Showing papers on "Modulation published in 1978"

Tamed Frequency Modulation, A Novel Method to Achieve Spectrum Economy in Digital Transmission

Abstract: This paper describes a new type of frequency modulation, called Tamed Frequency Modulation (TFM), for digital transmission. The desired constraint of a constant envelope signal is combined with a maximum of spectrum economy which is of great importance, particularly in radio channels. The out-of-band radiation is substantially less as compared with other known constant envelope modulation techniques. With synchronous detection, a penalty of only 1 dB in error performance is encountered as compared with four-phase modulation. The idea behind TFM is the proper control of the frequency of the transmitter oscillator, such that the phase of the modulated signal becomes a smooth function of time with correlative properties. Simple and flexible implementation schemes are described.

System for data transmission by means of an angle-modulated carrier of constant amplitude

Abstract: A system having a transmitter and a receiver for transmitting binary data signals of a given symbol rate, an angle-modulated carrier signal of a substantially constant amplitude and a continuous phase being generated in the modulation stage of the transmitter, and the transmitted modulated signal being orthogonally, coherently demodulated in the receiver. The modulation stage of the transmitter is arranged so that the phase of the modulated signal changes in each symbol interval by an amount from the sequence -π/2, -π/4, 0, π/4, π/2 (rad.), which amount is determined for the relevant symbol interval by at least two successive data symbols, and the value of the phase within the relevant symbol interval is determined by a filtered version of at least these two succesive data symbols. These measures result in a system which, without sacrificing the remaining desired communication properties of FFSK-systems, utilizes the available frequency spectrum in a more efficient manner than FFSK-systems, because the modulated signal has both a narrower spectral main lobe and, for frequencies outside this spectral main lobe, considerably less power than the FFSK-signal. Consequently this system is very well suited for efficient data transmission over radio links.

Optical channel waveguide switch and coupler using total internal reflection

Abstract: Light beam switching and coupling in a four-port channel waveguide-horn structure has been accomplished using electrooptic modulation of the critical angle of a refractive index interface in a Y -cut LiNbO 3 substrate. The resulting double-pole double-throw switch/coupler is potentially capable of simultaneously providing a combination of desirable characteristics.

Calibration of optical modulator frequency response with application to signal level control.

Abstract: A swept frequency technique for measuring the frequency response of an optical intensity modulator and its application to modulation level control are outlined. The technique is precise and suited for wideband measurements. Experiments have been performed up to 2-GHz frequency. The measured response confirms the theoretical prediction. In the experiment on level control, a modulation depth flatness of ±0.3 dB was obtained for 0.1–2-GHz rf sweeping.

Reduction of resonancelike peak in direct modulation due to carrier diffusion in injection laser.

Abstract: It is shown theoretically that the relaxation oscillation in the direct modulation of the injection laser is reduced by transverse carrier diffusion when the stripe width is comparable to the carrier diffusion length. Modified rate equations are derived by considering the distribution of the light intensity, the carrier density, and the diffusion of the carrier. By small-signal analysis of the rate equations it is found that the height of the resonancelike peak in the modulation characteristics is a minimum when the transverse width in which both the light and the carriers are confined is around the diffusion length.

Interferometric waveguide modulator with polarization‐independent operation

Abstract: A Mach‐Zehnder interferometric waveguide modulator is demonstrated which simultaneously modulates both TE and TM modes. Two sets of electrodes are utilized to provide approximate independent control of vertical and horizontal field components across the channel waveguides. In Z‐X–cut LiNbO3 this is shown to provide two polarization‐independent points of operation. With Ti‐diffused waveguides operated at 0.633 μm we obtained 90–95% modulation with applied voltages of 1.3 and 44 V and also with −8 and −36 V. Electrode design and alignment are shown to be important factors in device operation.

