Wilkinson Divider Powers TV Transmitters

As digital technology infiltrates an increasing number of applications, RF/ microwave components are handling more bandwidth to keep pace. For broadcast UHF television, for example, a power divider must handle the entire frequency range from 470 to 806 MHz with relative low loss and good amplitude and phase characteristics to ensure the integrity of the transmitted digital signals. For this purpose, a three-way Wilkinson power divider was developed based exclusively on microstrip lines. Two prototypes will be presented, showing good performance with respect to essential parameters, including insertion loss and return loss.

In a UHF broadcast television application, signals must be split at the transmitter input to feed every power amplifier accordingly. Power dividers are passive devices used to realize this task, delivering in-phase signals for each power amplifier in the transmitter signal chain. Power dividers can be realized as different types of components, including directional couplers and resistive power dividers, which contain lumped elements only, and Wilkinson power dividers. A Wilkinson power divider can be implemented with transmission lines, including coaxial lines, stripline, and microstrip.¹ The Wilkinson power divider design presented here prorepresentvides extremely broadband coverage and can be used to handle the entire ultrahighfrequency (UHF) band, making it a suitable candidate for use with UHF television transmitters.

A basic Wilkinson power divider has three ports with quarter-wavelength transmission lines and a load for imbalance between ports (Fig. 1). It splits a signal applied at port 1 into two symmetrical signals at ports 2 and 3.

Performance is directly influenced by the impedance connected to the ports 2 and 3. In practice, the impedance of the output ports is 50 Ohms. So, the only way to change the ratio of signal division lies in the λ/4 sections. If different λ /4 sections are used and their impedance values are correctly calculated, different values of output signal levels can be obtained, and no part of the input signal will appear at the resistance R. The calculation of the resistance R is based on the impedances involved in the Wilkinson structure, as represent ed in eq. 1:

where Z_a and Z_b are the impedances presented by the λ/4 sections of the Wilkinson divider.^1,2

The microwave architecture can be implemented in three steps. The first step involves building a 1:2 asymmetrical Wilkinson power divider, with different λ/4 sections, allowing asymmetrical division of the input signal into 1:3 and 2:3 paths. Then, an ordinary 1:2 Wilkinson power divider is designed, to divide the 2:3 of the input signal equally between the output ports. Finally, the 1:3 Wilkinson divider is assembled by joining the asymmetrical structure of the first step with the symmetrical structure of the second step. Before implementing the microwave structure, simulations of both prototypes were performed using Eagleware software (now available from Agilent Technologies). The initial simulations showed good results for the 1:2 and 1:3 Wilkinson power divider structures (Fig. 2). All of the scattering parameter (S_i,j) results, including return loss and insertion loss, were satisfactory.

Both dividers were implemented using microstrip. Further details of its impedance calculation can be found in refs. 3 and 4. The substrate material was RO5880 from Rogers Corporation, with h = 0.762 (mm), e_r = 2.2, and thickness of 70 m. A minimum insertion loss and a good matching, in general with a strong demand better than 20 dB can be reached for the entire UHF band from 470 to 806 MHz.

Figure 3 shows the layout of the divider implemented in the laboratory. This 1:2 asymmetrical structure sends one-third of the input power to port 2 (a drop by 4.8 dB) and the remaining portion of the input power to port 3 (a drop of 1.77 dB). The asymmetrical Wilkinson power divider presents three different impedances values, as seen by the different thickness of the lines in the layout. Figure 4 shows the final prototype that was implemented. The 1:3 Wilkinson power divider implementation includes the 1:2 asymmetrical Wilkinson power divider. Initially, the power is asymmetrical divided and then an ordinary 1:2 Wilkinson symmetrical power divider is implemented to complete the power division. The new divider has four ports, an input and three outputs, all designed for similar levels of output power and phase. Figure 5 shows the constructed 1:3 Wilkinson power divider.

The 1:2 asymmetrical Wilkinson divider and the 1:3 symmetrical Wilkinson divider were simulated and designed for optimal results in both prototypes. Measurements were made with the Agilent E5062A vector network analyzer (VNA) from Agilent Technologies across the full UHF band. The results displayed in Fig. 6 show the S₂₁ values for the 1:2 asymmetrical Wilkinson power divider.

The S₂₁ results for the asymmetrical port were close to the theoretical value of 4.8 dB, corresponding to one-third of the input power. Figure 7 shows port isolation based on measurements of S₂₂. The impedance match is better than 23 dB across the UHF band.

Next, measurements for the 1:3 symmetrical Wilkinson power divider were performed. Figure 8 shows S₄₁ for one output port, with the other two ports bearing similar results. The new Wilkinson divider showed good symmetry among ports.

The insertion loss was close to the theoretical value, with a small imbalance between ports. Figure 9 shows the matching of this port, for S₄₄. Phase imbalances among ports are minimal. Table 1 shows experimental data, while Table 2 presents results of S_ij parameters for 1

REFERENCES

1. E. Wilkinson, "An N-Way Hybrid Power Divider," IRE Transactions on Microwave Theory and Techniques, Vol. MTT-8, January 1960, pp. 116-118.

2. J. A. Justino Ribeiro, Engenharia de Microondas Fundamentos e Aplicaes, First Edition, rica, Sao Paolo, Brazil, 2008.

3. David M. Pozar, Microwave Engineering, 2nd Ed., Wiley, New York, 1998.

4. K. C. Gupta, R. Garg, I. J. Bahl, and P. Bhatia, Microstrip Lines and Slotline, Artech House, Norwood, MA, 1996.

5. Q. Guo, Y. Ma, and J. Ju, "A Novel Broadband High-Power Combiner," in the Proceedings of the 2005 IEEE Asia Pacific Microwave Conference, Sushou, December, 2005.

6. S. Horst, R. Bairavasubramanian, M. M. Tentzeris, and J. Papapolymerou, "Modified Wilkinson Power Dividers for MMWave Integrated Circuits," IEEE Transactions on Microwave Theory and Techniques, Vol. 55, No. 11, November, 2007.