Embedded RF Components and Devices in 3D LCP Package:
Part 3: Power Dividers on LCP
By Swapan K. Bhattacharya, Stephen Horst, and John Papapolymerou, Georgia Institute of Technology
The Wilkinson power divider is one of the most commonly used components in wireless communication systems for power division and/or combination. Such power divider consists of two λ/4 branches of the transmission line and a termination resistor of 100Ω where λ is the wavelength of the transmission line. Traditionally, chip resistors are used in the power divider design consuming larger real estate and imposing higher parasitic effects in comparison to the embedded resistors. This article discusses the application of embedded thin-film resistors as power divider in the frequency range 27 to 110GHz on the liquid crystal polymer substrate. In the X-band (8 to 12.5 GHz) and Ka-band (26.5 to 40 GHz), a traditional Wilkinson power divider design can be implemented with a 100Ω resistor. However, the power dividers in the V-band (50 to 75 GHz) and W-band (75 to 110 GHz) require additional transmission lines between the resistor and the quarter-wave transformers. Excellent circuit performance at mm-wave frequencies are reported with these designs. The fabricated circuits on LCP substrate utilized commercially available thin resistive film on a carrier copper foil, therefore, paving the path for low cost processing technologies for high frequency applications.
Power dividers have a wide range of applications in microwave circuits such as push-pull amplifiers, balanced mixers, and antenna distributions.1 Simply stated, a power divider splits an input signal into two or more identical, potentially phase-shifted, output signals. Often there is a desire to isolate the output ports so that reflected signals do not propagate to other paths of the system. An elegant solution to this problem is the Wilkinson power divider.2,3 The Wilkinson power divider uses a resistor to isolate its two output ports, while maintaining a matched impedance at all ports (Figure 1a). When implemented in a planar 2D format on PWB or thin substrate, a common dilemma in Wilkinson power divider design lies in separating the quarter-wave transformer arms to prevent strong mutual coupling, yet keeping them close enough to place a resistor between them. At lower microwave frequencies, a simple U-shaped bend is sufficient to meet these objectives. Scaling such a design to millimeter wave frequencies introduces physical layout constraints that make a simple solution difficult to achieve.
Frequency scaling difficulties are caused by the properties of the microstrip transmission lines. The characteristic impedance (Zo) of these lines are largely a function of signal conductor width and substrate height, while the electrical length of the arm segment, θ, is primarily dependent on the effective wavelength at the desired frequency of operation.3. Therefore, as frequency increases, the length of the quarter-wave transformers decreases, while the width remains relatively constant. When these lines include several 90° bends, creating the necessary layout without overlap, RF performance becomes increasingly difficult to achieve in the physical space available.4
There are several ways to gain space for the transmission lines. One way is to reduce substrate thickness. This will decrease the conductor width necessary to achieve given characteristic impedance, but will also add to the conductor losses present in the circuit.5 Another approach is to decrease the physical space required by the resistor. A separate packaged resistor component has traditionally been used for Wilkinson divider designs. These packaged component form factors come in fixed sizes, add undesired parasitics at microwave frequencies, and restrict multilayer system integration. For these reasons, it is desirable to have embedded resistors that can be scaled to be as large or as small as necessary. Lamination of thin-film resistive foil on LCP and subsequent etching to define embedded resistors can be an attractive option in contrast to current surface mount technology.
Design Considerations at Ka-band
The standard Wilkinson design places the resistor between the branches of the output ports to isolate any return signals between them. This differential placing also makes the Wilkinson divider useful in creating baluns.6 In the forward case, the divider operates by splitting the input signal with a simple T-style junction. Each path then travels through a λ/4 transformer with an impedance designed to step the signal up to twice the system impedance. This signal is then placed across a resistance of twice the system impedance through the superposition of odd and even modes. This halves the signal to the original system impedance with half of the original input power on each branch. In the reverse case, an incoming signal on an output port is presented with aλ/4 transformer and a resistor with twice the system impedance. In the odd mode, the transformer will appear open and the resistor will be halved to the system impedance due to the virtual short created by the symmetry. In the even mode, the resistor is effectively eliminated from the circuit by the virtual open, while the λ transformer presents the system impedance from the symmetrically doubled input port. Summing the two modes together, in an ideal case, the incoming signal on an output port is terminated at the system impedance with no reflections.
