Seok Joo Doo

Effective Suppression of IV Knee Walk-Out in AlGaN/GaN HEMTs for Pulsed-IV Pulsed-RF With a Large Signal Network Analyzer

IV knee walk-out in AlGaN/GaN high electron mobility transistors (HEMTs) on a Sapphire substrate is analyzed using dynamic radio frequency (RF) load-lines acquired with a large signal network analyzer (LSNA) for both continuous-wave (CW) and pulsed-IV/RF excitations. When thermal effects and traps are bypassed using pulsed-IV biasing and pulsed-RF excitations, the IV knee walk-out observed in CW load-lines is found to be effectively suppressed and the device delivers the maximum output power expected for class A operation. It is also demonstrated using pulsed-IV/RF measurements at various substrate temperatures that the IV knee walk-out primarily arises from thermal effects at high bias rather than trapping in the on-wafer devices characterized

A 3-5 GHz fully differential CMOS LNA with dual-gain mode for wireless UWB applications

Ultra-Wideband (UWB) is an emerging new technology for wireless personal area networks (WPAN) and is based on IEEE 802.153a standard. The frequency range of operation is between 3.1-10.6 GHz, which is divided into several band groups. The LNA in our paper is operable at the first band group of UWB that is 3.1-4.8 GHz range. The LNA is designed and laid out in a 0.18 mum CMOS process and with 1.8-V power supply. Simulation shows the LNA achieves 2-GHz bandwidth with minimum gain variation, two voltage gain modes of 20dB and 15dB respectively and noise figure around 5.2dB, while draws 15 mA current

NVNA Techniques for Pulsed RF Measurements

In this article, we have reviewed various advanced broadband and pulsed measurement techniques which have been developed for sampler-based NVNAs. We have seen that the RF modulation capability of sampler based NVNAs was not limited by the IF bandwidth of its receiver. Two methods for pulsed-RF, pulsed-bias measurements were also presented. The method based on the concept of multiple recordings enables one to achieve pulsed-RF measurements with 0 dB desensitization (no dynamic range degradation) even for very low duty cycles. Real-time active load-pull techniques for the rapid design of microwave power amplifiers were also presented. Finally, it was demon strated that pulsed-RF real-time active-load-pull could also be performed for the rapid design of pulsed-RF PAs or the characterization of transistors with reduced memory effects. These various measurements with sampler-based NVNAs were made possible thanks to the use of advanced ADC triggering techniques.

Pulsed multi-tone measurements for time domain load pull characterizations of power transistors

Real-time active load-pull (RTALP) characterization of power transistors with a large signal network analyzer (LSNA) has the benefit of greatly accelerating the design of power amplifiers. However RTALP has been so far limited to devices operating under CW excitations and constant biasing and was therefore mostly applicable to transistors with small memory effects. In TDMA communication or radar systems, amplifiers are excited by pulsed or modulated RF signals and operate under pulsed biasing. For such applications the measurement of modulated/pulsed RF signals under pulsed biasing/RF conditions is required for a more realistic understanding of the behavior of transistors with memory effects. This paper demonstrates that under appropriate conditions it is possible to effectively combine these two excitation modes while performing large-signal measurement by sub-sampling down-conversion with a LSNA. In this demonstration, the acquisition of CW two-tone and three-tone excitations is performed using multiple-recording with pulse duty rate of 1.5% and 0.15%. The pulse-recorded measurements for the three-tone excitation are found to be consistent with CW measurements (100% duty rate). This preliminary work indicates that either intermodulation or real-time active load-pull measurements under pulsed operation can be readily realized for RF samplers with sufficiently short settling time while operating under pulsed RF modulation.

High Efficiency RF Power Amplifier Designed With Harmonic Real-Time Active Load-Pull

In this work, a high efficiency p-HEMT radio frequency power amplifier (PA) is designed using a new multiharmonic real-time active load-pull using the large signal network analyzer. This technique synthesizes a large set of instantaneous load mismatches to quickly find the optimal harmonic impedances, so as to achieve high PA efficiency in a shortened design cycle. At 2 GHz a demo power amplifier implemented with a p-HEMT demonstrated a power added efficiency (PAE) of 68.5% for 18.0 dBm output power, while achieving a maximum PAE of 75% below the 1 dB compression point for 18.6 dBm output power.

