Broadside Coupler Channels 1 To 10 GHz

As Fig. 3 shows, a 3-dB coupler can be realized by using two 8.34-dB couplers connected in tandem. In this configuration, 3 dB = 20log and 8.34 dB = 20log.

Parameters C₁, C₂, and C_N are the coupling coefficients of each individual quarter-wavelength coupler stage, given by Eq. 1:

where Z_oe = even-mode impedance of each coupled line; Z_oo = odd-mode impedance of each coupled line; and Z₀ = 50 Ω.

Stripline was chosen as the transmission-line medium for the multisection coupler. The broadside coupling approach was also selected to realize the cascaded couplers (Fig. 4).

For a broadside stripline structure with δ = 0, the even-order and odd-order impedances are calculated by means of the equalities in Eq. 2¹²:

where: η₀ = the characteristic impedance of free space, which is equal to 120π.

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Since it is desirable to achieve maximum bandwidth with the coupler, the highest possible coupling coefficient must be achieved for the center section. To do this, the separation of the inner conductors must be kept to a minimum, and the separation of the two ground planes must be at a maximum for maximum coupling. Using a circuit substrate with low relative dielectric constant helps to achieve maximum coupling as well.¹¹

To meet these requirements, a suitable circuit-board material was needed to implement the coupler values of Table 1. Low-loss RT/duroid 5880 dielectric material from Rogers Corp., with relative dielectric constant (e_r) of 2.2, was selected for the coupler, using the material in thickness of 10 mils for the middle layer and 31 mils for the two other layers. The three dielectric layers were sandwiched between two metal plates to form the stripline coupler structure.

The multisection hybrid coupler requires a structure with various "jump" discontinuities in coupling values, known as a coupling profile . This is defined as the value of the coupling factor as a function of the distance along the coupler's propagation axis. To ensure that the 3-dB, 90-deg. coupler design provides suitable performance from 1 to 10 GHz, it will be designed and simulated over an initial bandwidth of 1 to 18 GHz.

By referring to the tables of coupling values found in refs. 2 and 13, it was found that a 19-section coupler would provide the needed performance. Those tables make it easy to obtain the even-mode impedances and corresponding coupling coefficients for each section. Once those values have been found, the LineCalc transmission-line calculator in ADS can be used to find the dimensions for each coupled section. Optimization of these values was also performed in the ADS software. These optimized dimensions were then used for the final design and simulation performed in the HFSS 3D EM software.

Figure 5 shows the 19-section coupler with tandem structure in the HFSS design environment. In Fig. 5, if port 1 is considered the input, then ports 2, 3, and 4 will be the direct, coupled, and isolated ports, respectively. Table 2 shows the final optimized values for length, line width (W), and offset between the up and down transmission lines (δ) in the broadside structure for each section of the coupler.

Figure 6, Figure 7, and Figure 8 present the results of simulations performed with HFSS, including direct-path insertion loss (S₂₁ in dB) and coupled-path insertion loss (S₃₁ in dB) in Fig. 6.

The insertion loss of both the direct and coupled paths is about 3 dB, indicating that the structure is performing as a 3-dB coupler. Figure 7 shows the phase difference between the direct-path and coupled-path channels in the coupler which, for a quadrature hybrid, should be about 90 deg. Figure 8 shows simulated return loss (S₁₁ in dB) and isolation (S₄₁ in dB) for the multisection coupler design. The simulation responses reveal that the amplitude imbalance is very good, at 0.6 dB, while the phase imbalance is acceptable, at 90 6 deg. The return loss and isolation are both better than 12 dB from 1 to 18 GHz (although the coupler was nominally designed for use from 1 to 10 GHz).

To fabricate the coupler, a commercial microwave substrate with relative dielectric constant, e_r, of 2.2 and circuit values of b = 72 mil and S = 10 mil was employed. RT/duroid 5880 substrates were used in fabricating the prototype coupler. Figure 9 shows the prototype hybrid coupler, measuring just 120 x 44 x 14 mm.

To create the broadside stripline coupler, the circuit pattern was formed on one side of two thicker laminate layers; another substrate layer was then placed between these two layers. The three substrate layers were then sandwiched between two aluminum plates to form the final stripline structure.

Measurement results for the fabricated coupler, shown in Fig. 10, Fig. 11, Fig. 12, and Fig. 13, show good agreement with the earlier simulations.

Figure 10 displays the measured scattering parameters from 1 to 18 GHz while Figs. 11(a) and 11(b) show the measured amplitude difference 21 (in dB) S₃₁ (in dB)> and phase difference 21) arg(S₃₁)> between the direct and coupled ports. The fabricated hybrid coupler offers respectable performance over the expanded frequency range of 1 to 18 GHz, in close agreement with simulated performance, and with good performance over the designed frequency range of 1 to 10 GHz.

Figure 12 shows the scattering parameters from 1 to 10 GHz, while Figures 13(a) and 13(b) show the measured amplitude and phase differences, respectively, between the direct and coupled ports.

Finally, Fig. 14 offers a comparison over the frequency range from 1 to 10 GHz. The measurements show amplitude imbalance maintained within 1.5 dB while the phase imbalance is controlled within 90 7 deg. from 1 to 10 GHz. In close match with simulations, measured return loss and isolation exceed 14 dB from 1 to 10 GHz.

S. MOHAMMAD HASSAN JAVADZADEH, Ph.D. Candidate, Sharif University of Technology, Tehran, Iran; e-mail: [email protected].

S. MOHAMMAD SAEED MAJEDI, Ph.D. Candidate, Ferdowsi University, Mashhad, Iran; e-mail: [email protected].

FOROUHAR FARZANEH, Professor, Sharif University of Technology, Tehran, Iran; e-mail: [email protected].

Acknowledgment

This project was supported by Baregheh Communication Co. (Tehran, Iran).

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