Adaptive electronically steerable phased array

Abstract: An adaptive antenna system which enhances the desired signal in the output of the system while reducing the interference from signals arriving from directions other than that of the desired signal The antenna system operates in conjunction with a modulation/demodulation system for spreading and despreading the spectrum of the desired signal By a series of deterministic perturbations of phase shifters or other signal varying devices attached to the antenna elements, the system measures the effects of these perturbations: (1) on the wide-band energy in the output of the antenna, prior to spectrum despreading, and (2) on the narrow-band energy following despreading, and then adjusts the phase shifters or other signal varying devices so as to enhance the desired signal while reducing the interfering signals

Design of efficient coding and modulation for a Rayleigh fading channel

Abstract: The design of a coding/modulation structure for digital communications over a Rayleigh fading channel, the structure of the corresponding decoder, and the error rate performance of the resulting system are considered. Emphasis is on the use of constant weight codes for constructing equal energy waveforms for transmission over the channel. The performance gains that are achieved by the integrated coding/modulation approach relative to conventional methods for obtaining diversity are illustrated via some examples. Of special interest is the use of a concatenated coding technique for forming codes of large distance and hence high diversity. A new decoding algorithm is applied to enable efficient decoding of the concatenated code. An example is included that shows a performance increase of several dB resulting from concatenation.

An Improved Method for Digital SSB-FDM Modulation and Demodulation

Abstract: A new method for digital implementation of SSB-FDM modulation and demodulation is presented, which utilizes the FFT algorithm to reduce the multiplication rate. The hardware realization of the presented method is shown to be the simplest; it consists of only an FFT processor and a set of complex bandpass filters which operate at the same rate as the input baseband sequences; no signal conversions are required prior to FFT processing. Simple and practical approaches are presented for design and implementation of the EFT processor and the set of complex bandpass filters. Design examples, which are made for implementation of a TDM-FDM translator at the supergroup level, show that the proposed method can attain the minimum multiplication rate among methods previously reported.

Precision phase modulators utilizing cascaded amplitude modulators

Abstract: The present invention relates to a precision phase modulator wherein a carrier signal of constant amplitude and frequency and a modulating signal are applied to the two inputs of the first one of a cascade of linear suppressed-carrier amplitude modulators. Each subsequent modulator of the cascade arrangement concurrently modulates the output signal of the previous modulator of the cascade with the modulation signal. The input carrier signal and the output signal of each modulator of the cascade is tapped-off and weighted, the weighted carrier input and outputs of the even-numbered modulators and the weighted outputs of the odd-numbered modulators are separately added and are combined in quadrature to provide a phase modulated output signal.

Range-gated pulse doppler radar system

Abstract: A range-gated radar system which includes a phase modulator that modulatesn r.f. generator with a signal which may be binary coded. In a preferred embodiment, the binary signal is a periodic signal of 2 N -1 bits per period. Starting at some arbitrary point in the sequence, K contiguous bits are transmitted then M contiguous bits are passed over, then K more bits are transmitted, and so on. The quantities M+K and 2 N -1 have no primary factors in common. The radar system may be a doppler radar system using balanced digital processing which involves separating the upper and lower sideband components of modulation that exist in the return radar signal as a result of the motion of scatterers and subtracting them.

Magnetostriction measurement by means of strain modulated ferromagnetic resonance (SMFMR)

Abstract: A novel method for measuring magnetostriction constants is presented. A strain, periodic in time, applied to the sample, causes a modulation of the ferromagnetic resonance line position. The height of the signal obtained after phase-sensitive detection is proportional to the strain modulation depth. The appropriate magnetostriction constant λ is obtained by comparing the height of the SMFMR signal with that of the FMR line, as recorded by means of magnetic field modulation. Features of the new technique are: An illustrative example (YIG layer on GGG substrate) is given.

Theoretical analysis of spectral modulation behaviour of semiconductor injection lasers

Abstract: The influence of the spectral width on the modulation behaviour of semiconductor injection lasers has been investigated theoretically on the basis of multimode rate equations. Analytical expressions are derived yielding an effective rate of spontaneous emission for a suitable description of dynamic characteristics of injection lasers. This effective emission rate may drastically exceed the total rate of spontaneous emission into the lasing modes. In the case of large signal modulation it has been shown that a large modulation depth as required for PCM-transmission also yields a considerable spectral broadening.

Signal enhancement by field modulation in optically-detected magnetic resonance experiments

Abstract: Field modulation can be used in optically-detected magnetic resonance experiments to enhance inhomogeneously broadened signals by factors that can be as large as twenty or more. The conditions under which such enhancements are expected are discussed and illustrated with results obtained for ZhSe:F and ZnS:Al. The use of chopped field modulation to eliminate unwanted non-resonant background signals is also described.

