Interference effects on Space Station Freedom and Space Shuttleorbiter Ku-band downlinks

The Space Shuttle orbiter (SSO) Ku-band single access return (KSAR) link and the Space Station Freedom (SSF) KSAR link via the tracking and data relay satellite system (TDRSS) use the same carrier frequency. The interference between spacecraft is minimized by opposite antenna polarizations and by TDRSS antenna beam pointing, but if the SSF and SSO are in close proximity, it is expected that mutual interference will be significant. It is shown that a simplified analytical approach will yield adequate accuracy for the expected range of operating conditions. Relative degradation in bit-energy-to-thermal-noise power spectral density ratio to achieve a 10^-5 coded bit-error probability is determined to be 4 dB for the Ku-band SSO-to-TDRS I-channel return link with a 4.5-dB effective signal-to-interference total power ratio (S/I) when the Ku-band SSF-to-TDRS return link interferes. For the Ku -band SSF-to-TDRS return link, both analysis and simulation results yield a relative signal degradation of 0.4 dB at the effective S/I=21.6 dB     

Photodetector nonlinearity limitations on a high-dynamic range 3GHz fiber optic link

The performance of a dc to 3 GHz externally modulated link utilizing balanced high-power photodetection is presented. Nonlinearity measurements of high power photodiodes show 1 dB compression currents in excess of 55 mA and an output third-order intercept point of +32 to +34 dBm. These high current photodetectors permit the use of high power lasers as external modulator sources for low noise fiber optic links with only small degradations in the predicted link dynamic range. An externally modulated link with a 240 mW Nd:YAG laser, a dual-output 4-V V, (dc) modulator, and balanced 100 mA total photocurrent yielded a link noise figure from 15.5 to 17.5 dB, a spur-free dynamic range of 119.5 dBHz^2/3, and a 1-dB compression dynamic range of 168.4 dBHz     

Fiber-optic microwave transmission using harmonic laser mixing,optoelectronic mixing, and optically pumped mixing

Three fiber-optical link configurations are proposed for use in microwave and millimeter-wave signal transmission. Harmonic laser mixing, optoelectronic mixing and optically pumped mixing are successfully utilized to achieve high carrier frequencies in fiber-optic links. The performance of harmonic laser diode mixers is experimentally investigated in the X-band. The p-i-n photodiode is used as an optoelectronic microwave mixer and an optically pumped microwave mixer, and the microwave characteristics of these mixers are demonstrated. These three fiber-optic link configurations show promise in transmitting microwave and millimeter-wave frequencies     

Maximum dynamic range operation of a microwave external modulationfiber-optic link

We fully analyze the analog performance of an external modulation fiber-optic link. We express relevant figures of merit-including gain, noise figure, third-order intermodulation distortion, AM compression, and dynamic range-in terms of the microwave scattering matrices of the modulator and detector circuits, and we predict the modulator bias condition promoting optimum link performance. Our predictions match the measured gain, noise figure, and dynamic range of an experimental 870-930 MHz external modulation fiber-optic link. Maximum spurious-free dynamic range-77 dB·MHz^2/3 (117 db·Hz^2/3 )-occurs when the modulator is biased at its halfwave voltage, where the optical throughput is nearly pinched off     

Nonlinearity of p-i-n photodetectors

At higher operating frequencies, the field dependence of the carrier velocity in p-i-n photodetectors generates harmonics and intermodulation products that can degrade the dynamic range of RF fiber-optic links. The authors present both a perturbational theory and measured harmonic data for a p-i-n photodiode operated at very high power densities which show that this and other detector nonlinear effects need not seriously compromise link performance. In particular, neither transit-time nor static nonlinearities in p-i-n photodiodes need limit the dynamic range of fiber-optic links operating below 5 GHz. The fact that the theoretical bandwidth of the photodiode, with all parasitic capacitance and inductance ideally removed, is 17 GHz, suggests that comparable spur-free performance should be achievable at X and Ku-band frequencies, once packaging parasitics are reduced     

An analytic and experimental comparison of direct and externalmodulation in analog fiber-optic links

Analytic lumped-element small-signal models of directly and externally modulated analog fiber-optic links are derived. Incremental modulation efficiency is defined and used to compare the performances of these modulation techniques. In experiments to optimize link RF-to-RF gain and noise figure, the measurements obtained agreed with calculations to within ≈1 dB. The externally modulated link was operated with two different impedance matching circuits. With a low-pass match the bandwidth was 150 MHz, and the link transducer gain was 1 dB; with a bandpass match the bandwidth was 22 MHz, the link transducer gain was 11 dB, and the noise figure was 6 dB. The directly modulated link was operated with a low-pass match. In this case, the bandwidth was 1 GHz, the link transducer gain was -14 dB, and the noise figure was 33 dB. These experimental results were achieved with no amplification,     

Linewidth-insensitive coherent AM optical links: design,performance, and potential applications

