Applications of Ferroelectrics for Communication Antennas HRL Research Laboratories Microelectronics Laboratory Jonathan Lynch, jjlynch@hrl.com Joe Colburn, colburn@hrl.com Dave Laney, dclaney@hrl.com Amit Nagra, UCSB Bob York, UCSB This work supported by DARPA FAME Program 6/16/00 1
Low Cost ESA, Fact or Fiction? Goal: To develop and produce an affordable electronically scanned, high gain antenna with polarization diversity Many technologies lend themselves to an ESA solution Constraints on cost, polarization, frequency, and sidelobes limit technology areas to traditional ESAs, reflectarrays, and smart lenses. Due to the large number of elements required, individual electronic component placement is costly. Size constraints often prohibit the use of inefficient antenna solutions, given a particular G/T spec. Large area fabrication of electronic components is key to low cost solution For a high gain antenna with 000 elements, the cost of the electronics must be pennies per element. Pick and place of components can significantly increase cost. Large area (simultaneous) fabrication of electronic devices can potentially meet stringent cost goals. MEMS offer the potential for large area fabrication of low loss phase shifters Barium Strontium Titanate (BST) is a ferroelectric material that has potential for large area fab of phase shifters Conventional ESA Multifaceted arrays Smart Lens Antenna
Reflect Arrays May Provide a Solution Reflect array antennas may provide a low cost ESA solution The array collects incoming radiation and reflects it to a collector feed. The feed contains a single LNA, power amplifier, polarization switches, etc. The array consists of a large number of elements, each with a low loss electronic phase shifter. Minimal phase shifter insertion loss is critical to avoid the need for multiple LNAs. Low loss phase shifter solutions Two technologies offer great promise in providing a low cost phase shifter solution, and can potentially be fabricated en masse. Microelectromechanical devices: RF MEM switches can be used to provide very low loss phase shifters with discrete phase states. Ferroelectric materials, such as Barium Strontium Titanate (BST), can potentially provide low loss phase shifters with continuous tuning Reflect Array Architecture MEM or BST Ground Plane Center Conductor Ground Plane CPW Phase Shifter 3
Ferroelectric Phase Shifters BST Ground Plane Input θ θ θ θ Output Center Conductor Ground Plane CPW Phase Shifter jb 0 jb 1 jb jb N 1 θ = β L = electrical length of T - line section (rad) bn = ω CnZo R rl = L Z Efficient phase shifters can be produced using a transmission line that is periodically loaded with variable capacitors (e.g. York/Nagra, UCSB). The effective propagation constant on the loaded transmission line is controlled by the variable capacitances Variable capacitors are realized using a ferroelectric, such as Barium Strontium Titanate Coplanar waveguide offers convenient fabrication Tuning voltage is applied to the center conductor of the CPW line CPW losses can be minimized with substrate micromachining Simple structure is compatible with large area fabrication With the proper design, the impedance match can be maintained over a wide tuning range and bandwidth o 4
Uniformly Loaded Transmission Line θ θ θ θ θ 1 j b jb High performance phase shifters can be realized using low loss transmission lines uniformly loaded with shunt capacitors jb Resulting slow wave structure has a complex characteristic impedance (eigenvector) that must be matched to the complex load. Making the first and last capacitor values half of the others satisfies impedance match conditions for the imaginary part of the characteristic impedance. Choosing the proper load resistance matches the real part of the characteristic impedance. Simulations show that a transmission line loss tangent of.005 and Capacitor loss tangent of.01 gives maximum insertion loss of ~1.8 db for 360 degree phase shift (neglecting metal losses) True time delay response permits wideband ESA operation Structure can be made compact by meandering line jb jb 1 j b 5
Passband/Stopband Characteristics of the Uniformly Loaded Line The uniformly loaded transmission line has a repeating pattern of passbands and stopbands. The passband/stopband behavior is often referred to as Bragg scattering and the structure a photonic bandgap structure in comparison to scattering of electrons or photons in a crystal lattice. Microwave engineers know it simply as a filter. The frequency response can be determined by plotting the frequency dependence of the shunt elements on the same graph (shown in red). b 4 0 4 Z b = o ω L e j b= ω C cot θ bandstop bandpass π/ π π 3π e j tan θ θ 6
Phase Shifter Optimized for Impedance Match Given a certain transmission line section electrical length θ, values for b and r L can be chosen to optimize the input match. The optimum values produce a return loss that is stationary with respect to small changes in the capacitor values. The optimum capacitor susceptance b and the optimum load resistance r L are given by: bg b= cot θ, = sin θ r L bg The approximate number of sections required for a 360 degree phase shift is given by: N Cmax + Cmin 1 π Cmax C cos min θ bg 0.5 1 1.5 Return loss (db) Return loss (db) Return loss (db) f=10 GHz 0 50 0 0 normalized capacitance f=6 GHz 40 0.5 1 1.5 0 0 normalized capacitance f=17 GHz 40 0.5 1 1.5 normalized capacitance 7
Non-uniform Capacitance Distributions A number of non-uniform capacitance distributions were investigated Uniform distribution with load matching A significant return loss improvement can be achieved by cutting the first and last capacitance values in half (load matching). Maximally flat distribution High return loss can be maintained over a large bandwidth and tuning range with a slight increase in insertion loss. Tchebycheff distribution This distribution tended to by more sensitive to capacitance variations, and therefore gave inferior performance over wide tuning ranges. Other distributions We also investigated power law, geometric series, and binomial distributions. Results were generally inferior to matched uniform and/or maximally flat. Uniform distribution with values chosen for optimum impedance match generally gave the best results. 8
Conclusions Of all the possible ESA architectures, the reflect array offers many potential cost advantages. Low insertion loss phase shifters are the key to achieving a low cost ESA using a reflect array. Low insertion loss allows the use of one LNA and one power amplifier for the entire array. By fabricating large numbers of phase shifters simultaneously the ESA cost can be made extremely low. Thin film ferroelectrics can be used to realize low loss phase shifters, and can be fabricated en masse. Loaded transmission line phase shifters using ferroelectrics can be designed to give excellent return loss, insertion loss, and phase shift over wide bandwidths. The key parameter is ferroelectric loss tangent. When the thin film ferroelectric loss tangent falls below ~0.01 in the millimeter wave range, low cost ESAs will become a reality. 9