Simsek Demir

Frequency Diverse Array Antenna with Periodic Time Modulated Pattern in Range and Angle

A general analysis of a frequency diverse transmit array antenna with a periodic modulated pattern in range, angle and time is presented. This antenna array system makes a continuous scanning in range and angle without using any phase shifters. The scanning is achieved using the frequency diversity by inserting a small amount of progressive frequency shift to each antenna element. The theory shows that there is the same modulation pattern in time, range and angle by taking the remaining two parameters fixed. The simulation results for radiation patterns of a binomial distribution array are presented. The expressions for determining the position and the angular bearing of a target for this type of antenna array system are given.

A Monolithic Phased Array Using 3-bit Distributed RF MEMS Phase Shifters

This paper presents a novel electronically scanning phased-array antenna with 128 switches monolithically implemented using RF microelectromechanical systems (MEMS) technology. The structure, which is designed at 15 GHz, consists of four linearly placed microstrip patch antennas, 3-bit distributed RF MEMS low-loss phase shifters, and a corporate feed network. MEMS switches and high-Q metal-air-metal capacitors are employed as loading elements in the phase shifter. The system is fabricated monolithically using an in-house surface micromachining process on a glass substrate and occupies an area of 6 cm times 5 cm. The measurement results show that the phase shifter can provide nearly 20deg/50deg/95deg phase shifts and their combinations at the expense of 1.5-dB average insertion loss at 15 GHz for eight combinations. It is also shown by measurements that the main beam can be steered to required directions by suitable settings of the RF MEMS phase shifters.

Exploitation of Linear Frequency Modulated Continuous Waveform (LFMCW) for Frequency Diverse Arrays

Frequency diverse arrays (FDA) are recently introduced. Not only the phased array and frequency scanning arrays, but also FDA provides electronic beam scanning ability. Moreover, electronic beam scanning is provided without the use of phase shifters or vector modulators. Nevertheless, reported studies on this concept do not provide practically implementable results. In this paper, fundamental expressions of the FDA operation and its typical implementation schemes are reviewed, and an ultimately useful scheme for implementing and FDA is introduced based on linear frequency modulated continuous waveform (LFMCW). The mathematical foundations of LFMCW FDA are developed and used to design a basic proof of concept structure. This structure is also implemented and the measurements related to the implementation are presented with discussions on the results.

A mathematical characterization and analysis of a feedforward circuit for CDMA applications

Feedforward is known to be one of the best methods for power amplifier linearization due to its superior linearization performance albeit with relatively poor power efficiency. Here we present the derived closed-form expressions, which relate the main channel power and distortion products at the output of a simple feedforward circuitry to the circuit parameters. Consequently, a mathematical handy tool is achieved toward specifying the circuit parameters rapidly for optimum linearity performance and efficiency. The main and error amplifiers used in the feedforward are assumed to have a third-order AM/AM nonlinearity. The derived expressions are verified by simulation. The conditions for the validity of the model choice are highlighted. For the matched and lossless case, a compact relationship is obtained, which clearly demonstrates the tradeoff between nonlinearities of main and error amplifier for a given system linearity and output power. Delay mismatch is also included in the analysis.

Multipath Characteristics of Frequency Diverse Arrays Over a Ground Plane

This paper presents a theoretical framework for an analytical investigation of multipath characteristics of frequency diverse arrays (FDAs), a task which is attempted for the first time in the open literature. In particular, transmitted field expressions are formulated for an FDA over a perfectly conducting ground plane first in a general analytical form, and these expressions are later simplified under reasonable assumptions. Developed formulation is then applied to a uniform, linear, continuous-wave operated FDA for the particular case of linear frequency increments, and closed-form solutions are established. Time dependence of the FDA array factor is next eliminated by calculating the average power received by an isotropic antenna at a given observation point. Field and power derivations are repeated for a conventional phased array to establish a performance benchmark. Numerical simulations are conducted for special test cases to demonstrate the advantages of FDAs over conventional phased arrays in terms of multipath propagation.