The use of coherent detection in analog optical links offers several advantages over direct detection: improved receiver sensitivity, inherent frequency translation, and the ability to utilize angle modulation and separate wavelength division multiplexed (WDM) signals. In this paper, we investigate an externally modulated coherent AM optical link. We study the dynamic range of the coherent AM link, considering receiver noise, laser phase noise, laser relative intensity noise (RIN), and system nonlinearities. With proper selection of the receiver's IF bandwidth, the coherent AM link can be made insensitive to the laser linewidth. For optical powers less than 5 mW, RIN of less than -160 dB/Hz reduces the spurious-free dynamic range (SFDR) by less than 3 db with the use of a balanced receiver. The external modulator nonlinearity is the dominant nonideal effect; it reduces the SFDR by 5-19 dB from the theoretical limit for 100% modulation index. We compare the performance of the coherent AM link with that of a conventional direct detection link for two applications: point-to-point links and distribution networks. When the received optical power is less than 1 mW, the coherent link can provide higher SFDR than the direct detection link. Thus, coherent links are well-suited for long distance point-to-point links and FM video distribution systems     

Broad-band electronic linearizer for externally modulated analogfiber-optic links

Linearization of analog fiber-optic links using a novel CMOS linearization circuit is reported. A 17-dB suppression of the third-order intermodulation product (IMP3) has been achieved at the modulation index of 49.6 % and over a broad-band frequency range from dc to 1.3 GHz. In an experimental link with a noise floor of -124 dBm/Hz, the spurious free dynamic range (SFDR) is improved by 14 dB from 85 dB/Hz^2/3 to 99 dB/Hz^2/3     

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     

Noise caused by semiconductor lasers in high-speed fiber-opticlinks

Theoretical and experimental results are presented for the signal-to-noise (S/N) ratio caused by mode partition noise, intensity noise, and reflection-induced noise in optical data links. Under given conditions an additional noise source with a S /N ratio of 20 dB will cause a power penalty of 1 dB in order to maintain a 10^-9 bit error rate. From numerical simulations the authors predict the maximum allowable dispersion in the presence of mode partition noise to be approximately 40% of a clock period. This figure is almost independent of bit rate and laser structure and agrees well with the measurements and with results of other workers. Numerical simulations of a buried-heterostructure and a TJS laser were carried out at four bit rates from 565 Mbit/s to 4.5 Gbit/s and the measurements were done at 2.2 Gbit/s using a TJS laser     

Long microwave delay fiber-optic link for radar testing

A long fiberoptic delay line is used as a radar repeater to improve radar testing capabilities. The first known generation of 152 μs delayed ideal target at X-band (10 GHz) frequencies having the phase stability and signal-to-noise ratio (SNR) needed for testing modern high-resolution Doppler radars is demonstrated with a 31.6-km experimental externally modulated fiberoptic link with a distributed-feedback (DFB) laser. The test application, link configuration, and link testing are discussed     

Calibrated imaging radar polarimetry: technique, examples, andapplications

The authors developed a calibration procedure for imaging radar polarimeters and applied it to a set of images acquired by the NASA DC-8 multifrequency radar system. The technique requires the use of ground reflectors of known cross-section for absolute calibration, that is, solution for σ⁰; however, the image data themselves can usually provide all information necessary for phase calibration and for antenna crosstalk correction. The accuracy of the approach, as measured by calculating the cross-section residuals of known targets in each calibrated scene, is on the order of ±1-2 dB at the P- and C-band, but improves to ±0.5 dB at the L-band. The authors present the results of applying this technique to radar scenes of lava flows of varying roughness, temperate and tropical rain forests, and ocean water surfaces. They also present several example applications which are feasible with calibrated data but which would be difficult to implement with uncalibrated data     

Fiber-optic magnetic-force phase modulator

A fiber-optic magnetic-force phase modulator using an aluminium-jacketed current-carrying fiber coil placed in a uniform external magnetic field is presented. The modulator has a center frequency of 30 kHz with a modulation efficiency of 2.4 rad/mA and a Q of 4.6. A theoretical model is developed for the modulator, and its predictions are compared with experimental results. An accurate and simple method of characterizing the phase modulator response is described     

An X-band spacecraft transponder for deep spaceapplications-design concepts and breadboard performance

The design concepts and measured performance characteristics of an X-band breadboard deep-space transponder (DST) for future spacecraft applications are summarized. The DST consists of a double-conversion, superheterodyne, automatic phase tracking receiver, and an X-band exciter to drive redundant downlink power amplifiers. The receiver acquires and coherently phase tracks the modulated or unmodulated X-band uplink carrier signal. The exciter phase modulates the X-band downlink signal with composite telemetry and ranging signals. The measured tracking threshold, automatic gain control (AGC), static phase error, and phase jitter characteristics of the breadboard DST are in good agreement with the expected performance. The measured results show a receiver tracking threshold of -158 dBm and a dynamic signal range of 88 dB     

