Naofumi Okubo

Structural determination of multilayered large-signal neural-network HEMT model

This paper reports on the structure of a large-signal neural-network (NN) high electron-mobility transistor (HEMT) model as determined by a pruning technique and a genetic algorithm. The bias-dependent intrinsic elements of an HEMTs equivalent circuit are described by a generalized multilayered NN whose inputs are the gate-to-source bias (V/sub gs/) and the drain-to-source bias (V/sub ds/). Using C/sub gs/ data as an example, we began by experimentally examining some of the features of the multilayered NN model to obtain rules-of-thumb on choosing training parameters and other information for succeeding studies. We then developed and studied a novel pruning technique to optimize the C/sub gs/ NN model. Excessively large NN configurations can be reduced to an appropriate size by means of a weight decay, which is based on the analysis of a synaptic connections activity. Finally, we employed a genetic algorithm for the same purpose. By representing the configuration of a standard multilayered NN as a chromosome, the optimum configuration of a C/sub gs/ model was obtained through a simulated evolution process. For this approach, the configuration of an NN that simultaneously represents seven intrinsic elements (C/sub gs/,R/sub i/,...,C/sub ds/) of an equivalent circuit was also shown for comparison to previous work. We successfully obtained simplified NN models using both approaches. The advantages and disadvantages of these two approaches are discussed in the conclusion. To our knowledge, this is the first report to clarify the general process of building an NN device model.

An FM-CW radar module with front-end switching heterodyne receiver

The authors have developed a 60-GHz FM-continuous wave (CW) radar module that generates sidebands by switching a high electron mobility transistor (HEMT) front-end. The module also uses heterodyne detection for FM-AM conversion noise reduction. The modules signal to noise ratio was 20 dB better than a previously designed homodyne FM-CW radar module.<<ETX>>

A 15/60 GHz one-stage MMIC frequency quadrupler

We have developed a 15/60 GHz one-stage MMIC frequency quadrupler using a 0.25-/spl mu/m AlGaAs/GaAs HEMT. The HEMT was characterized by our empirical large-signal model, in which charge conservation and dispersion are taken into consideration. We included this model in a commercially-available harmonic balance circuit simulator, and designed the one-stage quadrupler. The fabricated MMIC quadrupler has a conversion gain of -5 dBm with -5 dBm of output power for a 0 dBm input signal.

New method of analyzing BER performance of GFSK with postdetection filtering

This paper describes the theoretical performance of a frequency-hopping system using Gaussian-filtered FSK (GFSK), which is a promising candidate for 2.4-GHz wireless local area networks (WLAN). We have improved previous methods to calculate the bit-error rate (BER) performance of the GFSK system by considering the effect of a postdetection filter in the first-order approximation. The improved method is applicable when the single-sided bandwidth of the postdetection filter is wider than the bandwidth containing 99% of the GFSK signal energy. It is used to calculate the BER performance as well as the dependence of the BER on the level of interchannel interference. The calculated performance of the GFSK system suggests that the postdetection filter is very effective in reducing interchannel interference. The feasibility of a GFSK system with a 1-Mbd symbol rate and 0.32 modulation index is confirmed by the combination of a predetection filter with a 0.85-MHz bandwidth and a postdetection filter with a 0.8-MHz single-sided bandwidth.

A new empirical large-signal HEMT model

We propose an empirical large-signal model of high electron mobility transistors (HEMTs). The bias-dependent data of small-signal equivalent circuit elements are obtained from S-parameters measured at various bias settings. And C/sub gs/, C/sub gd/, g/sub m/, and g/sub ds/, are described as functions of V/sub gs/ and V/sub ds/. We included our large-signal model in a commercially available circuit simulator as a user-defined model and designed a 30/60-GHz frequency doubler. The fabricated doublers characteristics agreed well with the design calculations.

A 6-GHz 80-W GaAs FET Amplifier with TM-Mode Cavity Power Combiner

A novel 8-way divider/combiner using TM/sub 010/- and TM/sub 020/ -mode cavities was developed. This divider/combiner has an insertion loss of 0.2 dB and a bandwidth of 600 MHz in the 6-GHz communications band. For broadening the operating bandwidth of the divider/combiner, two techniques of double cavities and tight coupling are described. Degradation of power-combining efficiency is also discussed when input signals into a power combiner have variations in amplitude and phase. By using this divider/combiner, an experimental 6-GHz 80-W GaAs FET amplifier with a combining efficiency of 85 percent was demonstrated to investigate the feasibility of a solid-state high-power amplifier.

38-GHz Band High-Power MMIC Amplifier Design using Improved Load-Pull Method

To facilitate accurate amplifier design, the large-signal impedance of a GaAs FET with a 0.25-¿m-long gate was measured using an improved load-pull method. A new MMIC transformer was used in the measurement to transform the FETs impedance into the coverage range of a twin sleeve tuner. The transformer as well as a wafer prober improved measurement accuracy. Two types of high-power amplifiers were designed using the measured large-signal impedance: one is composed of four FET cells with 400-¿m-wide gates and the other is composed of two FET cells with 600-¿m-wide gates. At 38 GHz and at 1-dB gain compression level, the former has an output power of 25.1 dBm and the latter 23.5 dBm.

Genetic Determination of Large-Signal HEMT Model

This paper reports on a general approach to build a large-signal, neural network HEMT model using a genetic algorithm. By representing the configuration of a neural network model as the chromosome of a virtual creature, we looked for an optimum network configuration by simulating the evolution of a group of these virtual creatures (a population). We successfully designed neural networks representing bias-dependent intrinsic elements of a HEMTs equivalent circuit. We also verified the reliability of this technique by searching for the optimum model from different initial conditions.

A 50-GHz low-noise amplifier designed using noise parameters from an equivalent circuit

Development is reported of a 50-GHz low-noise amplifier for a trial model of a millimeter-wave personal satellite communication system. The amplifier has a 3.0-dB noise figure and 22-dB gain at 50 GHz, including its input circulator and package interfaces. To design the amplifier, new equivalent circuit which effectively models the noise and scattering parameters of HEMTs is proposed.