Showing papers on "Interference (wave propagation) published in 1983"

ECG Enhancement by Adaptive Cancellation of Electrosurgical Interference

Abstract: A technique utilizing a combination of adaptive noise canceling and conventional signal processing is developed to enhance electrocardiographic monitoring in the operating room by reducing the noise interference that is created by an electrosurgical instrument. Significant amounts of interference are eliminated by radio frequency shielding, passive and active low-pass filtering, and optical isolation. A digital adaptive canceler using the least mean-square algorithm of Widrow and Hoff is used to reduce the remainder of the interference, yielding an improvement in signal-to-noise ratio of approximately 110 dB. Clear electrocardiograms have been obtained with electrocautery equipment in operation.

Adaptive A/D Converter to Suppress CW Interference in DSPN Spread-Spectrum Communications

Abstract: The non-Gaussian nature of CW interference is exploited to suppress the CW via the A/D converter following chip demodulation in a direct sequence pseudonoise (DSPN) communication link. The modulation is coherent BPSK. The A/D converter quantizes to 2 bits, sign and magnitude. A scheme of threshold adaptation and postquantization weighting gains great advantage from making very reliable decisions on a relatively small percentage of the demodulated chips. The bit error rate performance in CW interference generally surpasses that of an ideal analog DSPN correlator. The performance in Gaussian noise is within 0.6 dB of ideal analog.

Image plane detector for the Dynamics Explorer Fabry-Perot interferometer

Abstract: A compact twelve-channel photon-counting device based on existing Generation II imaging technology has been developed for use as the image-plane detector of the Dynamics Explorer Fabry-Perot interferometer. The device has an S-20 photocathode, three-microchannel plate electron multiplication stages, and an equal-area concentric-ring segmented anode whose geometry mimics that of the interference ring pattern produced by a plane etalon. The twelve channels sample equal and contiguous intervals in the spectrum. The purpose of the development has been to utilize the signal multiplex advantage of a multichannel detector in the measurement of Doppler shifts and line-broadening effects for naturally occurring atmospheric emission features of low intensity. The design, testing, calibration, and flight performance of the novel detector system are presented. In addition, measured emission line profiles at high resolution from the satellite instrument are presented to illustrate the operation of the device.

Rejection of Pulsed CW Interference in PN Spread-Spectrum Systems Using Complex Adaptive Filters

Abstract: In this paper, the performance of direct sequence QPSK spread-spectrum systems using complex adaptive filters in the presence of pulsed CW interference is analyzed. Both adaptive prediction error filters and adaptive transversal filters with two-sided taps are considered. It is shown that the time constant of the tap weight adaptation in the interference off-interval is usually much greater than the time constant in the on-interval, and that this is beneficial for the system since it results in retaining the rejection property of the filter. Under steady-state conditions, the tap weights are calculated. Analytical expressions for the signal-to-noise ratio improvement under the least favorable interference condition are given. It is shown that the performance of the two-sided transversal filter is better than that of the prediction error filter.

Review of Support Interference in Dynamic Tests

Abstract: Introduction T problem of support interference has, of course, existed as long as the wind tunnel. Although numerous investigations of support interference have been performed since the one reported by Per kins, very few review reports have been published. The well-known one by Whitfield only considered the effects of the support in static wind tunnel tests. In this paper, the early work on static support interference will be reviewed very briefly before going into the more complex present-day problems, i.e., dynamic support interference and the special problems associated with static and dynamic tests at very high angles of attack.

Ultrasonic imaging system with correction for velocity inhomogeneity and multipath interference, using an ultrasonic imaging array

Abstract: An ultrasonic imaging system (50) has a means (70) connected to compare the signals received by the transducer elements (54-1 through 54-21). A means (130) is connected to inhibit or otherwise modify gain of selected ones of the signals based on the comparison to reduce multipath ultrasonic wave interference, refraction or obstruction image distortion or degradation. A transducer array (52) is formed as sectors of circles with substantially equal arc lengths. A digital signal processing circuit for the system (50) has a plurality of dual port RAMs (66-1 through 66-21) as delay lines and a cross-correlation means (70) including output addressing circuit (130) controlled by a microcomputer (128). The output addressing circuit (130) generates addresses for output of information from the dual port RAM delay lines (66-1 through 66-21) to accomplish time delay correction and elimination or other gain interference, refraction or obstruction.