High-Power 20-100-MHz Linear and Efficient Power-Amplifier Design

This paper presents modeling and analysis of the realization of high-power broadband RF power amplifiers (PAs) and power combiners. In the scope of the study, a model is proposed for a transmission line with a ferrite core, which is the basic building block for the transmission line transformers (TLTs) used in broadband high-power amplifiers and power combiner/divider networks as a building block. Simulated performances of the designed networks using the proposed model possess high conformity with the empirical performance of the corresponding realized networks. Therefore, broadband TLT networks with low insertion loss, high power-handling capability, and low return loss are realized. RF power combiner/divider networks with wide frequency bandwidth and high isolation are also implemented. By using class-B biased pushpull network topology, a linear and wideband RF PA is realized with high drain efficiency at high output power levels. In order to broaden the operating frequency range of the indicated amplifier, a novel technique for TLT utilization that matches the output impedance of the RF power transistor in wideband is proposed and realized. Measurement results satisfy the design specifications.

A 15–40-GHz Frequency Reconfigurable RF MEMS Phase Shifter

This paper presents a novel frequency reconfigurable phase shifter using the RF microelectromechanical systems (MEMS) technology. The phase shifter is based on the triple-stub circuit topology composed of three stubs that are connected by two transmission lines that are all implemented as distributed MEMS transmission lines. The insertion phase of the circuit is controlled by changing the electrical lengths of the stubs and the connecting transmission lines, while having ideally zero reflection coefficient at all times. The phase shifter has theoretically no specific limits on the frequency; in other words, it can be reconfigured to work at any frequency between 15-40 GHz, with adjustable phase steps, while providing a constant time delay within a 2%-3% instantaneous bandwidth around any selected center frequency in the above-mentioned band. The phase shifter is fabricated monolithically using a surface micromachining process on a quartz substrate and occupies 10.8 mm × 5.9 mm. Measurement results show that the phase shifter has average phase error of 1.6 °, 3.7 °, and 4.7 °, average insertion loss of 3.1, 5, and 8.2 dB and average return loss of 19.3, 15.8, and 13.7 dB at 15, 30, and 40 GHz, respectively. To the best of our knowledge, this paper demonstrates the first phase shifter in the literature that can work at any given frequency in a targeted band with adjustable phase steps.

A monolithic phased array using 3-bit DMTL RF MEMS phase shifters

This paper presents a phased array system designed at 15 GHz employing 3-bit distributed MEMS transmission line (DMTL) type phase shifters which are monolithically integrated with the feed network of the system and the radiating elements on the same substrate. The phase shifter can give 0deg-360deg phase shift with 45deg steps at 15 GHz which is used to obtain various combinations of progressive phase shift in the excitation of radiating elements. The phased array is composed of four linearly placed microstrip patch antennas. In order to monolithically integrate the patch antennas and phase shifters, tapered lines with low return loss from microstrip to coplanar waveguide (CPW) have been designed. The design of the phased array system and its components is given. Since the DC biasing schema of a MEMS system is also an important issue in terms of the RF losses, the paper also addresses the effect of the bias lines on the loss characteristics of the phase shifters. Moreover, the process steps, which are used in the fabrication of the phased array, are also summarized

Design of triple-band reconfigurable microstrip antenna employing RF-MEMS switches

A triple-band inset-fed reconfigurable microstrip antenna is designed. The switching between the three different frequency bands is achieved through the use of RF-MEMS switches placed along the inset and along the slot placed on the radiating patch.

A Reconfigurable RF MEMS Triple Stub Impedance Matching Network

This paper presents a reconfigurable triple stub impedance matching network using RF MEMS technology centered at 10GHz. The device is capable of covering impedances on the whole Smith Chart. The device structure consists of three variable length stubs which are designed as distributed MEMS transmission lines and two lambdag/8 length CPW transmission lines connecting the stubs. The variable length stubs are implemented with 12 MEMS switches over CPW lines and CPW lines connecting the switches. lambdag/8 spacing between the stubs is selected to obtain a uniform distribution of the impedance points on the Smith Chart. Initial measurement results of the fabricated structure show a good agreement with the simulation results