Showing papers on "Modulation published in 1977"

High density digital transmission system

Abstract: A high density bandlimited digital transmission system with a transmitter and a receiver connected by a radio path with an end-to-end channel transfer function selected to form at the receiver either raised cosine pulses or partial response pulses. The transmitter is partitioned or divided into an active pulse forming network and a passive pulse shaping and bandlimiting network. The active and passive networks are concatenated and in combination provide an optimal transmitter-receiver combination for raised cosine or for partial response signaling. Transmit and receive filters provide optimal signal pulse shaping while meeting specified out-of-band signal suppression. In-phase and quadrature phase carriers (I and Q channels) work with rectangular or half-wave sinusoidal modulating pulses. Further, time-offset or time-coincident pulses on each of the respective I and Q channels are used. Multi-level embodiments are also disclosed. Receivers using time-offset modulation forms allowing coherent detection using VSB and SSB techniques with resulting simplification of the receiver structures. This mode of operation is possible without the use of sideband separation filters at the transmitter. Only the receiver structure is affected. Preferred embodiments are described that closely approach optimal performance and achieve information densities in the range from 1.75 to 4 bits per second per Hertz.

Optimal reception of digital data over the Gaussian channel with unknown delay and phase jitter

Abstract: Asymptotically optimum (in the sense of minimum per-symbol error rate) receiver structures for data communication over the white Gaussian channel with unknown time delay and carrier phase jitter are developed. The receiver structures apply to the following suppressed-carrier modulation systems: double sideband (DSB), quadrature amplitude modulation (QAM) with an arbitrary constellation, vestigial sideband (VSB) and single sideband. The resulting minimum error probability receivers are asymptotically equivalent to maximum-likelihood digital {\em sequence}-estimating receivers. The optimum structures implicitly derive joint maximum-likelihood estimates of the unknown parameters and of the sequence of data symbols. It is shown that the parameter estimates can be obtained from two data-directed stochastic approximation algorithms. Unlike traditional theoretical treatments of this communication situation, which have separated the highly important carrier phase and timing recovery problem from the detection problem, a unified theory is presented from which the complete ideal receiver structure can be deduced.

Optimal codes for digital magnetic recording

Abstract: Channel coding is universally employed in digital magnetic recording to match certain properties of the coded data to the characteristics of the recorder channel. In this paper, attention is focused primarily on the digital sum variation and d.c. content of various codes. The older, more familiar examples, such as NRZ, Manchester, enhanced-NRZ and Miller are compared with the more recently developed group, zero modulation and M2 codes.

Modulation behavior of semiconductor injection lasers

Abstract: GaAs double heterostructure semiconductor injection lasers which now exhibit more than 25000 h cw room temperature lifetime are of great interest for future use as directly modulated transmitters for high bit-rate fiber optical communications. The effects limiting this application are modulation distortions, spectral width and additional spectral broadening in the case of modulation and spontaneous fluctuations of the output power. The dynamic and spectral behavior of injection lasers, the methods of high bit-rate modulation and the improvement of the high bit-rate modulation capability by coupling two lasers are discussed.

Suppression of relaxation oscillation in light output of injection lasers by electrical resonance circuit

Abstract: It is shown that relaxation oscillation in the light outputs of injection lasers can be suppressed by an external electrical circuit. This method can be applicable to high-speed pulse modulation. For this purpose, an electrical resonant circuit connected to the laser diode (LD) and tuned to the relaxation oscillation is considered as an example. Frequency response of direct modulation and the device-transient characteristics are analyzed. These results were verified experimentally. A brief discussion on the upper limit of the repetition frequency of pulse modulation is given and it is shown theoretically that stable pulse modulation of an LD up to a few Gbit/s may be obtained by applying this method.

Bias control circuit for light modulators

Abstract: A bias control circuit for light modulator includes a feedback control loop through which a measure of the modulated laser beam is processed to derive an error correction signal which is superimposed on the normal bias potential to shift the bias point in the same direction as the direction of shift of the operating curve of the light modulator due to the heat generated therein during operation. A feed-forward control loop is also provided to cancel an error signal in the feedback control loop resulting from an intentional gradial variation of the laser beam intensity prior to the modulation so that the average light intensity of the modulated laser beam is correspondingly varied.

