UWB Power Divider Has Stopband Filtering

UWB Power Divider Has Stopband Filtering

This multilayer power divider features good impedance matching at all ports with high isolation and excellent amplitude and phase balance across a wide bandwidth.

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A power divider with bandpass response and high isolation between ports was developed for use in ultrawideband (UWB) communications systems. The design, which incorporates two slotline resonators, achieves excellent amplitude and phase balance between channels and ports.

Featuring a frequency range of 3.8 to 10.0 GHz, is well suited for UWB applications. One of the resonators helps improve isolation between output ports, while the other helps generate a tunable transmission zero so that a wide upper stopband could be obtained.

UWB power divider  parameters
1. This diagram shows the parameters of the UWB power divider as used for simulations with the HFSS finite-element software.

Interest in UWB systems and components has increased steadily since about 2002, when UWB communications systems concepts were first introduced and developed. A number of components for UWB applications have been designed and fabricated since that time.1-6 Power dividers are among the most crucial of RF/microwave devices, since signals must be split and channeled through communications signals for proper detection and processing.

To meet the requirements for UWB systems, a number of UWB power dividers have been developed, with different configurations based on traditional Wilkinson power dividers.7-9 For the sake of shrinking the dimensions of power dividers for UWB applications, slotline techniques have been studied and applied. As an example, a single-wavelength slotline was applied to structure a noncoplanar, in-phase UWB compact power divider with a flat passband over the UWB frequency range.10 By adding one circular stub at the end of each output branch as compensatory circuits, it was possible to extend the operating frequency range of the power divider, and also helped to achieve a flat passband.11,12

However, due to the inherent properties of three-port networks, a lossless and reciprocal three-port network cannot obtain good impedance matching at all ports simultaneously. As a result, for the UWB power divider designs that had been developed with slotline techniques, the impedance matching at the output ports was poor, with low isolation between output ports.

Even worse, the bandwidths of the stopbands for these designs were relatively narrow,13,14 enabling undesired signals to enter into systems employing these UWB power dividers. Of course, additional filtering can be added to the systems to suppress the unwanted signals, but this adds to the cost of the system.

To meet the needs of UWB systems for power dividers at reduced costs and compact sizes, a novel compact in-phase multilayer power divider with wide stopband was designed using slotline techniques. The output branches of the proposed UWB power divider are placed on the same layer symmetric to the input arm to achieve in-phase output signals.

To improve the impedance-matching limitations that are traditionally suffered by UWB power dividers based on slotline techniques, an isolation resistor was added to the power divider circuitry as used in a traditional Wilkinson power divider. To obtain a wide stopband with the power divider, a one-half-wavelength slotline resonator was placed at the input port to generate another transmission zero in the stopband, resulting in a wide stopband range.

The results from computer simulations and measurements agree quite closely for this UWB power divider, showing that the multilayer circuit design achieves good impedance matching at all ports, with high isolation between ports and excellent amplitude and phase balance between output ports over the UWB frequency range from 3.8 to 10.0 GHz.

Wideband filter
2. This schematic diagram shows the wideband filter based on a half-wavelength slotline that is incorporated into the UWB power divider.

Figure 1 shows the configuration of the proposed in-phase UWB power divider, which consists of three layers (top, middle, and bottom layers). The input port (1) is located in the bottom layer, while the output ports (2 and 3) are placed in the top layer. The slotlines are etched in the middle layer, which is the ground plane. The length of the first slotline is decreased relative to slotlines used in previous UWB power-divider designs so that an isolation resistor could be installed at the ends of the output branches.

Two short-circuited circular slotline stubs and one open-circuited circular microstrip stub, which act as compensatory circuits, were included to reduce passband amplitude ripple. The short microstrip stubs at the ends of the output branches can also help increase the passband flatness, at the same time making installation of the isolation resistor easier.

Slotline frequency response
3. The plot shows the frequency response for the slotline as a function of length.

