Research Summary

Professor Mortazawi's research projects have been funded by ,ARL/BAE, Motorola, NASA, Lockheed Martin, ARO MURI, DARPA, MAFET-III, and DARPA, FAME and NSF programs.

1)      Spatial/quasi optical power combining amplifiers and transmit-receive arrays: The motivation for this work has been the lack of efficient solid state devices that can generate high enough power in the millimeter and sub-millimeter-wave region for radar, remote sensing and spaceborne communication.  This problem can be alleviated by spatially combining the power produced from many solid-state devices (conventional circuit level combiners suffer from high losses at mm-wave frequencies).  We have developed millimeter-wave circuits based on multiple layer structures that directly produce and amplify microwave beams Furthermore, we designed, constructed and delivered 33 GHz hard horn feeds for the excitation these amplifiers.  A software package was developed for analysis of hard horns and was provided to Lockheed-Martin.  Our effort within this project was fundamental in achieving the program goals (MAFET III review, March, 1999, Reston, Virginia).  This project has been funded by an ARO MURI and a DARPA MAFET III subcontract.

His contributions to this area are:

a)             
Developed a technique for the near field excitation of quasi-optical power amplifier arrays in order to design practical and efficient power amplifiers insertable into the already existing systems.  This technique is based on the design of “Hard Horn Feeds” which provide a uniform amplitude and phase across the array and has been adopted by Lockheed Martin Corporation.  Fig. 1a shows the general concept for the excitation of spatial amplifier array via hard horn feeds.  Figs 1b and 1c show the predicted and measured nearfield aperture power distribution for a 33 GHz hard horn.  Indeed by using this technique one can distribute the microwave energy uniformly across a large array.

Fig. 1a: A multilayer spatial amplifier array, Figs. 1b &1c: simulated and measured nearfield power distribution for a 33 GHz hard horn feed

b)      Developed the general concept and designed a multilayer spatial power amplifier array.  The most important feature of this amplifier array is the heat sinking and EM isolation provided by a ground plane separating the driver and the final amplifier stages (Fig. 1a). The most recent results include the design and construction of a 25 W Ka band (33 GHz) constrained fed spatial power amplifier through our collaboration with Lockheed/Martin (Figs. 2a,b, and c).  A 50 W array is currently being fabricated.  Furthermore Army Research Laboratory has shown a great interest in a slotted waveguide based spatial power amplifier design that we have developed.

Figs. 2a,b and c  A 25 W, Ka band spatial amplifier array with hard horn feed

Fig. 2d An X band power amplifier array for the fundamental study of spatial power combining

2)      Extended resonance power amplifiers and oscillators:  One of the distinguishing features of this technique is that resonance is achieved through mutual interaction of multiple devices connected to a transmission line.  This effectively places multiple solid state power devices in shunt.  There is no need for complications normally added in the design of conventional hybrid type power amplifiers.  The extended resonance circuits are very compact since they eliminate the need for matching circuits for the individual devices.  This type of structure is compatible with MMIC fabrication.  The extended resonance holds strong potential in wireless applications.  We have designed and demonstrated successfully extended resonance amplifiers at 1 GHz (class E with 70% power added efficiency) and at X band (10 GHz) and at Ka Band (33 GHz).  Based on our designs we have developed a new linearization technique that has a potential to impact the basic design of efficient power amplifiers in cellular phones (the most power hungry element in a cellular phone handset is the power amplifier stage. The design of highly efficient and linear power amplifiers is an important area of research in RF Communications.).  Photographs of a 1 GHz, 4 Watts class E and a 400 mW, 33 GHz power amplifier are shown in Figs. 3a and 3b.

Figs. 3a,b Photographs of a 1 GHz 4 Watts class E and a 400 mW, 33 GHz power amplifier

3)                  Tunable low loss thin film BST (barium strontium titanate) varactors for RF and microwave applications:  This project is currently supported by a DARPA FAME grant.  The main thrust of this project is to develop voltage controlled capacitors using very low loss thin film ferroelectric material.  This is because commonly used varactor diodes suffer form high losses at RF frequencies.  Furthermore, they require high voltages (20 to 30 V) to achieve 2:1 capacitance tuning.  The new voltage controlled thin film BST capacitors can achieve 3:1 tuning at voltages as low as 4 to 5 volts.  Several areas that will greatly benefit from incorporation of such tunable capacitors are: a) the design of efficient DC to DC converters, b) low loss IF and RF tunable filters and c) low phase noise oscillators for wireless applications.  Other applications of this material such as tunable antennas and phase shifters are currently under investigation.