A design method of an ultrawideband transition from doublesided parallel stripline (DSPSL) to coplanar waveguide (CPW) is proposed based on analytical expressions of characteristic impedance. The conformal mapping is used to obtain the characteristic impedance for each section of the transition within 3.7% accuracy as compared with the EM simulation results. An efficient and clear guideline for the design of the transition is proposed. The implemented transition performs less than 0.6 dB insertion loss per transition for frequencies from 40 MHz to 12 GHz and less than 1.2 dB insertion loss to 27 GHz, which well exceeds the previous results in the literature.
Balanced transmission lines such as coplanar stripline (CPS), slot line, and doublesided parallel stripline (DSPSL) have frequently been used for applications such as antenna feeds, double balanced mixers, power dividers, and so on [
On the other hand, coplanar waveguide (CPW) consists of three conductors on the top surface of a dielectric substrate, where the center conductor is a signal strip separated from two ground planes on each side by a narrow gap. As one of the unbalanced lines, CPWs have frequently been used as transmission lines in a variety of microwave circuits [
In the previous literature, designs for various DSPSLtoCPW transitions have been reported in [
In this paper, an efficient and clear guideline for the design of DSPSLtoCPW transition based on analytical expressions of characteristic impedance is presented. The conformal mapping method is applied to obtain the analytical expressions, and details of the analysis process are described. The proposed transition is also designed and implemented to exhibit ultrawideband performance from 40 MHz to 27 GHz.
The proposed structure of the DSPSLtoCPW transition is illustrated in Figure
Proposed configuration of the DSPSLtoCPW transition: (a) perspective view, (b) top view, and (c) bottom view.
The electric field lines at each crosssection in Figure
Electric field lines at each crosssection along the transition.
The dimensions of the designed transition are summarized in Table
Summary of the transition dimensions (unit: mil).









24  17  75  140  5  5  100  35 
In this paper, analysis of characteristic impedance and effective dielectric constant of the proposed DSPSLtoCPW transition is performed separately for two adjoining transitional structures: DSPSLtoCBCPW transition (AA′ to CC′) and CBCPWtoCPW transition (CC′ to EE′). Firstly, for the sections from AA′ to CC′, analytical expressions for characteristic impedances of the transitional structure can be obtained from [
Transitional structure from AA′ and CC′.
Next, the CBCPWtoCPW transition (sections from CC′ to EE′) is designed using the guideline in [
Top plane trace of the CBCPWtoCPW transition (
The configuration of the CPW structure with a ground aperture on the bottom layer is shown in Figure
The ground planes are in infinite extent, and all conductors are perfectly conducting with zero thickness.
The aperture is assumed to be symmetrical with respect to the center of the signal line.
The RF signal along the structure propagates as a quasiTEM mode.
The dielectric boundaries, which can be considered as perfect magnetic walls, exist.
Configuration of the CPW transitional structure: (a) the whole structure and (b) divided structures for analysis.
In this analysis, the influence of the bottom ground aperture is carefully considered as the aperture width increases, since the field leakage from the aperture may not be negligible. For the analysis, the structure is divided into three regions as shown in Figure
An analytical expression for Region I of the CPW can be found in [
Since Region II is also symmetric, only the right half region is considered in obtaining the capacitance as shown in Figure
Conformal mapping transformation for Region II.
The shape of Region III is carefully chosen to account for the effect of leakage field through the ground aperture on the bottom layer. The amount of leakage field flowing out of the ground aperture on the bottom layer will depend on the aperture width. In order to calculate the amount of capacitance caused by this leakage, the structure for Region III is modeled as Figure
Model of Region III with consideration of leakage field: (a) field distributions and (b) equivalent circuit model.
Figure
From (
The characteristic impedance and effective permittivity of the DSPSLtoCPW transitional structure (CC′ to DD′) can be obtained as the sum of capacitance contributions of the three regions. The total capacitance is obtained as the sum of (
The effective dielectric constant can be obtained as the ratio of the total capacitance to the calculated capacitance with air filling instead of the dielectric substrate
Figures
Characteristic impedance of the CBCPWtoCPW transition (CC′ to DD′) as a function of
With the proposed design process, the shape of the CBCPWtoCPW transition can be easily obtained using the analytical expression of characteristic impedance, (
Characteristic impedances of the CBCPWtoCPW transition (CC′ to DD′). The substrate is RT/Duroid 4003 with 8 mil thickness (dielectric constant: 3.38).
In order to demonstrate the performance of the proposed transition, a backtoback configuration of the DSPSLtoCPW transition was fabricated as shown in Figure
Picture of the fabricated ultrawideband DSPSLtoCPW transition in a backtoback configuration including DSPSLtomicrostrip transitions for ease of measurement: (a) top view, (b) bottom view, (c) measurement using universal test fixture, and (d) measurement setup.
The measured and simulated results of the return loss and insertion loss of the transition are compared in Figure
Simulated and measured insertion loss and return loss of the transition.
Table
Performance summary of DSPSLtoCPW transitions.
Bandwidth (GHz)  Insertion loss  Return loss  Dielectric constant of the substrate  

[ 

<1.8 dB  >10 dB  9.6 
[ 

<1.4 dB  >10 dB  6.15 
[ 

<1 dB (Some resonant points exist)  >10 dB (Some resonant points exist)  3.38 
This work 

<1.2 dB  >10 dB  3.38 
A design method of a novel ultrawideband DSPSLtoCPW transition based on analytical expressions of characteristic impedance is presented. In addition to reported analytical expressions of characteristic impedance for some sections of the transitional structure, conformal mapping is used to accurately obtain new analytical expressions of characteristic impedances for the CBCPWtoCPW transitional structure. Therefore, in this paper, an efficient and clear guideline for the transition design is established. The implemented transition using the proposed design process has demonstrated to possess ultrawideband performance excelling the previous reported results due to good impedance matching, smooth field transformation, and ground continuity. The transition design approach proposed in this paper is expected to find multitude of applications to circuits requiring balanced lines and CPW structures.
This research was supported by the National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012034753).