Pingshan Wang

Pulsed wave interconnect

Pulsed wave interconnect is proposed for global interconnect applications. Signals are represented by localized wave-packets that propagate along the interconnect lines at the local speed of light to trigger the receivers. Energy consumption is reduced through charging up only part of the interconnect lines and using the voltage doubling property of the receiver gate capacitances. In a 0.18-/spl mu/m CMOS technology case study, SPICE simulations show that pulsed wave interconnect can save up to 50% of energy and /spl sim/30% of chip area in comparison with the repeater insertion method. A proposed signal splitting structure provides reasonable isolations between different receivers. Measured S-parameters of 3.8-mm interconnect lines fabricated through CMOS foundry showed that the distortion and attenuation of a pico second signal are much less serious than the theoretical predictions. Pulsed wave interconnect also enables time division application of a single line to boost its bit rate capacity. The use of nonlinear transmission lines (NLTL) is also proposed to overcome pulse broadening and attenuation caused by dispersion and frequency-dependent losses. Pulsed waves on an NLTL may be generated, transmitted, split and detected with components realizable in bulk and SOI CMOS technologies. Tapered NLTL can be used for pulse compression. NLTL edge sharpening abilities may be applicable for signal rise time control.

High-speed interconnects with underlayer orthogonal metal grids

On-chip high-speed interconnects with underlayer orthogonal metal grids, including grid-backed lines (GBLs) and grid-backed coplanar waveguides (GBCPWs), are characterized through s-parameter measurements. For GBL test structures, the presence of underlayer metal grids reduces dispersion by a factor of 4 while the local speed of light decreases by a factor of 2 in comparison to those of conventional microstrip lines. The dispersion reduction comes from suppressing higher order modes; the local speed of light reduction comes from a longer current return path. These characteristics are beneficial for compact CMOS analog circuit designs. Losses caused by substrate and conductor lines are restrained by shielding the substrate and by involving weaker electric fields. Resonance at a frequency characterized by that of a patch antenna was observed and needs to be considered in high-speed circuit designs. The grids have weaker effects in the case of CPWs, where the side ground plate effects are significant. A signal transmission example shows that dispersion and frequency-dependent losses are important in determining the signal rise edge. Semi-empirical distributed resistance-inductance-capacitance-conductance (RLCG) equivalent circuit models are constructed for the interconnects below the resonant frequencies.

The interaction of symmetric and asymmetric modes in a high-power traveling-wave amplifier

A three-dimensional (3-D) model has been developed for the investigation of the coupling of the lowest symmetric and asymmetric modes in a high-power, high-efficiency traveling-wave amplifier. We show that in a uniform structure and for an initially nonbunched beam, the interaction efficiency of the asymmetric mode may be much higher than that of the symmetric mode. It is also shown that the coupling between these two modes is determined by a single parameter that depends on the beam characteristics; its value varies between zero when no coupling exists and unity in case of maximum coupling. For a beam that is uniform at the input end, this parameter varies linearly with the guiding magnetic field. In case of a bunched beam, it decreases linearly with the increasing phase-spread of the bunch. Because of the interaction, the radius of the beam increases linearly with the power associated with the asymmetric mode at the input end; it increases rapidly in the case of an initially uniform beam relative to the case of a prebunched beam. Selective damping to suppress the asymmetric mode is described and analyzed.

Symmetric and asymmetric mode interaction in high-power traveling wave amplifiers: experiments and theory

High-power microwave amplifier operation has been studied for use in a number of applications. The performance of the amplifiers has been marred, in some cases, by pulse shortening of the microwave signal. A possible source of the shortening is the loss of the beam due to hybrid HEM/sub 11/ mode interaction with the beam. In this paper, we describe experiments which investigate high-power operation and the effects of HEM modes on the amplifier performance. We report the high-output powers (>50 MW) with efficient (>54%) amplification of microwaves in an X-band traveling wave amplifier. In some experiments, peak power levels exceeding 120 MW were measured at an efficiency of 47%. The excitation of the asymmetric hybrid electromagnetic mode was monitored carefully, but does not seem to have a critical impact on the main interaction process in spite of the fact that its dispersion curve almost overlaps that of the symmetric interacting mode. Theoretical analysis of the interaction in a tapered traveling wave structure indicates that, even if the amount of power in the asymmetric modes at the input of the structure is comparable to that in the symmetric mode, the asymmetric modes cause no power reduction in the symmetric mode. For the case of off-axis beams the TM/sub 01/ output power may drop by about 30% and the power in the hybrid mode reach about one third of that in the symmetric mode. In order to avoid hybrid mode excitation it is necessary to suppress the reflections from both ends of the output structure several decibels below the gain level of the asymmetric mode.

