Sanjay Raman

Off-axis properties of silicon and quartz dielectric lens antennas

The theoretical far-field patterns and Gaussian-beam coupling efficiencies are investigated for a double-slot antenna placed off aids on extended hemispherical silicon and quartz lenses. Measured off-axis radiation patterns at 250 GHz agree well with the theory. Results are presented that show important parameters versus off-axis displacement: scan angle, directivity, Gaussicity, and reflection loss. Directivity contour plots are also presented and show that near-diffraction limited performance can be achieved at off-axis positions at nonelliptical extension lengths. Some design rules are discussed for imaging arrays on dielectric lens antennas.

Cognitive Radio and Networking Research at Virginia Tech

More than a dozen Wireless @ Virginia Tech faculty are working to address the broad research agenda of cognitive radio and cognitive networks. Our core research team spans the protocol stack from radio and reconfigurable hardware to communications theory to the networking layer. Our work includes new analysis methods and the development of new software architectures and applications, in addition to work on the core concepts and architectures underlying cognitive radios and cognitive networks. This paper describes these contributions and points towards critical future work that remains to fulfill the promise of cognitive radio. We briefly describe the history of work on cognitive radios and networks at Virginia Tech and then discuss our contributions to the core cognitive processing underlying these systems, focusing on our cognitive engine. We also describe developments that support the cognitive engine and advances in radio technology that provide the flexibility desired in a cognitive radio node. We consider securing and verifying cognitive systems and examine the challenges of expanding the cognitive paradigm up the protocol stack to optimize end-to-end network performance. Lastly, we consider the analysis of cognitive systems using game theory and the application of cognitive techniques to problems in dynamic spectrum sharing and control of multiple-input multiple-output radios.

Single- and dual-polarized millimeter-wave slot-ring antennas

Single- and dual-polarized dielectric lens-supported slot-ring antennas have been developed for operation at millimeter-wave frequencies. The antennas are fed with a coplanar waveguide (CPW) to be compatible with uniplanar mixers and low-noise amplifiers, and the feedline is shown to have a minimal effect on the antenna performance. The measured antenna patterns agree well with theoretical results and have symmetric main beams, low sidelobe levels (<-15 dB), low cross polarization (<-20 dB), and 27 dB directivity. A 2/spl times/2 array of single-polarized slot-ring antennas for monopulse applications demonstrates excellent patterns at 94 GHz with -3 dB crossover power levels in both elevation and azimuth scans. The dual-polarized slot ring antenna patterns are nearly identical to those of the single-polarized antenna, and two-port isolation is as low as -25 dB. The dielectric lens-supported slot-ring antenna is an excellent candidate for compact, low-cost millimeter-wave systems with fixed or variable polarization capabilities.

A W-band dielectric-lens-based integrated monopulse radar receiver

An integrated monopulse receiver has been developed for tracking applications at W-band frequencies. The receiver is based on dielectric-lens-supported, coplanar-waveguide-fed slot-ring antennas integrated with uniplanar subharmonic mixers. The design center frequency is 94 GHz and the IF bandwidth is 2-4 GHz. The measured DSB conversion losses of the individual receiver channels range from 14.4 to 14.7 dB at an LO frequency of 45.0 GHz and an IF of 1.4 GHz. This includes the lens reflection and absorption losses, backside radiation, RF feedline loss, mixer conversion loss, and IF distribution loss. Excellent monopulse patterns are achieved with better than 45 dB difference pattern nulls using IF monopulse processing. This translates to sub-milliradian angular accuracy for a 24 mm aperture. Better than 25 dB nulls are possible over a 600 MHz bandwidth.

DC offsets in direct-conversion receivers: characterization and implications

Since DC-offsets can have a large, negative impact on the performance of direct-conversion receivers, it is important to determine the offset performance of a given design. This article discusses one technique that can be used to characterize and measure dc-offsets in DCR circuit applications. DC-offsets are a primary concern in the design of DCRs and must be characterized in determining the performance of a particular receiver design. By measuring the dc level at the output of a DCR front-end in separate steps, the source of the offsets - device mismatches, LO self-mixing, and second order nonlinearities - can be resolved.

