Effects of solar transit in Ku-band VSAT systems

Behavior of networks of very small aperture satellite terminals (VSATs) operating at Ku band during the solar transit period, is compared to more traditional C or Ku-band satellite networks. Based on analyses and experiments, it is explained why solar transit outages are rarer in Ku-band VSAT systems than conventional satellite communications systems. In many cases, Ku-band VSAT systems will operate through periods of Sun transits without any significant increase in transmission error rates or incidences of link outages     

Evolution of the large Deep Space Network antennas

The evolution of the largest antenna of the US NASA Deep Space Network (DSN) is described. The design, performance analysis, and measurement techniques, beginning with its initial 64-m operation at S-band (2295 MHz) in 1966 and continuing through the present Ka-band (32-GHz) operation at 70 m, is described. Although their diameters and mountings differ, these parabolic antennas all employ a Cassegrainian feed system, and each antenna dish surface is constructed of precision-shaped perforated-aluminum panels that are secured to an open steel framework     

1/f noise in MODFETs at low drain bias

The 1/f noise in normally-on MODFETs biased at low drain voltages is investigated. The experimentally observed relative noise in the drain current S_I/I² versus the effective gate voltage V_G=V_GS-V_off shows three regions which are explained. The observed dependencies are S_I/I²∝V_G ^m with the exponents m=-1, -3, 0 with increasing values of V_G. The model explains m =-1 as the region where the resistance and the 1/f noise stem from the 2-D electron gas under the gate electrode; the region with m=0 at large V_G or V_GS≅0 is due to the dominant contribution of the series resistance. In the region at intermediate V_G , m=-3, the 1/f noise stems from the channel under the gate electrode, and the drain-source resistance is already dominated by the series resistance     

Calibration of the CCRS airborne scatterometers

A series of experiments and associated analyses which were designed to lead to an end-to-end calibration of the Canada Centre for Remote Sensing (CCRS) fanbeam scatterometers are described. The method followed was originally introduced in 1984 by A. Yizhar et al. for the Ku-band scatterometer at one incidence angle. This work was extended to yield a full calibration for the Ku-band and C -band scatterometers over the complete range of incidence angles accessible to the instruments. An array of 12 trihedral corner reflectors was deployed in a grassy field near Ottawa. The CCRS CV-580, equipped with two scatterometers, repeatedly overflew the array collecting radar replicas of the targets proportional to the unknown two-dimensional antenna pattern. Data from inertial navigation systems and aerial photographs from a Wild RC-10 mapping camera were used to determine the exact track of the aircraft during the acquisition. This data, with a field survey, alloyed the reduction of the scatterometer data from the reflector array to yield the unknown antenna pattern of the instruments. The cross-polarized antenna patterns were then deduced from the like-polarized results. The results show consistency within 0.5 dB and overall calibration accuracy is estimated to be better than 1 dB     

The effects of the thermal protection system on the Space ShuttleS-band quad antennas

A theory is presented to determine the effects of the multiple dielectric thermal protection system on the Space Shuttle orbiter's S-band quad antennas. A ray optics approach is developed which indicates internal reflections from the orbiter metallic skin and multiple dielectric layers. The results of the computer program developed show that the beamwidth of the two upper quad antennas is narrowed by a thermal protection system. The results also show that the thermal protection system will cause significant depolarization of a circularly polarized signal as the angle from the normal to the antenna surface is increased. Heating of the thermal protection surface or degradation due to multiple missions may also be predicted by incorporating changes in the complex permittivity as a function of temperature. Heating and multiple mission effects are found to be less than 1 dB     

A study of performance degradation of small-cell radio systemscaused by improper antenna positioning

A report is presented on the results of an extensive computer-simulation study of the cochannel interference caused by imprecise antenna locations. Interference problems are investigated for various percentage tolerances (which determine tolerance circles around ideal locations) and spatial distributions (which determine actual antenna location inside the tolerance circles) using the tenth percentile of the S/I distribution of the mature system as a criterion. One result shows that system performance is highly sensitive to large-percentage tolerances in later stages of cell-splitting. After analyzing the results, a method was developed to predict the performance of a general mature system with any number of cell sites, any cell radius, any set of percentage tolerances, and any set of spatial distributions     

Interfaces for high speed fiber-optic links: analysis andexperiment

An analysis of directly and externally modulated fiber-optic links is presented. The theoretical analysis is based on the signal flow diagram of the interface circuits to the laser diode, Mach-Zehnder electro-optic modulator, and p-i-n photodiode. System parameters such as gain, noise figure, two-tone intermodulation distortion, and spurious free and compression dynamic range are expressed as a function of frequency and operating point of the laser and external modulator. Two directly and externally modulated fiber-optic links were designed and fabricated to verify the analytical models. The direct modulation FO link was developed at the Ku-band (11.6-12.4 GHz), whereas the external modulation link was at L-band (875-925 MHz). Spurious-free dynamic ranges of 95.8 dB Hz^2/3 and 113 dB Hz ^2/3, respectively, were achieved. The predictions based on the analytical models match the measured results     