System for reducing the effects of power supply switching

Abstract: A power supply arrangement that substantially eliminates the effects of the sidebands and harmonics at the switching frequency of the power supply on the RF carrier of an amplifier or other unit being supplied power. The switching frequency of the power supply arrangement is continuously varied in frequency at an appropriate rate and deviation with the result that a band of frequencies replaces the fundamental switching frequency of the power supply and all harmonics so that noise created by the switching frequency signal and the RF signal are then similarly diffused. The net power present as spurious noise in the amplifier unit is constant with the maximum amplitude of the noise as viewed, for example, in a doppler filter being substantially reduced. In a radar transmitter, for example, the transmitted pulse frequency appears as PRF sidebands of the CW frequency and the extraneous modulation from the switched power supply provides sidebands of the PRF sidebands. At each of these PRF sidebands, the switching frequency modulation, in accordance with the invention, causes the sidebands resulting from the switching frequency to be spread over a relatively wide band so as to substantially decrease the amplitude of the spurious signals.

A Look at Crosstalk in Quadrature-Carrier Modulation Systems

Abstract: A method is presented for comparing the performance of different quadrature-carrier modulation techniques. It is based on the calculation of the mean-square crosstalk between adjacent digital communications channels. This method was used to determine the crosstalk of a number of constant envelope modulation schemes, including Staggered Quaternary Phase Shift Keying (SQPSK) and Minimum Shift Keying (MSK). The results, as have previous results, show the improved performance of MSK over SQPSK. By utilizing the general equation for mean-square crosstalk as a measure of performance, a search was made for constant envelope modulation methods with reduced crosstalk. A number of methods were found which have better mean-square crosstalk performance than MSK. Finally this paper presents a method for reducing degradation due to crosstalk by using mismatched windows in the standard correlation detector. An example of this technique for detection of MSK signals is analyzed.

Magnetomer using a thin magnetic film optical waveguide with a.c. modulation and automatic nulling

Abstract: The present invention relates to devices intended to measure magnetic fields, and also known as magnetometers. It consists in using a zeroing method to measure the influence of the Faraday effect of the field being measured, upon a radiated energy beam propagating in guided fashion through a thin film of magnetic material.

Spectra of PSK Signals with Overlapping Baseband Pulses

Abstract: This paper derives a new power spectrum formula for constant-envelope PSK signals. The only signal constraints are that the baseband pulses be of finite duration and the pulse amplitudes be mutually independent. The computer program used to implement the formula is made highly efficient by the use of fast Fourier transform (FFT) algorithms. The key contribution of the new result is the high degree of baseband pulse overlap permitted before the computation cost becomes excessive. In some computation approaches, this cost grows as LN, where L is the number of modulation levels and N is the number of data periods spanned by the pulse. In the approach developed here, the cost is independent of L and grows roughly linearly with N . The cost improvement, which is substantial for large L and N , is exploited here by using the new approach for a wide variety of baseband pulses. Results are given which demonstrate the potential value, in terms of narrower spectra and lower sidelobes, of using overlapping pulses.

High speed frequency modulation of far infrared lasers using the Stark effect

Abstract: Electronic frequency tuning of an optically pumped far infrared waveguide laser has been achieved by using the Stark effect. Frequency modulation with a 50-kHz modulation frequency and an index greater than 1 has been observed as well as a maximum modulation frequency of 300 kHz.

Distortion sound effects circuit

Abstract: A distortion sound effects circuit producing an output signal corresponding to an input signal but containing one of four selected types of distortion. A compressor which includes a voltage-controlled amplifier first compresses the audio input signal within preselected limits, and the distortion operations are performed on the compressed audio signal to advantageously render the subsequent distortion operations insensitive to gross amplitude fluctuations of the input signal. The compressor includes means for full-wave rectifying the compressed audio signal. The full-wave rectified signal is provided as a distorted output signal containing hard-even distortion of the type commonly achieved by severely, asymmetrically clipping a signal. A distortion circuit includes single voltage controlled square-low amplifier to which the compressed audio signal is applied performs the other three types of distortion. Soft-odd distortion is achieved by slightly and symmetrically overloading the input stage of the amplifier by an amount less than would result in clipping at the output stage. Hard-odd distortion is achieved by symmetrically and severely overloading the input of the amplifier to cause clipping at the output stage. Soft-even type distortion is achieved by squaring the compressed audio signal through modulation of the amplifier gain with the compressed audio signal while it is also applied to the amplifier signal input. A mixer having three inputs respectively coupled with potentiometers respectively fed by the amplifier output, the full-wave rectifier output and the undistorted, compressed audio signal output of the compressor circuit enables a musician to mix the three signals in any desired proportion.