A Comprehensive Analysis of Quadrature Signal Synthesis in Cross-Coupled RF VCOs

This paper presents a linear model for cross-coupled quadrature voltage-controlled oscillators (QVCOs) together with a simple generalized proof for the condition of quadrature oscillation. The analysis provides insight into the oscillation mechanism and the previously observed frequency shift when the magnitude and phase of the coupling signal are deliberately modified by a complex coupling coefficient Kc. It is demonstrated that the steady-state oscillation condition GMRp=1 holds only if GM is replaced by an effective large-signal transconductance GMeff that is a function of the coupling transconductance G Mc and the imaginary part of Kc. Closed-form expressions for the phase imbalance and amplitude error in presence of mismatch between the LC tank circuits as well as device transconductances are also derived. We introduce the new concept of quadrature resistance/quadrature conductance (Rquad/Gquad), an incremental element synthesized by the coupling transistors, and show that its magnitude is responsible for the frequency shift and quadrature oscillation. A one-port model of the QVCO is studied and the loading effect of Gquad on the tank is examined. Particularly, it is shown that Gquad degrades the open-loop quality factor and worsens the phase noise. The analysis in this paper can be applied to most QVCO topologies presented in the literature

Quality factor and inductance in differential IC implementations

Monolithic inductors are widely used in microwave ICs and RFICs. Design considerations for inductors on low-resistivity substrates have been described. A /spl pi/-equivalent model, used to account for various loss mechanisms, can be helpful in understanding differences in the definition of L and Q for single-ended and differential structures that exist in the literature. Specific uses for the equations in this article have been outlined. The L and Q for an example monolithic, differential inductor were extracted from measured two-port S-parameters using equations for differential configurations on low-resistivity substrates.

A 60-GHz uniplanar MMIC 4/spl times/ subharmonic mixer

A uniplanar GaAs monolithic microwave integrated circuit /spl times/4 subharmonic mixer (SHM) has been fabricated for 60-GHz-band applications using an antiparallel diode pair in finite ground coplanar (FGC) waveguide technology. This mixer is designed to operate at an RF of 58.5-60.5 GHz, an IF of 1.5-2.5 GHz, and an LO frequency of 14-14.5 GHz. FGC transmission-line structures used in the mixer implementation were fully characterized using full-wave electromagnetic simulations and on-wafer measurements. Of several mixer configurations tested, the best results show a maximum conversion loss of 13.2 dB over the specified frequency range with a minimum local-oscillator power of 3 dBm. The minimum upper sideband conversion loss is 11.3 dB at an RF of 58.5 GHz and an IF of 2.5 GHz. This represents excellent performance for a 4/spl times/ SHM operating at 60 GHz.

Analysis and Design of a CMOS Phase-Tunable Injection-Coupled LC Quadrature VCO (PTIC-QVCO)

This paper presents the design, analysis, and characterization of a low-power, low-phase-noise, phase-tunable injection-coupled LC quadrature oscillator (PTIC-QVCO). Two LC VCOs are superharmonically coupled in quadrature phase via a frequency doubler that injects a synchronizing signal at the common source node of the negative transconductor stage. Conceptual and analytical models of the circuit are introduced to derive the conditions for quadrature operation and examine the circuit parameters affecting the phase imbalance due to mismatched VCOs. Additionally, a tunable tail filter (TTF) is incorporated to calibrate the residual quadrature imbalance in presence of a 3-sigma variation in the device parameters and drive the oscillator to its optimum phase noise performance. To validate the proposed approach, measurements have been carried out on a 9 GHz prototype implemented in a 0.18 mum RF CMOS process. With core current consumption of 5 mA at 1.8 V supply voltage, the circuit achieves a measured phase noise figure-of-merit ranging from 177.3 to 182.6 dBc/Hz at 3 MHz offset along the 9.0 to 9.6 GHz frequency tuning range. Quadrature phase correction of plusmn110 at 9 GHz is demonstrated.

FET diode linearizer optimization for amplifier predistortion in digital radios

This letter presents the initial results of a study on a diode-based power amplifier (PA) linearization technique. The optimization of a diodes parameters to match a MESFET PA is discussed. The relevance of AM-AM and AM-PM curves to linearity requirements and modulation format is also discussed. As much as a 6.7-dB improvement in ACPR is made in the upper channel using a series diode linearizer.