Amplitude-modulated noise: the detection of modulation versus the detection of modulation rate.

Abstract: Modulation threshold, that is, the modulation depth required to discriminate a sample of amplitude‐modulated (AM) noise from a sample of unmodulated noise, was measured as a function of modulation rate (16–320 Hz), modulator waveform (sine or square), and the bandwidth of the AM noise (0.5–8.0 kHz). Modulation threshold increases monotonically with modulation rate, sine‐wave thresholds are greater than square‐wave thresholds, and threshold rises as the bandwith of the AM stimulus decreases. These effects all support the use of some form of energy detection model to explain modulation threshold. The modulation thresholds were compared with pitch thresholds gathered under precisely the same conditions. Pitch threshold or, alternatively, rate threshold was taken to be the modulation depth required to decide which of two samples had the higher modulation rate; the rate difference was 20%—just over three semitones. In the region above about 70 Hz, rate threshold is essentially a constant multiple of modulation threshold, indicating that the primary constraint on rate threshold is the audibility of the modulation. Below 70 Hz, rate and modulation threshold diverge; it is argued that the limit on rate threshold in this region is probably the length of the correlation required to extract the periodicity.

Modulated carrier amplifying system

Abstract: MODULATED CARRIER AMPLIFYING SYSTEM Abstract of the Disclosure A system providing linear amplification of a signal including a pulse duration modulation of the signal upon a carrier signal, the system further including a plurality of amplifying units which are sequentially activated by the modu-lated carrier. The amplifying units are arranged in the form of a bridge circuit, the nodes of which are coupled to a load, the amplifying units including transistors which are driven alternately in states of conduction and nonconduction for maximizing power coupled to the load while minimizing power dissipated in the transistors. A filter connected between the amplifying units and the load attenuates signals at the fre-quency of the carrier so that the waveform of the signal applied to the load duplicates the waveform of the signal at the input of the system.

Analysis of Intermodulation Noise in Frequency Converters by Volterra Series

Abstract: Frequency converters produce intermodulation noise in the desired signal band which may be a serious problem for communications systems using amplitude modulation. In this paper, we introduce the Volterra series with time-varying kernels to treat intermodulation in frequency converters with one two-terminal nonlinearity. The method gives exact results up to the order calculated (third order here) for any nonlinearity expressable as a power series, will treat frequency dependence in the nonlinearity as well as the terminations, and leads to a convenient algorithm for computer calculation. The mathematics provides a physical picture of intermodulation of a specific order as being produced by the modulation of lower order products through the nonlinearity. In fact, the solution for a given order of intermodulation currents or charges is the solution of a set of linear equations where the driving functions are intermodulation currents of lower order. A program has been written for the specific but important case of an abrupt junction varactor upconverter. Results for an upconverter that may be used for single-sideband operation in the common carrier microwave band show that the largest contribution to intermodulation comes from currents which are at the sum and difference frequencies of the input (IF) signal corresponding to currents above the input port in frequency and currents in the bias circuitry. This paper documents previously unpublished work (1972) done as part of the exploratory study of single-sideband modulation on long-haul microwave radio transmission.

Modulation current control of laser diodes

Abstract: A circuit which senses the time intervals during which a total current applied to a laser diode exceeds a second breakpoint value on the light-current characteristic curve and which generates an error signal that may be fed back to adjust a current modulator.

High repetition rate driver circuit for modulation of injection lasers

Abstract: An injection laser modulator comprises a self-biased field-effect transistor (FET) and an injection laser to provide a quiescent state during which lasing of the injection laser occurs in response to a high repetition rate signal of pulse coded modulation (PCM). The modulator is d.c. coupled to an input pulse source of PCM rendering it compatible with an input pulse referenced to ground and not being subject to voltage level shifting of the input pulse. The modulator circuit in its preferred and alternate embodiments provides various arrangements for high impedance input and low impedance output matching. In addition, means are provided for adjusting the bias of the FET as well as the bias of the injection laser.