Starting with a 25Ω/square resistor foil, the resistor was made as small as possible to meet the 100Ω resistor required by the divider. This yields a 4:1 ratio with dimensions of 400µm in length by 100µm in width shown in Figure 1b. The quarter-wave transformers were then wrapped around the resistor in a semi-circular fashion. Numerical field solver simulations revealed that the best configuration using the maximally compact topology shown was achieved by making the bend radius as large as possible without impeding on the 50µm lines needed for the output ports. Finally, the output and input line lengths were set to give an arbitrarily determined 180° phase shift at the output at 35GHz. Disregarding the CPW-to-microstrip transitions present in Figure 1b, the circuit measures approximately 1.99mm in width by 1.63mm in height which is significantly smaller than the divider presented by Antsos.7
Measurements of the Wilkinson divider were taken using a four port Agilent 8364B PNA. Figure 2a shows the insertion loss slightly less than 0.5dB throughout Ka-band, from 27 to 40GHz. Return loss for each port, (Figure 2b) stays below 20dB with the exception of the input port at the band fringes. Isolation between the output ports, (Figure 2c) is on par with the return loss measurements. An analysis of the phase through each branch of the divider shows a constant shift of 2.5° from the designed reference.
Modified Wilkinson Power Divider at V- and W-bands
A modification of the Wilkinson power divider is constructed with simple planar implementation while maintaining performance.5 By adding transmission lines between the resistor and quarter-wave transformers of the traditional design, the conditions of being reciprocal, isolated between the output ports, and matched at all ports were met. The design is particularly useful at millimeter-wave frequencies where reduced physical dimensions make a circuit configuration suitable for low-cost package-level implementation which is difficult using traditional methods. The designed circuit (Figure 3a) gives 0.3-dB excess insertion loss, 19-dB isolation, and 50° bandwidth at V-band. At the W-band, the circuit gives 0.75-dB excess insertion loss, 24-dB isolation, and 39% bandwidth. Fabricated power divider circuits are shown in Figure 3b. The measurement results up to 110 GHz5 are presented in Figures 4a through 4d. The results compare well with W-band power dividers realized on silicon substrate using MCM-D technology.8 The resistive material was 20Ω/square. NiCr deposited on low-loss Benzocyclobutene (BCB) dielectric.
Conclusions
Wilkinson power dividers on LCP can be implemented at millimeter wave frequencies with excellent circuit performance using commercially available 25Ω/square resistive foils. Traditional Wilkinson power divider design can be utilized up to Ka-band. For V-band and W-band regimes, an extension of the design with addition of transmission lines can be implemented with ideal RF performance characteristics. Therefore, mm-wave Wilkinson power dividers can be realized on organic substrate without the need of discrete resistors.
Acknowledgments
The authors would like to thank Rogers Corporation for supplying the LCP substrate used in this research, and Ticer Technologies for supplying the resistor foils.
References
1. A. Mohra, “Compact dual band Wilkinson power divider”, 25th National Radio Science Conference, 2008.
2. E. J. Wilkinson, “An N-way hybrid power divider”, IEEE Transactions on Microwave Theory and Techniques, Vol MTT-8, No. 1, 1960, pp 116-118.
3. D.M. Pozar, Microwave Engineering, 3rd ed., John Wiley and Sons, 2005.
4. S. Horst, S. K. Bhattacharya, J. Papapolymerou, M. Tentzeris, “Monolithic low cost Ka band Wilkinson power divider on flexible organic substrates”, 57th Electronic Components and Technology Conference, Reno, May 2007, pp 1851-1854.
5. S. Horst, R. Bairavasubramanian, J. Papapolymerou, M. Tentzeris, “Modified Wilkinson Power Divider for Millimeter-Wave Integrated Circuits”, IEEE MTT, Vol 55, No 11, 2007, pp 2439-2446.
6. M.A. Antoniades and G.V. Eleftheriades, “A Broadband Wilkinson Balun Using Microstrip Metamaterial Lines”, IEEE Antennas and Wireless Propagation Letters, Vol. 4, Issue 11, 2005, pp. 209-212.
7. D. Antsos, R. Crist, and L. Sukamto, “A Novel Wilkinson power divider with predictable performance at K and Ka-Band” IEEE MTT-S International Microwave Symposium Digest, 1994, pp. 907