Automatic Evaluation Of Holographic Interferograms By Reference Beam Phase Shifting

Abstract: In most cases experimental investigations in fracture mechanics using holographic interferometry demand a spatial resolution better than a fraction of the wavelength of light. High accuracy can be achieved by phase measurements rather than mere fringe counting. A variation of the phase is performed by heterodyne techniques using a frequency shift of the reference waves or by phase shifting of the reference waves during reconstruction. A method was proposed to determine the three dimensional deformation field of object surface points by solving three systems of equations, each one defined by the data of four phase shifted fringe patterns. By means of this method fringe pattern disturbances can be eliminated, an accurate determination of interference phases, even between fringe maxima, is performed, and the sign of the deformation vector can be achieved. Furthermore, this method is well suitable to be automated in the data generation, recording, and evaluation processes, respectively. These aspects will be discussed and for the first time results of an experimental application of this method in practice, containing all the necessary intermediate steps, are presented.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A dual differential interferometer

Abstract: A dual two-beam differential interferometer that measures both the amplitude and orientation of propagating, broadband surface acoustic waves (32). Four beams (16,17,21,22) are focused on a surface (31). The four reflected beams (16,17,21,22) are separated into two pairs. The two pairs (16,21,25) are detected to produce two signals (X,Y) that are used to complete amplitude and orientation.

A New Optical Interferometer for Absolute Measurement of Linear Displacement in the Subnanometer Range

Abstract: A new optical interferometer system for the absolute determination of lattice constant of a crystal is designed and constructed. It can measure linear displacement accurately within the range of 94% of a period of the interference fringes with long term fluctuation of 50 pm for 14 hours and short term fluctuation of 30 pm. Its high visibility, compact and simple structure will allow various applications to precision measurement of dimension in both scientific and industrial fields.

Wave propagation phenomena

Abstract: The phenomenon of wave propagation is encountered frequently in a variety of engineering disciplines. For the design of antennas the interaction with electromagnetic waves has to be known. For earthquake analysis the elastodynamic wave propagation is essential. Knowledge of surface waves of liquids is necessary for the design of harbours and dams, and for the design of pressure vessels and piping networks in several branches of industry pressure transient analyses are required. What these wave propagation phenomena have in common is that they describe the motion of a disturbance with a definite speed (velocity of light, velocity of sound). The specific properties of these waves are reflection,diffraction and interference,-well known phenomena in physical optics.

The Effect of the Interference of Traveling and Stationary Waves on Time Variations of the Large-Scale Circulation.

Abstract: It is hypothesized that the interference of stationary and traveling waves of the same longitudinal can cause some of the observed time variations in the large-scale circulation. To explore this hypothesis the eight-winter average structure of a regularly occurring, westward propagating disturbance which we earlier called the “16-day wave” is further documented. Energy quantities are calculated as this 16-day wave moves in and out of phase with the stationary or time-mean wave. The resulting time variations are similar to some already reported in the literature. Eddy heat momentum transport associated with energy conversions have phase relationships between pressure levels that can be approximately predicted by a simple linear superposition of the observed stationary waves and traveling external Rossby waves. In further support of the hypothesis, cross-spectral results determined from independent data show a reasonable agreement with these predictions.

Real-time diffraction interferometer

Abstract: A real-time diffraction interferometer for analyzing an optical beam comprises converging means (13) for bringing the beam to a focus at focal point (14), and an apertured grating structure (20) positionable adjacent the focal point (14). The apertured grating structure (20) comprises a transparent substrate (10'), an obverse surface of which is coated with a translucent coating (11) except for a pinhole-sized spot (12) that is left uncoated so as to function as an aperture in the coating (11). A reverse surface of the substrate (10') has a lenticulate surface configuration, which functions as a diffraction grating. The beam incident upon the apertured grating structure (20) is separated into a major portion, which is transmitted with attenuated intensity through the translucent coating (11), and a minor portion, which is transmitted with undiminished intensity through the pinhole aperture (12). The major portion of the beam is diffracted into spatially separated diffraction components, and the minor portion of the beam is diffracted by the pinhole aperture (12) so as to acquire a spherical wavefront. Interference patterns produced by interference of the spherical wavefront with each of the wavefronts of the zeroth order and the positive and negative first-order diffraction components of the intensity-attenuated beam transmitted by the coating (11) are separately imaged on conventional solid-state photodetectors (21, 22 and 23).

White-light interferometer

Abstract: A conventional interferometer arrangement (e.g., test glass 12) is illuminated with white light and the resultant colored interference field is recorded by a color TV camera (16). The three output signals (R, G, B) of the color TV camera are directly, or after linear combination, converted into digital values and subsequently serve to address a digital storage (179) in which the phase difference of the interfering waves is stored for each R, G, B combination. The values read from storage are reconverted into analog signals and displayed as so-called pseudo-interference fringes on a TV monitor (18). A computer (19) for the further processing of the read data may be connected to the storage output.

