSPP coupled lines. (b) Microstrip-to-DS SSPP transition in which $H_T$ = 0.20 mm, $D_T$ = 0.40 mm, $G_T$ = 0.30 mm, $L_T$ = 0.40 mm, and $W$ = 1.00 mm. (c) Simulated insertion and return losses. (d) Simulated coupling ($|S_{31}|$) and isolation ($|S_{41}|$).

Figure 6

Design of microstrip-to-DS SSPP transition. (a) Dispersion curves at different positions along transition. (b) Simulated S-parameters of back-to-back transition.

Figure 7

Fabricated transmission lines. (a) Microstrip straight line. (b) DS SSPP straight line. (c) Microstrip coupled lines. (d) DS SSPP coupled lines.

Figure 8

Measured (solid) and simulated (slotted) results of microstrip and DS SSPP lines. (a) $|S_{21}|$. (b) $|S_{11}|$.

Figure 9

Measured (solid) and simulated (slotted) results of microstrip and DS SSPP coupled lines for $G_{\mathit{cl}}$ = 1.00 mm. (a) $|S_{21}|$. (b) $|S_{31}|$. (c) $|S_{11}|$.

Figure 10

Measured results of microstrip and DS SSPP coupled lines for $G_{\mathit{cl}}$ = 0.10 (dotted), 0.75 (solid), and 1.00 mm (slotted). (a) $|S_{21}|$. (b) $|S_{31}|$.

Figure 11

Microwave ring resonators. (a) Layout of DS SSPP ring resonator. (b) Fabricated microstrip ring resonator. (c) Fabricated DS SSPP ring resonator. Both resonators have the same coupling structure in which $W_c$ = 0.20 mm, $L_c$ = 4.05 mm, and $G_c$ = 0.10 mm.

Figure 12

Calculated $Q$-factors of ring resonators at fundamental (slotted) and second harmonic (solid) resonant frequencies. (a) $Q_L$ and $Q_0$. (b) $Q_C$, $Q_D$, and $Q_R$.

Figure 13

Measured (solid) and simulated (slotted) S-parameters of (a) microstrip and (b) DS SSPP ring resonators.