James H. Schaffner

Distortion in linearized electrooptic modulators

Intermodulation and harmonic distortion are calculated for a simple fiber-optic link with a representative set of link parameters and a variety of electrooptic modulators: simple Mach-Zehnder, linearized dual and triple Mach-Zehnder, simple directional coupler (two operating points), and linearized directional coupler with one and two dc electrodes. The resulting dynamic ranges, gains, and noise figures are compared for these modulators. A new definition of dynamic range is proposed to accommodate the more complicated variation of intermodulation with input power exhibited by linearized modulators. The effects of noise bandwidth, preamplifier distortion, and errors in modulator operating conditions are described. >

Passive Millimeter-Wave Imaging Module With Preamplified Zero-Bias Detection

An analytical model and supporting measured data are presented for a preamplified W-band radiometer with a zero-bias detector appropriate for commercial millimeter-wave imaging cameras. Basic radiometer parameters, including RF bandwidth, are computed directly from simple low-frequency measurements and compare well with those obtained from RF measurements. A detailed analytical model shows how radiometer performance depends on internal component parameters, such as low-noise amplifier gain, noise factor, reflection coefficient, detector responsivity, etc. The measurements suggest that performance is sufficient for operation without a Dicke switch or mechanical chopping. A measured noise equivalent temperature difference of 0.45 K was obtained, assuming a single sensor is scanned across a focal plane, forming 32 pixels with 3.125-ms integration time per pixel. This sensitivity is considered sufficient by commercial manufacturers to obtain quality images in low-contrast (e.g., indoor) environments.

A tunable impedance surface performing as a reconfigurable beam steering reflector

We describe a reconfigurable microwave surface that performs as a new kind of beam steering reflector. The surface is textured with an array of tiny resonators, which provide a frequency-dependent surface impedance. By tuning the individual resonators, the surface impedance, and thus the reflection coefficient phase, can be varied as a function of position across the reflector. Using a reflection phase gradient, the surface can steer a reflected beam. As an example, we have built a simple mechanically tuned surface in which physical motion of only 1/100 wavelength generates a sufficient phase gradient to steer a reflected beam by /spl plusmn/16 degrees. To steer to greater angles, the surface can be configured as an artificial microwave grating, capable of /spl plusmn/38 degrees of beam steering. The concept of the tunable impedance surface demonstrated here can be extended to electrically controlled structures, which would permit more elaborate reflection phase patterns, and provide more capabilities, such as the ability to focus or steer multiple beams.

Measurements and models for 38-GHz point-to-multipoint radiowave propagation

This article presents results of a wide-band measurement campaign conducted at 38 GHz. The objective of the research was to determine multipath and time varying channel behavior of short-hop millimeter-wave point-to-multipoint radio links during various weather events. 73963 power delay profiles (PDPs) were captured on three links, each comparable to proposed local multipoint distribution systems (LMDS) in a campus environment. Multipath was observed in unobstructed LOS links during rain but not during clear weather. Short-term variation of the received signal over 1-2 min observation periods is described by a Rician distribution with a K factor which varies as a function of rain rate. Measured rain attenuation exceeds Cranes (1996) model predictions by several decibels. A novel prediction technique is presented that applies canonical antenna patterns and site specific information to estimate worst case multipath channel characteristics including relative power, time of arrival (TOA), and angle of arrival (AOA) of each multipath component. New metrics, the excess delay zone and relative power zone, are defined and contour plots are developed to determine potential reflectors from an area site map. These results and models provide useful guidelines for the design of millimeter-wave wireless communication systems.

Reconfigurable aperture antennas using RF MEMS switches for multi-octave tunability and beam steering

The requirements for increased functionality within a confined volume will place greater burdens on electromagnetic platforms for air, space, and sea over the next few decades. An important piece of the any solution to these new requirements are transmitting and receiving apertures that can handle multi-octave bandwidths with beam steering capability. The ability of an aperture to be reconfigured for a particular mission will become essential. New types of devices are being developed which will enable the realization of these reconfigurable apertures. This paper presents a discussion of how one of these new devices, the RF MEMS switch, can be utilized to change the phase and frequency characteristics of conventional antenna elements to perform beam steering over a wide range of microwave frequencies.

Intermodulation distortion in high dynamic range microwave fiber-optic links with linearized modulators

Linearization of integrated optic intensity modulators significantly reduces the two-tone intermodulation distortion. The resulting intermodulation distortion produced by these modulators then varies as the input power to the fifth-order link system, the overall intermodulation product is a combination of third-order and higher-order terms. The authors determine the dynamic range of a cascaded microwave network consisting of a preamplifier, a high-dynamic-range fiber-optic link with a highly linear modulator, and a postamplifier. An expression is found that relates the intermodulation power at the output to the relative suppression from the signal level. As an example, a hypothetical 10-GHz low-distortion fiber-optic link that has a dynamic range of 125 dB in a bandwidth of 1 Hz is cascaded with various preamplifiers, and it is shown that the dynamic range of the system is reduced by as much as 20 dB, depending on the third-order intercept of the amplifier.

A Method for Improving the Efficiency of Transparent Film Antennas

This letter presents an optically transparent antenna, which is made of conductive AgHT coated film, with an improved efficiency by applying a highly conductive coating or metallization in the form of a very narrow strip to selective antenna areas of high current density. A measurement showed that the proposed method improved the maximum gain of a fabricated transparent antenna using the AgHT film from -5 dBi to 0 dBi at 2.2 GHz while preserving much of the optical transparency. A 3D electromagnetic model using a full-wave method of moments was developed to investigate the effect of width of the highly conductive narrow strip on the efficiency of the transparent antenna.

Wave-coupled LiNbO/sub 3/ electrooptic modulator for microwave and millimeter-wave modulation

A new technique of phase velocity matching in electrooptic modulators was demonstrated. The results show that the phase velocity mismatch due to material dispersion in traveling-wave LiNbO/sub 3/ optical waveguide modulators can be greatly reduced by breaking the modulation transmission line into short segments and connecting each segment to its own surface dipole antenna. The array of antennas is then illuminated by the modulation signal from below at the proper angle to produce a delay from antenna to antenna that matches the optical waveguides delay. A phase modulator 25 mm in length with five antennas and five transmission line segments was operated from 4.6 to 13 GHz with a maximum phase modulation sensitivity of over 100 degrees /W/sup 1/2/.<<ETX>>

60 GHz and 94 GHz antenna-coupled LiNbO/sub 3/ electrooptic modulators

Antenna-coupled LiBbO/sub 3/ electrooptic modulators can overcome the material dispersion which would otherwise prevent sensitive high-frequency operation. The authors previously demonstrated the concept with a phase modulator at X-band. They have extended this demonstration to a narrowband 60-GHz phase modulator and broadband amplitude modulator designs at 60 and 94 GHz, respectively.<<ETX>>

Spur-free dynamic range measurements of a fiber optic link with traveling wave linearized directional coupler modulators

Two-tone testing was performed on a laboratory fiber optic link with LiNbO/sub 3/ traveling wave linearized directional coupler modulators. The third-order distortion was measured at 500 MHz and 1000 MHz, and the spur-free dynamic range was determined. Two modulator configurations, with either one or two passive bias sections, were tested and the results were compared with measurements of simple directional coupler modulators. A 9.5-11 dB improvement in the dynamic range was found in the links employing the linearized modulators.<<ETX>>