Figure 2 shows the half-wavelength slotline resonator, consisting of a UWB filter with microstrip lines. This filter can achieve several transmission poles for an UWB operating frequency range and generate one transmission zero at stopband. By adjusting the length of the slotline, the transmission zero will shift towards a high-frequency band or a low-frequency band. Figure 3 offers simulated results for the UWB filter designed for this power divider, indicating that the transmission zero will move toward the lower-frequency band with increasing length and move toward a higher-frequency band with decreasing length.

UWB power divider
4. This is a photograph of the UWB power divider as it is fabricated on commercial PCB material.

The prototype multilayer in-phase power divider with bandpass response and high isolation was designed and fabricated on 4350B printed-circuit-board (PCB) material from Rogers Corp. with relative dielectric constant of 3.48, loss tangent of 0.0037, and thickness of 0.508 mm. Figure 4 shows photographs of the proposed power divider. The size of the fabricated power divider is 24 × 30 mm.

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Simulating the Divider

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The UWB power divider was computer simulated with the aid of Version 13.0 High Frequency Structure Simulator (HFSS) electromagnetic (EM) finite-element simulation software from ANSYS Corp. The dimensions of the power divider’s parameters from the simulation software (and as detailed in Fig. 1) are: wm = 1.15 mm, wm1 = 1.4 mm, wm2 = 0.91 mm, d = 0.5 mm, lst = 6.92 mm, la = 1.72 mm, ls1 = 16.09 mm, ls = 6.21 mm, lm1 = 4.93 mm, lm2 = 6.64 mm, lm3 = 9.62 mm, ws = 0.23 mm, ws1 = 0.26 mm, rs = 0.93 mm, and rm = 1.86 mm. The isolation resistor is R = 100 Ω. For the sake of compatibility with other systems, the port impedances are fixed at 50 Ω.

Return loss
5. This is the return loss for the UWB power divider at all ports and isolation between output ports.

Figures 5, 6, and 7 provide comparisons of simulated and measured results for the UWB power divider, showing fairly good agreement across the wide frequency range. In fact, light differences can be traced from machining errors. The measured return losses are more than 17 dB for port 1, more than 15.5 dB for port 2, and more than 15.4 dB for port 3 across the frequency band from 3.8 to 10.0 GHz. Isolation between output ports is better than 16 dB. Insertion losses, including losses from SMA connectors, are about 1 ± 0.7 dB and 1 ± 0.6 dB. The amplitude unbalance is ±0.2 dB for the same frequency range while the phase unbalance is about ±2 deg.

Transmission characteristics
6. These are the transmission characteristics of the proposed UWB power divider.

In addition, the group delay exhibits good flatness across the bandwidth, with maximum difference of just 0.13 ns across the wide frequency band. Signal suppression is about 20 dB for the frequency band from 11.5 to 17.5 GHz, and attenuation reaches 20 dB at 2.5 GHz, ensuring that the power divider can reject undesired signals.

Phase differences and group delay
7. These plots show the phase differences and group delay for the proposed UWB power divider.

As was demonstrated, it is possible to construct a novel in-phase multilayer power divider with a bandpass response and high isolation by using slotline techniques. The width of the stopband was successfully extended by introducing a one-half-wavelength slotline resonator to the power-divider circuitry. The measured and simulated results for the UWB power divider show good impedance matching at all ports by introducing one resistor at the ends of the output branches. The UWB power divider shows good amplitude and phase balance between its output ports from 3.8 to 10.0 GHz, making it a suitable candidate for a variety of UWB applications.

Acknowledgments

This work was sponsored by the National Natural Science Foundation of China (Grant No. 61006026) and the Fundamental Research Funds for the Central Universities (Grant No. ZYGX2012J030).

Long Xiao, Engineer

School of Electronic Engineering, University of Electronic Science and Technology of China, Chengdu, People’s Republic of China, 611731

Hao Peng, Engineer

School of Electronic Engineering, University of Electronic Science and Technology of China, Chengdu, People’s Republic of China, 611731

Tao Yang, Engineer

School of Electronic Engineering, University of Electronic Science and Technology of China, Chengdu, People’s Republic of China, 611731

References

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