Fernanda S. Yamasaki

Soliton Generation Using Nonlinear Transmission Lines

In recent years, there has been great interest in the study of nonlinear transmission lines (NLTLs) for high-power radio frequency (RF) generation. The periodicity of the NLTL accounts for dispersion effects, whereas its nonlinear elements (inductors and/or capacitors) are responsible for the nonlinear processes. Both of these mechanisms acting simultaneously on a propagating pulse allow the generation of high-frequency oscillations at the output. The objective of this paper is to study the quantifiable characteristics of these lines for RF generation by means of SPICE circuit simulation and on basis of experiments. The RF generation at 40 MHz is demonstrated through measurement of the fast Fourier transform of the RF signal extracted at the load and by comparison with the corresponding simulated spectrum. It is expected that the technique presented here can be useful for the design of NLTLs to drive compact RF antennas for space applications and defense mobile platforms.

State of the art of nonlinear transmission lines for applications in high power microwaves

Nonlinear transmission lines have been used with great success in pulse sharpening for obtaining output fast pulse rise times in nanosecond and picosecond ranges. Nowadays recent research on this field has shown a great prospect for RF generation using these devices in aerospace application such as in ultra-wide band radar for communications, remote sensing and surveillance due to less attenuation and electromagnetic interference. Also other important applications are envisioned in high power microwave sources for battlefield communication disruption and space vehicle systems as they can replace electronic tubes without the need of vacuum and heating. A nonlinear transmission line basically consists of a lumped LC line using a nonlinear element (variable inductance or capacitance) or a continuous transmission line using a nonlinear medium (ferroelectric or ferrimagnetic). In this paper the state of art for both configurations are discussed as well as their main characteristics and limitations.

RF generation using nonlinear transmission lines for aerospace applications

As an alternative to replace microwave tubes used nowadays in compact space systems for satellites communications, this work examines the feasibility of using nonlinear transmission lines (NLTL) consisting of nonlinear components as varactor diodes, or capacitors modeled nonlinearly, as ideal circuits, without ohmic losses and real ones, with ohmic losses. All simulations were confirmed using equations found in the literature to verify the models validity. It was also verified that high voltages prevent the use of varactor diodes in NLTLs. This makes the selection for ceramic capacitors more suitable in such applications.

Prospects of building capacitive nonlinear lines using ceramic PZT for high-frequency operation

The objective of this paper is the study of ceramic dielectrics for nonlinear transmission line (NLTL) applications in high-voltage (HV) compact pulsed power systems at high frequencies. Barium and strontium titanate (BST) ceramics have been used with great success as excellent dielectrics in the construction of high voltage (HV) commercial ceramic capacitors with reduced dimensions because of their high dielectric constant. However, the attempts to reach frequencies above 500 MHz up to 1 GHz using BST ceramics in nonlinear lumped lines have been unsuccessful due to the loss in the dielectric material that limits the operating frequencies up to about 250 MHz. On the other hand, these results indicated that the use of a lower loss nonlinear dielectric of reduced permittivity could be the solution for achieving higher frequencies in nonlinear capacitive lines. Therefore, the main point of this work is to characterize another type of ceramic known as PZT (Lead Zirconate Titanate), which is promising for NLTL applications.

Influence of Input Pulse Shape on RF Generation in Nonlinear Transmission Lines

Nonlinear transmission lines (NLTLs) are nonlinear LC ladder networks that can act as a nonlinear and dispersive medium, thus allowing the generation of soliton bursts. Several papers have been published indicating that NLTLs offer a new option for pulse shaping and RF generation. In this paper, we investigated the response of an NLTL driven by three different pulse shapes: a rectangular, a half sine, and a triangular waveform. The performance of the NLTL was evaluated through time- and frequency-domain analysis of the RF signal sampled at the 29th section of a 30-section capacitive NLTL. The frequency of the generated RF signal is correlated with the shape of the input signal, whereby the output frequency can be adjusted by the amplitude-time characteristics of the pulse at the input of the capacitive NLTL. Higher frequency oscillations, around 30 MHz, were generated by the rectangular wave train due to its shorter rise time. The propagation of solitons along the NLTL is influenced by the input pulse rise time. Namely, the rectangular pulse showed smaller delay time as it had the shortest rise time (less than 10 ns). Maximum efficiency was obtained for the input pulsewidth of 150 ns under a duty cycle of 1/3 for the three pulse shapes (rectangular, half sine, and triangular), and this combination of parameters yielded the highest RF conversion efficiency of the NLTL. The capacitive NLTL showed a higher RF conversion efficiency (16.4%) when driven by the rectangular input signal.