Channel depletion boundary modulation magnetic field sensor

Abstract: A high sensitivity, low noise, broad bandwidth channel conduction field sensor device is described. The conductive channel is configured to create an exceptionally narrow, undepleted conduction channel of approximately filamentary form. The filamentary conductive channel so formed is provided with a source at one end of the channel and two or more laterally spaced drains at the other end thereof. Electric or magnetic fields may be utilized to deflect a stream of charge carriers traversing the conductive channel from the source toward the drains utilizing the depletion width modulation effect of the fields upon the boundaries defining the conductive channel portion. Modulation of the depletion zone width and depth along the channel sides effectively moves the stream of carriers and the conductive channel area to overlap one drain more than another. This develops a differential drain current balance which can be utilized to provide an output signal. Width and length criteria for defining a filamentary channel structure are described for the ultimate desired configuration and size which are to be obtained. As noted, operation of the device is based upon modulation of the width and depth of the depletion zone boundaries defining the conductive channel. An increased signal output is obtained by reducing the width of the channel to eliminate excess carriers normally found in wide channel devices and, further, by making the depletion zones as large a portion of the total channel width as can be obtained.

Light emitting element drive system

Abstract: In a system in which a light emitting element such as a semiconductor laser, light emitting diode or the like is modulated directly by a drive current corresponding to an input modulation signal, the drive current is selected to correspond to an integrated value of the input modulation signal, thereby compensating for deterioration of the modulation characteristic based on a thermal effect in the light emitting element to obtain a desired optical output waveform.

A Worst-Case Crosstatk Comparison Among Several Modulation Schemes

Abstract: The effects of worst-case crosstalk (from a time-domain point of view) on unsynchronized frequency division multiplexed (FDM) constant envelope modulations are considered assuming coherent hard-decision receivers and no filtering for bandwidth constraint. Continuous-phase frequency shift keying (FSK) is found to be superior to various forms of phase shift keying (PSK).

Light emitter with laser diode control - uses detector output signal and has circuit providing output and bias control signals for diode

Abstract: From the output of a detector comprising a photodiode (1) and a wide-band amplifier (2) are derived a first signal (P1), corresponding to the level of maximum modulation, and a second signal (P0), corresponding to the minimum level of modulation. The second signal (P0) is subtracted from the first signal (P1) in a differential amplifier (9). The output (PM) represents the value of modulation. This output (PM) is fed to one input of a comparator (10), and a modulation reference signal (PMref) is fed to another input. The output of the comparator (10) is used to keep the laser to its predetermined output. The second signal (P0) is also compared in a second comparator (11), with a reference threshold value (P0 ref) representing the required emission threshold. The output of the second comparator (11) is a threshold error signal which is used to adjust the bias voltage of the laser diode.

Suboptimal Continuous Shift Keyed (CSK) Demodulation for the Efficient Implementation of Low Crosstalk Data Communication

Abstract: A suboptimal receiver of constant envelope, continuousfrequency MSK-type modulations called Continuous Shift Keying (CSK) is proposed that halves demodulator complexity with no more than a 1.6 dB loss in detectability in AWGN. The technique is especially applicable to a frequency division multiple access system composed of many unsynchronized user terminals.

Design Procedure for Wide Bandwidth Acousto-Optic Modulator

Abstract: In this paper, we give the design equations and a procedure for the complete specification of the design parameters for a wide bandwidth acousto-optic modulator (AOM). Although the modulating medium, transducer material, and bonding agents determine the modulator performance, we concentrate here on the question of how to design an AOM, given specifications on its minimum modulation bandwidth, diffraction efficiency, and the maximum ellipticity of the output beam.

A Digital Filter-Modulation Combination for Data Transmission

Abstract: It is shown that it is possible to generate a band-limited PSK signal using a ROM and a four-quadrant multiplying D/A Converter.

Method and apparatus for transmitting auxiliary channel over digital communications system

Abstract: An auxiliary channel, for example, an engineering orderwire, is derived onn existing time-division multiplex carrier system by modulating the pulse repetition rate of the digital bitstream. The modulating signal itself may be an analog or a digital signal. The modulator and demodulator both include a first-in, first-out shift register acting as a buffer, and the desired modulation is effected by varying the rate at which the digital information is fed out of the FIFO buffer at the transmitting end of the carrier system.