Steered Beam and LMS Interference Canceler Comparison

Abstract: The performance of two kinds of interference cancelers is compared, namely, a constrained steered beam interference canceler (CSBIC) and a least mean square interference canceler (LMSIC). For simplicity, this is done for a two-element array. In our comparison we use the array output desired signal-to-interference power ratio (SIR) and the desired signal-to-noise ratio (SNR). These power ratios are obtained for the CSBIC and LMSIC under two sets of conditions: 1) The error in the assumed angle of incidence for the CSBIC is small, and the LMSIC operates in a codetracking mode. 2) The error in the assumed angle of incidence for the CSBIC is large, and the LMSIC operates in a code-acquisition mode. Comparison of the corresponding power ratios obtained under these two sets of conditions then establishes the condition under which it is more desirable to use a CBSIC as compared with an LMSIC.

Experiments with active phase matching of parallel-amplified Multiline HF laser beams by a phase-locked Mach-Zehnder interferometer.

Abstract: Active phase matching of multiline HF laser beams by means of a phase-locked Mach-Zehnder interferometer was demonstrated by locking the interferometer to the central interference fringe at zero optical path length difference. The central fringe could be found by varying the spectral content of the input beam. Laser amplification in one leg of the interferometer decreased fringe visibility without adversely affecting locking. Single-line fringe patterns produced by an array spectrometer (while the interferometer was operated in its scanning mode) were analyzed to show that no significant dispersion occurred in the amplifier. The techniques developed have potential for measuring dispersion mismatch between larger parallel amplifiers. These experiments demonstrated in principle that a number of multiline HF amplified beams can be recombined and phase-matched to produce a high beam quality output beam.

Simple fiber-optic Fabry-Perot sensor

Abstract: In this paper we describe a simple interferometer which relies upon the interference of light reflected from the ends of a single length of a single-mode fiber. The fiber length thus forms a long-path plane mirror fiber Fabry-Perot (FFP) interferometer. In the scheme reported uncoated fiber ends are used, and the interference due to backreflected light from both ends is monitored. This configuration has an advantage in that the source and detector electronics are used near the same fiber end, while the other is left free. Measurements of the backreflected light also ensure good fringe visibility, and for uncoated fiber ends, near perfect cosine squared fringes are generated. In this way the sensor more closely represents a common path Michelson interferometer. A further advantage of this scheme may be realized by employing a multimode laser and matching the free spectral range of the FFP to the longitudinal mode spacing of the laser.

Performance of IJF-OQPSK Modulation Schemes in a Complex Interference Environment

Abstract: The error probability (P e ) performance of intersymbolinterference and jitter-free offset-QPSK (IJF-OQPSK) modems in a cochannel and adjacent-channel interference environment is evaluated using a computer simulation technique. Hardware design and experiments have been completed to verify the simulation results. The results indicate that a spectral efficiency of 1.5 bits/s/Hz can be obtained with hard-limited IJF-OQPSK channels. This is a significant improvement compared to hard-limited QPSK, OQPSK, and MSK systems. It is expected that the IJF-OQPSK scheme will have applications in low-cost power and bandwidth efficient earth-stations and terrestrial radio systems with transmit high-power amplifiers operating in saturation.

Aspherical Surface Testing With Shearing Interferometer Using Fringe Scanning Detection Method

Abstract: Described is a technique for accurately measuring the wavefront aberration of aspherical optical surfaces with a lateral shearing interferometer. A computer controlled interference phase measuring technique is employed, which provides greater accuracy and real time data analysis. Key elements of the present system are a lateral shearing interferometer with a prallel plate, a piezoelectric-driven mirror, an areal image detector, and a microcomputer system with a graphic display. The shearing interferometer gives a fringe pattern corresponding to the derivative of the wavefront, which is analyzed by the fringe scanning method. By integrating the drivative of the analyzed data, we have the wavefront aberration of the test optics over an aperture containing 32 x 32 element array. A rms accuracy of measurement is 1/32 wavelength is achieved on the evaluation of a f/4 aspherical mirror.

Probability of Error Measurement for an Interference Resistant Transform Domain Processing Receiver

Abstract: In this paper, the results of implementing a transform domain processing spread spectrum receiver are presented. Measured probability of error results for single tone interference conditions are compared to analytical results, and the improvement in system performance achievable by using a notch filter to attenuate the narrowband interference is demonstrated. The theoretical and experimental results agree very well. Measured probability of error results are also presented for an adaptive system where the notch filter tracks the center frequency of the interferer.