Simulation Studies of Distributed Nonlinear Gyromagnetic Lines Based on LC Lumped Model

Great interest has been devoted to the study of nonlinear transmission lines (NLTLs) for radio-frequency generation. The two better known configurations of NLTLs are a dispersive line consisting of sections with nonlinear components and a continuous nondispersive line called gyromagnetic line. The goal of this paper is to study the gyromagnetic line through the effects of changing the NLTL parameters. This is done based on an analytical model supported by SPICE circuit simulations and validation of results with numerical analysis, focusing on the pulse rise time compression. Different models are studied by comparing simulations with corresponding results found in the literature. Such a technique could be used for the design of NLTL for space applications and mobile defense platforms of compact size.

Key issues in design of NLTLs

Dielectric nonlinear element transmission lines (NLTLs) have demonstrated great potential in RF generation. Their main characteristic is that they do not require an electron beam to operate. However, there are some key points in their operation not clarified yet. For instance, they generate output oscillations with frequency near half the line Bragg frequency with unmatched load less than full characteristic impedance; however, for high resistive load this frequency appears to be near the Bragg frequency. In this paper, these points are checked experimentally by measuring the standard FFT of the signal along a varactor diode NLTL made with 30 sections, whose operation frequency is in the range of 25-40 MHz depending on the load.

Operation analysis of a novel concept of RF source known as gyromagnetic line

Gyromagnetic nonlinear transmission lines are interesting new devices used for RF generation since they are all solid state, lightweight and compact, neither requiring vacuum nor thermionic filament as in electronic tubes. Experiments with these lines have demonstrated their successful operation at L and S bands, thereby enabling them for applications in UWB pulsed radars in space vehicles and defense systems. There is also a great interest in compact solid-state high-power microwave sources for applications in small defense platforms (boats, trucks, etc.) to destroy the enemy electronic systems. Although the operation of these devices has been demonstrated exhaustively in recent years, their working principle is not quite well understood so far as it has been expected that the precession of the magnetic dipoles in the ferrite material given by the Larmor frequency, which predicts a proportional increase with the magnetic field bias. However, as observed experimentally the opposite occurs and the frequency decreases with the increasing of the magnetic field applied. Thus, the objective of this paper is to address this problem using the Landau-Lifshitz-Gilbert (LLG) equation with boundary conditions on the TEM mode propagation in the coaxial line. The formulation obtained for the precession frequency will be used to compare with experimental data found in the literature.

Key issues in design of nonlinear transmission lines

Summary form only given: Dielectric nonlinear transmission lines (NLTLs) have demonstrated great potential in RF generation. Their main characteristic is that they do not require an electron beam to operate. However, there are some key points in their operation not clarified yet. For instance, they generate output oscillations with frequency at half the line Bragg frequency for a high number of sections (n>30); however, for smaller n (=10) this frequency appears to be near the Bragg frequency1. In this work, it will be demonstrated that this effect has to do with the spreading of train of solitons travelling through a long line (i.e. with a high number of sections). In addition, it will be shown that for a line with higher n, the nonlinearity factor k of the dielectric can be very close to unity, which means that capacitors with more stable capacitance can be used. This effect has been observed in practice, as it is easy to produce a train of solitons with higher amplitudes for a line built with a large number of sections2. Finally, the points discussed above will be checked experimentally by measuring the standard FFT of the signal along a varactor diode NLTL made with 30 sections, operating at the output frequency around 50 MHz.

Operation of dielectric nonlinear transmission lines based on ceramic PZT slabs

De-poled PZTs (Lead-Zirconium-Titanate) were studied in the terms of nonlinearity properties, dielectric losses and breakdown. As shown in this paper de-poled PZTs have lower permittivity and dielectric losses compared to poled ones and BT (Barium Titanate) ceramic dielectrics. This was confirmed by measuring the permittivity, polarization curve and, tangent loss (δ) of this material. As discussed herein lower dielectric losses and permittivity may favor the replacement of BT tiles by PZTs in dielectric nonlinear transmission lines used for RF generation.