Seasat over-land scatterometer data. I. Global overview of theKu-band backscatterer coefficients

Statistics on the backscatter coefficient σ⁰ from the Ku-band Seasat-A Satellite Scatterometer (SASS) collected over the world's land surfaces are presented. This spaceborne scatterometer provided data on σ⁰ between latitude 80° S and 80° N at incidence angles up to 70°. The global statistics of vertical (V) and horizontal (H) polarization backscatter coefficients for 10° bands in latitude are presented for incidence angles between 20° and 70° and compared with the Skylab and ground spectrometer results. Global images of the time-averaged V polarization σ⁰ at a 45° incidence angle and its dependence on the incidence angle are presented and compared to a generalized map of the terrain type. Global images of the differences between the V an H polarization backscatter coefficients are presented and discussed. The most inhomogeneous region, which contains the deserts of North Africa and the Arabian Peninsula, is studied in greater detail and compared with the terrain type     

Comparison of decoding algorithms for trellis coded modulationusing Viterbi algorithm in the presence of intersymbol interference

The letter describes a comparison of some receiver structures suitable for trellis-coded modulation on channels causing intersymbol interference. Simulation results show that a Viterbi decoder treating trellis coding and intersymbol interference as one compound encoding mechanism yields the best performance, which at P_b=10^-4 is about 6 dB in S/N better than any other receiver structure     

Optimum field theory design of broad-band E-plane branchguide phase shifters and 180° couplers

Optimum rectangular waveguide E-plane branch guide phase shifters and 180° branch guide couplers are designed with the rigorous method of field expansion into normalized eigenmodes. The design includes both the higher order mode interaction between the step discontinuities and the finite step and branch heights. The phase shifter design applies the Schiffman principle to branch guide couplers where two ports are short-circuited. The 180° coupler design combines the advantage of the broadband potential of multiple-branch couplers with the low-insertion-loss qualities of E-plane stub-loaded phase shifters. A computer-optimized phase shifter prototype for the waveguide Ku-band (12-18 GHz) shows a 90°±1° differential phase shift with reference to an empty waveguide within about 23% bandwidth. Five-branch three-stub coupler prototypes, designed for 3±0.2 dB coupling, for the waveguide Ku- and Ka-bands (26-40 GHz) achieve a 180°±1° differential phase shift at the output ports within about 19% bandwidth, as well as more than 30 dB isolation and return loss. The theory is verified by measured results     

Wet antenna effect on VSAT rain margin

The contribution of wet antennas to the antenna signal path losses in a VSAT environment is treated theoretically. The current commercial VSAT systems operating in either C-band or Ku-band generally have their remote terminal antenna reflectors and the antenna feed horn radomes coated with hydrophobic materials. The aim is to prevent the antenna and radome surfaces from becoming wet during a rainfall. This precaution relieves the burden of added rain margin necessary on the link budget. The magnitude of the propagation loss when the antenna reflector and the antenna feed horn radome surfaces are wet is determined. The results can indicate whether the expense of applying and maintaining the hydrophobic materials on the VSAT remote antennas and radomes is justified under specific loss conditions     

High-frequency InP/InGaAs double heterojunction bipolar transistorson Si substrate

Self-aligned high-frequency InP/InGaAs double heterojunction bipolar transistors (DHBTs) have been fabricated on a Si substrate. A current gain of 40 was obtained for a DHBT with an emitter dimension of 1.6 μm×19 μm. The S parameters were measured for various bias points. In the case of I_C=15 mA, f _T was 59 GHz at V_CE=1.8 V, and f _max was 69 GHz at V_CE=2.3 V. Due to the InP collector, breakdown voltage was so high that a V_CE of 3.8 V was applied for I_C=7.5 mA in the S-parameter measurements to give an f_T of 39 GHz and an f_max of 52 GHz     

Modeling the gate I/V characteristic of a GaAsMESFET for Volterra-series analysis

The authors show that the Taylor-series coefficients of a FET's gate/drain I/V characteristic, which is used to model this nonlinearity for Volterra-series analysis, can be derived from low-frequency RF measurements of harmonic output levels. The method circumvents many of the problems encountered in using DC measurements to characterize this nonlinearity. This method was used to determine the incremental gate I/V characteristic of a packaged Aventek AT10650-5 MESFET biased at a drain voltage of 3 V and drain current of 20 mA. The FET's transconductance was measured at DC, and its small-signal equivalent circuit (including the package parasitics) was determined by adjusting its circuit element values until good agreement between calculated and measured S parameters was obtained. The FET was then installed in a low-frequency test fixture. Excellent results were obtained     