Passive ranging method and apparatus using interferometric scanning

Abstract: In a passive-type ranging system in which a pair of directional antennas mounted in spaced apart relationship along a baseline are synchronously scanned on a distant emitter of RF radiation for determining the range to the emitter as a function of a time differential t21 of the received emissions at the spaced antennas, an improved method and apparatus is disclosed in which each of the separate antennas is replaced by an electronically steered interferometer sensor. The pair of spaced, interferometer sensors are scanned in phase-locked synchronization to cause the characteristic multiple interference lobes and intervening nulls to sweep across the distant emitter. Certain of the sensors, lobes, or nulls, are used as highly directional indices for precision measurement of the range determining time differential t21. In order to maintain close phase synchronization between the scanning positions of the multi-lobe interference patterns, a pilot signal emitter is mounted on the same platform as the interferometer sensors and positioned along the boresight of the sensors for emitting a point source of a known signal to which the electronic steering signals that drive the sensors are phase-locked.

Adjacent cell interference in FH-MFSK cellular mobile radio system

Abstract: The effect of adjacent cell interference in cellular mobile system using FH-MFSK transmission is evaluated quantitatively. The performance of base to mobile communication in the system is analyzed, assuming perfect synchronization between users in all the cells. Analysis of the system employing no power control shows that the number of simultaneous users possible at average bit error probability P b of less than 1 × 10-3is reduced greatly from the corresponding figure for the isolated cell (which is about 170). It is then shown that a simple power control strategy could reduce the adjacent cell interference significantly. A reasonable knowledge of the distribution of users within a cell allows the optimization of the receiver threshold with respect to distance from the base. With this optimization, each cell could accomodate ≳ 115 users at P b < 10-3, the exact figure being dependent on the user distribution. The power control also helps to reduce the average power transmitted from a base.

Resolution system for interferometer

Abstract: Analog-to-digital conversion and a computer produce finer resolution for interferometer distance measurement. The system applies to a laser interferometer producing two detected signals with voltages 19 and 20 that vary to form two sine waves in quadrature as interference fringes occur. A pair of analog-to-digital converters 21 and 22 converts each of these signals into digital values subdividing each sine wave fringe cycle into a plurality of increments. An up/down counter 25 supplied with a significant digit from each of the converters counts fringes corresponding to changes in path lengths. A computer 30 arranged with access to the fringe count and the digital increments determines a distance measurement based on the fringe count and a fine resolution of the distance measurement based on final values of the signals at any subdivided fringe cycle increments after changes in the path lengths.

Self-Referencing Wavefront Sensor

Abstract: A modified Smartt point-diffraction interferometer employing phase-shifting electronic phase measurement techniques is described. Special techniques making it possible for the interferometer to give good visibility interference fringes for a large range of input wavefront tilts are discussed. A trade-off between acceptable values of wavefront tilt and light efficiency is presented.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Surface waves in backscattering and the localization principle

Abstract: It is shown that use of the localization principle indicates that the spikes observed in the backscattering from large dielectric spheres cannot be due to surface waves.

Method and interferometer for the measurement of short distances

Abstract: A method and apparatus for the measurement of short distances by means of an interferometer using incoherent light is described. An incoherent beam of light is transmitted via a beam-splitter (2) both to the reference point (3) and to the object (4) to be measured, and the position of the reference device is adjusted until interference is detected, at which time the distances passed by the beams are equal. The position of the reference device (3) is adjusted periodically by means of an automatic actuating device, which may be, e.g., a mirror attached to a loudspeaker mechanism, and the interference is detected by means of an electronic detector (5), as well as, at the moment of interference, the position of the actuating device is read, e.g., on the basis of the momentary value of the supply signal controlling the actuating device. By regularly monitoring the timing of the repeated interference moments in relation to the movement of the actuating device, it is possible, after calibration of the apparatus, to observe the changes in the position of the object (4) to be measured continuously.

Adjacent-Channel and Quadrature-Channel Interference in Minimum Shift Keying

Abstract: In this paper we present formulas for the computation of error probability in the presence of quadrature-channel or adjacentchannel interference in addition to intersymbol interference in a minimum shift keying system. The filters in the receiver and transmitter are arbitrary but with a finite number of poles. The effect of phase jitter in the main channel, phase and symbol timing misalignment in the interfering channels, and sampling time jitter is taken into account. The probability of error is averaged over the phase and symbol timing misalignment. Numerical results are presented for Butterworth filters in the receiver and transmitter with two, three, and four poles. Curves of error probability as a function of various variables (signal-to-noise ratio, bandwidth of receiver and transmitter filters, number of poles, channel frequency separation, phase jitter, sampling time, and symbol timing and phase misalignment) are presented. The method of this paper can readily be applied to other filters; hence, it can be used in the design and prediction of the performance of digital communication systems.