Microstrip and CPW Power Divider Design - Chapter 8

Microstrip and CPW Power Divider Design

PathWave Advanced Design System (ADS)

Theory

A power divider is a three-port microwave device that is used for power division or power combining. In an ideal power divider, the power going into port 1 is equally split between the two output ports, and vice versa for power combining. Figure 1 demonstrates this concept. Power dividers have applications in coherent power splitting of local oscillator power, antenna feedback network of phased array radars, external leveling and radio measurements, power combining of multiple input signals, and power combining of high-power amplifiers.

Objective

To design various types of power dividers at 3 GHz and simulate the performance using ADS.

T-Junction Power Divider

The different types of power dividers are T-junction, resistive, Wilkinson, and hybrid coupler. The T-Junction power divider is a simple 3-port network and can be implemented in any kind of transmission medium such as a microstrip, stripline, coplanar waveguide, etc. A 3-port network cannot be lossless, reciprocal, and matched at all the ports. Therefore, since a T-junction power divider is lossless and reciprocal, it cannot be perfectly matched at all of the ports. The T-Junction power divider can be modeled as a junction of three transmission lines.

Wilkinson Power Divider

The Wilkinson power divider is a robust power divider with the output ports matched, with the reflected power dissipated. This provides better isolation between the output ports when compared to the T-Junction power divider. The Wilkinson power divider can also be used to provide arbitrary power division. The geometry and transmission line equivalent of a Wilkinson power divider is shown in Figure 10. In this section, both a lumped element and distributed element Wilkinson Power divider will be simulated.

Results and Observations

Figure 13 shows that there is very little reflection into each port at the design frequency of 3 GHz. Figure 14 shows that there was also about half power going from the input port into each of the output ports. Note that the schematic simulation does not take into account the effect that the layout will have on the results. This will be shown in a later simulation. Figure 15 also shows that there is little coupling between the two output ports. This is a direct result of adding the isolation resistor.

Results and Observations

Figure 21 shows that there is more reflection into each of the ports when compared to the discrete component model. Figure 22 shows that there is still about half power going from the input port into each of the output ports. Figure 23 shows that there is less coupling between the output lines when compared with the T-junction power divider. This shows the isolation resistor was effective. None of these traces match up exactly with the discrete element simulation. This is to be expected, as the layout-based model takes into account electromagnetic effects, such as edge effects and added parasitics of the microstrip layer. This will negatively impact the performance of the power divider.

Coplanar Waveguide T-Junction Power Divider

Coplanar waveguides (CPW) are a type of waveguide that is fabricated on a printed circuit board. Unlike traditional schematics on a PCB, a waveguide relies on the spacing between the traces to guide the wave through the circuit. This principle applies in a CPW T-junction power divider. The layout below is similar to that of the T-junction power divider but must be slightly modified to operate as a waveguide.

Design Flow of CPW T-Junction Power Divider

1. Select an appropriate substrate of thickness (h) and dielectric constant (ℇr) for the design of the power divider.

2. Calculate the wavelength λg from the given formula λg = c√εrf

Where

• c is the velocity in air
• f is the frequency of operation of the coupler
• ℇr is the dielectric constant of the substrate

3. Synthesize the physical parameters (length and width) for the λ/4 CPW line with impedances of Z0 and √2 Z0.

CPW T-Junction Power Divider Simulation

1. Calculate the physical parameters of the CPW T-junction power divider using the parameters from the earlier section. Use LineCalc to synthesis the length, width, and gap of the 50 Ω (Z0) and 70.7 Ω (√2 Z0) lines. The LineCalc windows are shown in Figures 24 and 25. The physical parameters for the lines are as follows:

50 Ω Line:

• Width = 3 mm [fixed]
• Gap = 0.37 mm
• Length = 15.96 mm
• 70.7 Ω Line:
• Width = 1.5 mm [fixed]
• Gap = 0.69 mm
• Length = 15.67 mm

Results and Observations

Figure 30 shows that there is minimal reflection at the input port, but more reflection exists at the output ports. This is to be expected, as creating a match was not a consideration in the design of the power divider. Figure 31 shows that just under half power was sent to each of the output ports. This is to be expected, as the layout simulation takes into account the electromagnetic effects, which will lead to power dissipation along the waveguide. Figure 32 shows that coupling exists between the two output ports. While this is not ideal, it is expected, as there are no components in the design intended to reduce the coupling between the two ports (as had been done for the Wilkinson power divider).