Four phase to two phase correlator

Abstract: A four phase to two phase correlator is incorporated within the demodulator section of a spread spectrum modem to process a received RF signal modulated by a transmitted PN code and a digital data signal The correlator includes a first modulator for combining the channel 1 and channel 2 R code signals with the received RF input signal to generate a first channel 1 RF signal and a first channel 2 RF signal The first channel 1 RF signal is modulated by the transmitted PN code signal, the channel 1 R code signal and the digital data signal The first channel 2 RF signal is modulated by the transmitted PN code signal, the channel 2 R code signal and the digital data signal A second modulator combines the channel 1 and channel 2 PN code signals and the channel 1 and channel 2 R code signals with a first unmodulated RF signal to generate a second channel 1 RF signal which is modulated by the channel 1 PN code signal and by the channel 1 R code signal, and a second channel 2 RF signal which is modulated by the channel 2 PN code signal by the channel 2 R code signal A third modulator combines both channels of the first and second RF signals to generate a third RF signal which is modulated only by the digital data signal

Optimized Designs for Fiber-Optic Cable Television Systems

Abstract: System design trade-offs for the application of fiber optics to CATV are presented with the system partitioned into trunking and local distribution. Performance and cost comparisons for analog Intensity Modulation (IM), analog Frequency Modulation (FM), and digital (PCM) approaches are presented. The performance and cost effects of multiplexing are presented for all modulation bases. The paper concludes with a summary showing that multiplexed digital fiber optics systems are best for most of the trunking areas, with analog approaches recommended for local distribution.

Phase-shift modulation technique for the linerisation of analogue optical transmitters

Abstract: A phase-shift modulation technique is described for the linearisation of light sources in fibre-optic transmission systems. A 25 dB reduction in the 2nd-harmonic distortion has been achieved in specially fabricated GaAlAs double-heterostructure light-emitting dioded (l.e.d.s) compared with the distortion levels typical of conventional l.e.d.s.

Optical phase modulation instruments

Abstract: To obtain the refractive index of a fluid, each of two beams of light are transmitted at an angle through different ends of a vibrating quartz crystal to phase modulate the beams of light with respect to each other and then transmitted through two different flow cells before being combined at a point of nearly equal optical path length from the source of light, with the resulting interference pattern being transmitted onto a photocell to generate an electrical signal which varies periodically with the modulation of the two beams of light. The signal from the photocell and a signal representing the modulation of the crystal are compared in phase and a third signal indicates the refractive index of the effluent from the chromatographic column generated as a result of this comparison.

Selective spectral-line modulation technique for high sensitivity continuum-source atomic absorption spectrometry

Abstract: : A selective spectral-line modulation technique for performing continuum-source atomic absorption is described and its operation discussed. In the new technique, discrete droplets of a metal salt standard solution are periodically introduced into a source modulator flame. The atom clouds produced within this flame move through the radiation path of a continuum source, thereby selectively modulating only those atomic lines characteristic of the atoms within the cloud. This combination of continuum source and source modulator flame replaces a modulated line source within an otherwise typical atomic absorption spectrometer. The system is shown to enable multi-element analysis and exhibits relative freedom from both line and broad-band spectral interferences. Analytical curves and sensitivities obtained with the system for several elements are presented. Extensions of the system and its applications are briefly discussed. (Author)

Wideband modulation of the C13O216 laser R (18) line at 10.784 μm with an N14H3 Stark cell

Abstract: Intense Stark resonance absorptions near 10.7 μm of the R (18) and R (24) lines and a weaker absorption of the R (26) line of the isotopic C13O216 laser by N14H3 are reported. Wideband modulation experiments with an N14H3 Stark cell and the R (18) line are presented. Key modulation features are the 2‐ns rise time, high modulation depths, low power dissipation, short Stark modulator cell (10 cm), high saturation intensity (≳8 W/cm2), new spectral features of the modulated signal versus Stark bias, and a demonstration of 180‐Mbit random word modulation.

Radio direction finding system

Abstract: A radio direction finding system including a transmitter and associated antenna located at one end of a radio communication path and a receiver and associated antenna at the other end. One of the antennas is characterized by an omni-directional pattern. The other antenna is characterized by a rotating, directional antenna pattern which is periodically reversed in direction at a rate low compared to its rotation frequency, but high compared to changes in bearing occurring between the transmitter and receiver. The periodic modulation of the RF signal caused by the rotating antenna pattern is detected by the receiver which generates a demodulated output signal. This demodulated signal is averaged over a period including the two directions of antenna rotation, with the result that the bearing error portions cancel, yielding a bearing-indicating signal which is relatively free from receiver-introduced phase error.