Download Micromachined Acoustic Programmable Tunable Finite Impulse

School of Electrical, Computer and Energy Engineering
PhD Final Oral Defense
Micromachined Acoustic Programmable Tunable Finite Impulse Response (FIR) Filters
for Microwave Applications
by
Ameya Galinde
July 24, 2013
2:30 pm
GWC 305
Committee:
Dr. Abbas Abbaspour-Tamijani (chair)
Dr. Junseok Chae
Dr. George Pan
Dr. Stephen Phillips
Abstract
This dissertation proposes a miniature FIR filter that works at microwave frequencies,
whose response can ideally be digitally programmed to change its center frequency,
bandwidth, and response shape. Such a frequency agile device can find applications in
cellular communications and wireless networking. The basic concept of the FIR filter
utilizes a traveling wave tapped-delay line excited by capacitive transducers. By using a
low loss acoustic waveguide of appropriate geometry, a miniature device with acceptable
impedance can be fabricated. This filter can be programmed with a switching time on the
order of microseconds. The input RF signal is applied at various locations on the acoustic
waveguide at one end that excites waves of a propagating acoustic mode with varying
spatial delays and amplitudes which interfere as they propagate. The output RF signal is
picked up at various locations along the propagating structure at the other end, resulting
in a tapped delay line that is essential to the construction of an FIR filter. The basic
principle used for tuning in this design is that the frequency response of the input and
output transducer arrays can be shaped by controlling the DC voltage profile applied to
the individual transducer fingers in each of these arrays. Equivalent circuit modeling of
the capacitive transducer, acoustic waveguide and transducer-line coupling is presented
in this dissertation. A theoretical model is developed for the filter from a general theory
of an array of transducers exciting a waveguide. This model is used to obtain a set of
filter design equations that help in the overall filter design process. A MATLAB based
circuit simulator is also developed to simulate the filter responses. An example
waveguide structure is introduced to illustrate the overall design process. Design
parameters optimized using the design equations are presented for this structure.
Simulation results are presented and compared to values estimated by the theoretical
model. In general, it is observed that high substrate losses can significantly degrade the
filter performance in terms of the insertion loss and filter bandwidth. Microfabrication of
a filter similar to the example structure is briefly discussed. A few other filter structures
that were considered early on in the work are presented along with some earlier analysis
techniques. During the initial phase of this project, a filter structure based on ridges
etched in single crystal Silicon was approved for microfabrication after a basic analysis.
A semi-analytical method to obtain propagating elastic modes of a ridge waveguide
etched in an anisotropic crystal is then presented. This analysis was to aid in the design
and testing of the ridge waveguide based filters. Details of the fabrication of this filter
and the challenges faced are discussed in detail. Due to several unavoidable fabrication
hurdles, the final devices based on this particular ridge waveguide design were not in a
condition acceptable enough for testing. Finally, future work and a few other alternative
designs are discussed that can have a better chance of success. Analysis and modeling
work to this point has given a good understanding of the working principles, performance
tradeoffs and fabrication pitfalls of the proposed device. With the appropriate acoustic
waveguide structure, the proposed device could make it possible to realize miniature
programmable FIR filters in the GHz range.