A software radio is defined as a set of digital signal processing (DSP) primitives, a metalevel system for combining the primitives into communication system functions (transmitter, channel model, receiver, etc.), and a set of target processors on which the software radio is hosted for real-time communications. The performance of enabling hardware technologies is related to software radio requirements, portending a decade of shift from hardware radios toward software intensive approaches. Computational models and architecture are discussed, stressing the need for topological consistency of radio functions and host architectures. A layered topology-oriented design approach encapsulated in a canonical open architecture software radio model is presented. The model provides a unified mathematical framework for quantitative analysis of algorithm structures, host architectures, and system performance for CAD. >

Software radios: Survey, critical evaluation and future directions

Wide-band switches and true-time delay (TTD) phase shifters have been developed using distributed microelectromechanical system (MEMS) transmission lines for applications in phased-array and communication systems. The design consists of a coplanar waveguide (CPW) transmission line (W=G=100 /spl mu/m) fabricated on a 500 /spl mu/m quartz substrate with fixed-fixed beam MEMS bridge capacitors placed periodically over the transmission line, thus creating a slow-wave structure. A single analog control voltage applied to the center conductor of the CPW line can vary the phase velocity of the loaded line by pulling down on the MEMS bridges to increase the distributed capacitive loading. The resulting change in the phase velocity yields a TTD phase shift. Alternatively, the control voltage can be increased beyond the pull-down voltage of the MEMS bridges such that the capacitive loading greatly increases and shorts the line to ground. The measured results demonstrate 0-60 GHz TTD phase shifters with 2 dB loss/118/spl deg/ phase shift at 60 GHz (/spl sim/4.5-ps TTD) and 1.8 dB loss/84/spl deg/ phase shift at 40 GHz. Also, switches have been demonstrated with an isolation of better than 40 dB from 21 to 40 and 40 to 60 GHz. In addition, a transmission-line model has been developed, which results in very close agreement with the measured data for both the phase shifters and switches. The pull-down voltage is 10-23 V, depending on the residual stress in the MEMS bridge. To our knowledge, this paper presents the first wide-band TTD MEMS phase shifters and wide-band switches to date.

Distributed MEMS true-time delay phase shifters and wide-band switches

We describe active and nonlinear wave propagation devices for generation and detection of (sub)millimeter wave and (sub)picosecond signals. Shock-wave nonlinear transmission lines (NLTL's) generate /spl sim/4-V step functions with less than 0.7-ps fall times. NLTL-gated sampling circuits for signal measurement have attained over 700-GHz bandwidth. Soliton propagation on NLTL's is used for picosecond impulse generation and broadband millimeter-wave frequency multiplication. Picosecond pulses can also be generated on traveling-wave structures loaded by resonant tunneling diodes. Applications include integration of photodetectors with sampling circuits for picosecond optical waveform measurements and instrumentation for millimeter-wave waveform and network (circuit) measurements both on-wafer and in free space. General properties of linear and nonlinear distributed devices and circuits are reviewed, including gain-bandwidth limits, dispersive and nondispersive propagation, shock-wave formation, and soliton propagation. >

Active and nonlinear wave propagation devices in ultrafast electronics and optoelectronics

This paper describes the design and fabrication of distributed analog phase-shifter circuits. The phase shifters consist of coplanar-waveguide (CPW) lines that are periodically loaded with varactor diodes. The circuits are fabricated on GaAs using standard monolithic processing techniques. The phase velocity on these varactor diode-loaded CPW lines is a function of applied reverse bias, thus resulting in analog phase-shifting circuits. Optimally designed circuits exhibit 0/spl deg/-360/spl deg/ phase shift at 20 GHz with a maximum insertion loss (IL) of 4.2 dB. To the best of our knowledge, this is the lowest reported IL for a solid-state analog phase shifter operating at 20 GHz.

Distributed analog phase shifters with low insertion loss

The GaAs nonlinear transmission line (NLTL) is a monolithic millimeter-wave integrated circuit consisting of a high-impedance transmission line loaded by reverse-biased Schottky contacts. The engineering of functional monolithic NLTLs is considered. Through generation of shock waves on the NLTL, the authors have generated electrical step functions with approximately 5 V magnitude and less than 1.4 ps fall time. Diode sampling bridges strobed by NLTL shock-wave generators have attained bandwidths approaching 300 GHz and have applications in instruments for millimeter-wave waveform and network measurements. The authors discuss the circuit design and diode design requirements for picosecond NLTL shock-wave generators and NLTL-driven sampling circuits. >

GaAs nonlinear transmission lines for picosecond pulse generation and millimeter-wave sampling

We report traveling-wave amplifiers having 1-112 GHz bandwidth with 7 dB gain, and 1-157 GHz bandwidth with 5 dB gain. A third amplifier exhibited 5 dB gain and a 180 GHz high-frequency cutoff. The amplifiers were fabricated in a 0.1-/spl mu/m gate length InGaAs/InAlAs HEMT MIMIC technology. The use of gate-line capacitive-division, cascode gain cells and low-loss elevated coplanar waveguide lines have yielded record bandwidth broad-band amplifiers.

112-GHz, 157-GHz, and 180-GHz InP HEMT traveling-wave amplifiers

A second-harmonic generation is reported in the 26-40-GHz band through soliton propagation on a GaAs monolithic nonlinear transmission line. At 20-dBm input power, a 20-diode structure attained >

28-39 GHz distributed harmonic generation on a soliton nonlinear transmission line

We have fabricated active probes for on-wafer waveform and network measurements. The probes incorporate GaAs nonlinear transmission line (NLTL) based network analyzer (NWA) integrated circuits and low-loss quartz coplanar-waveguide probe tips. The active probes show step response falltimes of 2.7 ps when excited by a 0.7-ps falltime input, Using these active probes, we demonstrate both waveform measurements with 2.7-ps risetime and network measurements to 200 GHz. We discuss the probe tip and NWA IC design, the hybrid assembly and mechanical design, and system design considerations. On-wafer waveform and S-parameter measurements of monolithic millimeter-wave integrated circuits are demonstrated. >

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M. Case

Papers

Active and nonlinear wave propagation devices in ultrafast electronics and optoelectronics

GaAs nonlinear transmission lines for picosecond pulse generation and millimeter-wave sampling

112-GHz, 157-GHz, and 180-GHz InP HEMT traveling-wave amplifiers

28-39 GHz distributed harmonic generation on a soliton nonlinear transmission line

Millimeter-wave on-wafer waveform and network measurements using active probes