Pulsed-IV pulsed-RF measurements using a large signal network analyzer

A new pulsed-IV pulsed-RF measurement system using a large signal network analyzer (LSNA) is proposed to address the problem of desensitization afflicting conventional pulsed-RF measurement systems. Several extraction methods using the entire spectrum measured by the system are presented to extract non-desensitized pulsed-RF S-parameters of a transistor. The comparison of the calculated S-parameters using the least-square fitting in time domain with those using only the fundamental tone reveals the significant increase in dynamic range achieved by the proposed measurement scheme.

High-Power EMI on RF Amplifier and Digital Modulation Schemes

We present the performance of an RF amplifier and digital modulation techniques in the presence of high-power electromagnetic interference (EMI) to provide existing and next generation communication systems with critical information. An advanced measurement setup comprised of a large-signal network analyzer is used to characterize the adverse effects of EMI on the device characteristics of an RF power amplifier and the performance of digital modulation schemes. Furthermore, our analysis incorporated hybrid numerical tools, such as the hybrid S-parameter method to carry out an extensive EMI analysis of digital modulation schemes in the presence of complex structures, such as cylindrical cavities. Our studies yield critical information for the communication systems. For instance, our analysis suggests that digital modulation schemes are more susceptible to EMI than the RF power amplifier that processes the modulated signals. Power levels of the order of megawatts are required to have a notable impact on the device characteristics of an RF amplifier in the presence of a missile-like body, whereas, much lower power levels are sufficient to degrade the performance of a digital modulation scheme as long as it is within the bandwidth of the modulated signal. Our analysis further indicates that nonconstant envelope digital modulation schemes are more susceptible to EMI.

New Ultra-fast Real-time Active Load-pull Measurements for High Speed RF Power Amplifier Design

To accelerate the speed of RF power amplifier design, a novel ultra-fast real-time active load-pull measurement based on a large signal network analyzer (LSNA) is presented. Real-time load-pull conventionally uses a single-tone excitation at the transistors output to achieve a time varying load impedance. However this approach still requires multiple measurements at different power levels to map the Smith Chart and determine the optimal load termination. This paper proposes a modulated two-tone excitation method for mapping the Smith Chart in a single 10 ms LSNA measurement. Experiments for the lst and 2nd harmonic demonstrate that this ultra-fast real-time active load-pull permits to identify the optimal load impedance in one measurement.

Volterra characterization and predistortion linearization of multi-carrier power ampliflers

The linearization of RF power amplifiers (PA) can benefit from the availability of the generalized Volterra coeflicients characterizing its non-linear response. In this work a large-signal network analyzer is used to acquire the amplitude and phase of the 3rd intermodulation terms Ym3- and Ym3+ of an LDMOSFET PA. The frequency dependence and difference between Ym3- and Ym3+ reveals the memory effects of the RF amplifier. A new vectorial digital predistortion linearization with 3rd and 5th order corrections is implemented to account for the difference in memory effits in the lower and upper side bands. The two band predistortion . linearization can linearize independently each band of a 2-carrier WCDMA in a LDMOSFET RF amplifier providing up to 40 dBc ACPR. The extension from 2-carrier to multi-carrier power amplifiers could proceed by further dividing the bandwidth in additional bands.

Large signal network analyzer with trigger for baseband & RF system characterization with application to K-modeling & output baseband modulation linearization

The modeling and balancing of IQ modulators and the development of linearization techniques for power amplifiers (PA) such as output baseband modulation (OBM) benefit from the availability of linear and nonlinear MIMO Volterra system parameters involving both baseband and RF signals. A triggered LSNA system is reported for this purpose. By synchronizing the LSNA measurements with the generation of the baseband modulation, RF signals with a stable envelope can be acquired. Applied to an IQ modulator the triggered LSNA facilitates the measurement of the correlation between the I and Q signals at baseband and RF, effectively yielding the modulator K-model. Inverting the K-model of the IQ modulator, an IQ balancing of about 43 dBc and 46 dBc for the lower and upper side-bands, respectively, was achieved using 14 bit signed DACs. Finally the triggered LSNA finds applications in the linearization of RF PAs with input (IBM) or output (OBM) baseband modulation by facilitating the extraction of the Y/sub md-/ and Y/sub md+/ Volterra nonlinear parameters.