High-Frequency Domain-Wall Motion and Magnetization Rotation of Patterned Permalloy Films under External Magnetic Field Excitation

The incorporation of ferromagnetic materials into integrated microwave devices is a promising approach for the development of on-chip high-performance circuit components. Therefore, high-frequency domain-wall motion and magnetization rotation, which yield permeability, are of primar interst. However, so far it has not been attempted to physically separate high-frequency domain-wall motion and magnetization rotation that are under high-frequency magnetic field excitation. Nor have there attempts for the corresponding characterizations. In this work, patterned permalloy films are integrated with on-chip microstrip lines. Domain-wall motion and magnetization rotation are separated through aspect ratio and dimension control. The measured results show that high-frequency-field driven domain-wall motion is fast, different from current driven domain-wall motion. It is also shown that coupling effects are not important when the distance between two adjacent permalloy films is ∼ 1 μm despite their large lateral dimensions. The experimental results agree with simulation results.

Permalloy loaded transmission lines for high-speed interconnect applications

The mutual inductance and self-inductance of global interconnects are important but difficult to extract and model in deep submicrometer very large scale integration (VLSI) designs. The absence of effective mutual magnetic field shielding limits the maximum unbuffered interconnect line length. In this paper, we propose and demonstrate that permalloy-loaded transmission lines can be used for high-speed interconnect applications to overcome these limitations. Permalloy films were incorporated into planar transmission lines using a CMOS-compatible process. The line characteristics show that eddy-current effects are the limiting factors for the high-frequency permalloy applications when ferromagnetic resonance are restrained through geometry design. Patterning permalloy films effectively extends their application to above 20 GHz. The line characteristic impedances are about /spl sim/90 /spl Omega/. Under 50 mA dc current biases, the line parameters did not change much. Moreover, the patterned permalloy reduces the magnetic field coupling between two adjacent transmission lines by about 10 dB in our design. The demonstrated operation frequency range, current carrying capability and magnetic field shielding properties indicate that the permalloy loaded lines are suitable for high-speed interconnect applications in CMOS technologies.

High-frequency permalloy permeability extracted from scattering parameters

We report a ferromagnetic thin film characterization method and the measured microwave permeability of patterned permalloy films. The method incorporates ferromagnetic materials with transmission lines and extracts RLCG equivalent circuit elements from scattering parameters. The frequency-dependent effective permeability is then obtained from the incremental R and L caused by the ferromagnetic material. The measured high-frequency losses of thin permalloy (Ni80Fe20) films between 1 and 20 GHz show that geometry design restrains ferromagnetic resonance and eddy-current effects effectively. Above ∼22 GHz, mode conversion occurs in the test structures. Other broadband structures are necessary for further extractions.

A comparative study of high power, multistage, X-band TWT amplifiers

Our previous work on high-power efficient X-band TWT amplifiers has used a two stage device with bunching produced in a greater than light phase velocity region, immediately followed by a short low phase velocity output structure. The device is driven by a 7 mm diameter 750 kV, 450 A pencil electron beam. The structure, which has a 4 GHz bandwidth in the bunching section, produces an amplified output with a power in the range 20-60 MW. At higher output powers pulse shortening develops. A serious candidate for the pulse shortening is excitation of the HEM/sub 11/ mode in the structure. This mode overlaps the frequency domain of the desired TM/sub 01/ mode. We have designed and tested new amplifier structures in which the separation of these modes is substantially increased. The performance of the new amplifier(s) will be compared with that of the older device, and the relevance of the hybrid modes to pulse shortening assessed.

Short Pulse Generation With On-Chip Pulse-Forming Lines

We report our results on pulse-forming-line (PFL)-based CMOS pulse generator studies. Through simulations, we clarify the effects of PFL length, switch speed, and switch resistance on the output pulses. We model and analyze CMOS pulse generators with on-chip transmission lines (TLs) as PFLs and CMOS transistors as switches. In a 0.13- μm CMOS process with a 500- μm long PFL, post-layout simulations show that pulses of 10.4-ps width can be obtained. High-voltage and high-power outputs can be generated with other pulsed power circuits, such as Blumlein PFLs with stacked MOSFET switches. Thus, the PFL circuit significantly extends short and high-power pulse generation capabilities of CMOS technologies. A CMOS circuit with a 4-mm-long PFL is implemented in the commercial 0.13- μm technology. Pulses of ~116-ps duration and 205-300-mV amplitude on a 50-Ω load are obtained when the power supply is tuned from 1.2 to 1.6 V. Measurement connection setup is the main reason for the discrepancies among measurements, modeling, and simulation analyses.

Efficient operation of a high-power X-band traveling wave tube amplifier

We report experimental results demonstrating 54% power conversion efficiency (43% energy conversion efficiency), from a two-stage X-band traveling wave tube amplifier designed for high-power operation. The first stage of the amplifier is a 12-cm-long Boron Nitride dielectric section used to modulate the electron beam. The second stage consists of a long high-phase-velocity bunching section followed by a short low-phase-velocity output section. Output powers of up to 78 MW with narrow spectrum width were obtained with ∼700 kV, ∼200 A beam.