Quarter-micrometer gate ion-implanted GaAs MESFET's with an f1 of 126 GHz

The fabrication and characterization of a 0.25-μm-gate, ion-implanted GaAs MESFET with a maximum current-gain cutoff frequency f_t of 126 GHz is reported. Extrapolation of current gains from bias-dependent S-parameters at 70-100% of I _dss yields f₁'s of 108-126 GHz. It is projected that an f₁ of 320 GHz is achievable with 0.1-μm-gate GaAs MESFETs. This demonstration of f₁'s over 100 GHz with practical 0.25-μm gate length substantially advances the high-frequency operation limits of short-gate GaAs MESFETs     

I/Q channel reversal correcting properties of anSQPSK Costas loop with integrated symbol synchronization

An integrated-carrier loop/symbol synchronizer, using a digital Costas loop with matched arm filters to demodulate staggered quaternary phase-shift keyed (QPSK) signals, is analyzed. An expression is derived for the S curve, parameterized by bit synchronization error. This result suggests that the demodulator structure offers an inherent I/Q channel reversal correcting capability. Computer simulation results are presented that support this conclusion, and suggest that ambiguity resolution performance depends on the ratio of carrier and synchronization loop bandwidths     

Modeling hot-carrier effects in polysilicon emitter bipolartransistors

In self-aligned polysilicon emitter transistors a large electric field existing at the periphery of the emitter-base junction under reverse bias can create hot-carrier-induced degradation. The degradation of polysilicon emitter transistor gain under DC stress conditions can be modelled by ΔI_B∝I_R ^m+ntⁿ where n≈0.5 and m ≈0.5. The more complex relationships of Δβ(I _C, I_R, t) and β(I _C, I_R, t) result naturally from the simple ΔI_B model. Using these relationships the device lifetime can be extrapolated over a wide range of reverse stress currents for a given technology     

Design and performance of octave S/C band MMICT/R modules for multi-function phased arrays

A complex wideband transmit/receive module that achieves performance levels superior to any MMIC module is described. Peak performance within the octave 3.0 to 6.0 GHz band includes a power output of 21 W at S-band and 19 W at C-band, a noise figure of 3.9 to 5.0 dB, 30 to 38 dB of receive gain, 25 to 26 dBm output IP₃, 40 dB of gain control in 256 steps, dual receive channels with independent amplitude and phase control, and an 8-bit phase shifter with less than 1 degree calibrated RMS phase error. Total GaAs area is 146 mm² with 170 mm of total gate periphery. The module incorporates a compact digital interface, requires only three supply voltages, and utilizes advanced packaging techniques, resulting in a size compatible with a grating lobe free grid spacing     

Dual-chip GaAs monolithic integration Ku-bandphase-locked-loop microwave synthesizer

Two novel multifunction monolithic chips, GaAs microwave monolithic integrated circuit (MMIC) and large-scale integration (LSI) chips, have been developed to realize an extremely small and lightweight microwave synthesizer. The MMIC includes a voltage-controlled oscillator, a dual-output buffer amplifier, a balun, and dynamic/static prescalers. To integrate these functions on a single chip, each circuit has been drastically reduced in size by utilizing a uniplanar MMIC configuration. The LSI includes a dual-modulus prescalar, programmable counters, and a phase/frequency comparator. By incorporating these two monolithic chips in the structure, a Ku-band microwave synthesizer has been fabricated in an 11-mm×23-mm flat package. The synthesizer to which these multifunction chips were applied had a tuning range broader than 1 GHz in the Ku-band with a flatness within 2 dB_pp. In spite of low-Q monolithic circuitry, single-sideband (SSB) phase noise was as low as -70 dBc/Hz     

Stability analysis and numerical simulation of multideviceamplifiers

Stability analysis of multidevice amplifiers is made on a generalized circuit comprising two n-ports with S-matrices S (active devices) and S' (passive networks) connected at n interface ports. Open-loop transfer functions defined for a signal-flow graph and its (n-1) subgraphs of incident and reflected waves at the interface ports are expressed in terms of det M_n and its minors, where M_n=S'S-I_n and I_n is the n×n identity matrix. it is shown that the Nyquist plots of the n transfer functions completely characterize the number of right-half complex-frequency-plane zeros of det M_n, and hence the amplifier stability. Insertion of an ideal circulator and isolators at the interface ports enables one to calculate the Nyquist plots and voltage distributions of possible instabilities using commercially available linear circuit simulators. Numerical simulations for two types of parallel-operated GaAs FET amplifiers are performed to verify the usefulness of the analysis in design-phase check of multidevice amplifier stability