Continuous phase quadrature phase shift keyed signaling techniquewith coding

A continuous phase quadrature phase shift keyed (CPQPSK) modulation technique is presented. This method utilizes a conventional QPSK modulator and a phase trajectory converter to approximate M=4, h=1/4 continuous phase signal and allows low cost, low complexity, and high rate (>1 Gbit/s) CPM modem implementation for bandwidth efficient transmission through nonlinear satellite channels. Using a communications analysis computer program it has been found that CPQPSK has 99 percent out-of-band power of 0.8R (MSK has 99 percent out-of-band power of 1.2 R where R is defined as bit rate), continuous phase trajectories, and nearly constant envelope amplitude. Simulation of realistic hardware designs indicate that the CPQPSK will require an Eb/No of 14 dB to achieve a bit error rate (BER) of 10^-6. Forward error correcting techniques using block codes with an overhead of 10 percent indicate that the Eb/No requirements can be reduced to 11.2 dB for 10^-6 BER     

Evaluation of 4-Gbit/s optical fiber transmission distance with direct and external modulation

Transmission characteristics for a recently modulated measured distributed-feedbacked (DFB) laser and an externally modulated DFB laser using a Ti:LiNbO/sub 3/, Mach-Zehnder modulator at 4 Gb/s are discussed. The transmission characteristics are estimated by an advanced eye-pattern analysis method. The maximum measured fiber dispersion with a directly modulated laser is 100 to 140 ps/nm when the chirp power penalty is 1 dB. However, for external modulation, there is no power penalty after transmission over a 2220-ps/nm dispersive fiber. This confirms that external modulation has superior transmission characteristics. The modulation scheme for 4-Gb/s systems in terms of these results is discussed.<>     

A 16-b 320-kHz CMOS A/D converter using two-stage third-orderΣΔ noise shaping

The design and measured performance of a two-stage third-order ΣΔ (sigma-delta) analog-to-digital (A/D) converter is described. The A/D converter achieves a 96-dB dynamic range and a maximum signal-to-noise-plus-distortion ratio (S/(N+ D)) r.m.s./r.m.s. of 93 dB with 320-kHz output rate and an oversampling ratio of 64. An analysis of the integrator gain error is presented. The modulator is realized in a 1.2-μm double-metal single-poly CMOS process with an active area of 1.6 mm². This modulator operates from a 5-V power supply and a single reference voltage     

Microwave and millimeter-wave QWITT diode oscillators

The authors present DC, microwave, and millimeter-wave characteristics of different quantum-well-injection transit-time (QWITT) devices. Small-signal and large-signal device models are used to provide physical design parameters to maximize the output power density at any desired frequency of operation. A peak output power density of 3.5-5 kW/cm² in the frequency range 5-8 GHz has been obtained from a planar QWITT oscillator. This appears to be the highest output power density obtained from any quantum-well oscillator at any frequency. This result also represents the first planar circuit implementation of a quantum-well oscillator. Good qualitative agreement between DC and RF characteristics of QWITT devices and theoretical predictions based on small-signal and large-signal analyses is achieved. The device efficiency has been increased from 3% to 5% by optimizing the design of the drift region in the device through the use of a doping spike with optimized concentration, without compromising the output power at X -band. Self-oscillating QWITT diode mixers are also demonstrated at X-band in both waveguide and planar circuits. The self-oscillating mixer exhibits a conversion gain of about 10 dB in a narrow bandwidth and a conversion loss of about 5 dB if broadband operation is desired     

Microwave systems design for high-performance moving targetindicators in radars

Clutter cancellation of 65 dB and better is directly proportional to good radar stability, and since many hardware areas produce instabilities at various levels, the architecture of a radar requires special design considerations to support this high stability. The noise character and generation methods of these instabilities in the various hardware areas are described, and design solutions are given to eliminate them. Microwave delay line, a reliable, accurate method of measuring radar stability in L- and S-band radars, is described. The longest microwave delay line available for use at L -band and S-band frequencies is a 15-μs sapphire bulk acoustic wave (BAW) delay line. For higher-frequency radars, smaller delays must be used to keep the insertion loss down to a usable level. The question is raised as to the adequacy of this delay time to provide sufficient visibility for stability measurements of the stable noise. For transmitter measurements, it is adequate for the more common pulse widths, which are less than 15 μs. For LO measurements, the analysis shows that this delay does provide sufficient decorrelation for accurate LO noise measurements     

Prediction of slant path rain propagation statistics usingdual-polarized radar

The authors conducted a year-long experiment in which a dual-polarized S-band radar probed the volume surrounding two 11.45-GHz satellite downlink paths during rain. Accuracy was assessed by comparison to directly measured link attenuation with two 11-GHz beacon receivers 7.3-km apart at an 18.5° elevation angle, one colocated with the radar. Drop size distributions calculated from the radar horizontal reflectivity (Z_H) and differential reflectivity (ZDR) measurements were used to predict 11.45-GHz satellite beacon attenuations. The radar-predicted attenuations and those measured on the radio links agree, both on an event basis and in terms of annual cumulative distributions