Pulse frequency modulator and compressor for staircase FM radar systems

Abstract: A pulse compression radar which transmits a step-wise FM pulse and correlates at the receiver using a bank of delay lines (having progressively increasing delays) of the fiber optic type. One such delay line corresponds to each step of the frequency modulation program transmitted. The summed delay line outputs converted to electrical signals are summed to provide the compression desired. A suitably amplified stair-step received echo signal is applied to control the deflection of a Bragg cell illuminated from a laser source, the variable light ray deflection produced by the Bragg cell being focused progressively at the input of each of the discrete fiber optic delay lines. The same bank of fiber optic delay lines is employed in an oscillator loop, these delay lines being effectively electronically switched into place sequentially in the frequency determining feedback loop of the oscillator circuit. The same or a different laser source floods all inputs of the fiber optic delay lines through a light modulator in the feedback path, the desired stair-step FM transmitted signal being a modulation on the light energy employed in the oscillator configuration. Time-sharing transmit/receive and light source wavelength/color separation embodiments are shown.

Apparatus and method for filtering signals in a logging-while-drilling system

Abstract: The disclosure is applicable for use in a logging-while-drilling apparatus for obtaining subsurface measurements during drilling in a fluid-filled borehole. Acoustic carrier waves are generated downhole in the borehole fluid and are PSK modulated in accordance with digital data representative of downhole measurements. The PSK modulation is obtained by momentarily unidirectionally either decreasing or increasing the frequency of the acoustic carrier signal until either a desired phase lag (for a decrease in frequency) or phase lead (for an increase in frequency) is imparted to the acoustic carrier signal. An uphole receiving subsystem includes transducers for converting the modulated acoustic carrier waves to electronic signals and circuitry for demodulating the electronic signals to recover the measurement information taken downhole. In accordance with an important feature of the invention, a filter is provided for selectively filtering the electronic signals before the demodulation thereof. The filter is provided with a bandpass characteristic having a center frequency which is displaced from the nominal frequency in the directon of the unidirectional decrease or increase of frequency applied when originally modulating the acoustic carrier waves. The center frequency is preferably offset from the nominal carrier frequency by an amount which is a function of the bit rate of the digital information, preferably an amount equal to one-half the bit rate of the digital information. In the preferred embodiment of the invention, the bandpass characteristic of the filter means is skewed in the same direction as the offset.

Acoustic-optic coherent modulator and detection system

Abstract: The intensity of light beams transmitted through a tunable acousto-optic modulator is controlled by modulating the center wavelength of the filter passband. Such wavelength modulation can be obtained by modulation of the acoustic frequency or the crystal birefringence. An acousto-optic modulator of this type is sensitive to the temporal coherence of the light beam. When used in an optically coherent detection system, it can provide detection of very weak laser beams in the presence of strong incoherent background radiations.

Digital echo canceler for a modem for data transmission by means of modulation of a carrier

Abstract: Digital echo canceler for a modem for data transmission by means of phase and, possibly, also amplitude modulation of a carrier, the ratio between carrier frequency f o and modulation rate 1/T having the form P/Q, P and Q being integers. The echo canceler receives the binary data to be transmitted for generating an echo copy signal. An echo canceler of a simple implementation is obtained in that the echo canceler comprises means responsive to each symbol configuration at each instant iT for generating at least a number depending on the amplitude allotted to the carrier at said instant iT and for applying this number to at least a given section of an assembly of adaptive digital filter whose output signals are combined for forming a digital version of the echo copy signal, the coefficients of these filters being adjusted by an assembly of adjusting circuits receiving said error signal in digital form and arranged for minimizing a predetermined function of said error signal.

Fast Synchronization in a Spread-Spectrum System Based on Acoustoelectric Convolvers

Abstract: A wideband PN spread-spectrum modem has been implemented using a pair of acoustoelectric convolvers as programmable matched filters in combination with digital delay and coincidence gating. The convolvers provide the capability for fast synchronization of a burst-format message during a short 13-bit preamble. The spreading code is different for every preamble and data bit of a message. (Conventional receivers often require long synchronization times for non-repeating preamble codes or require the same code to be repeated for every preamble bit.) Efficient algorithms have been developed for coarse synchronization and bit alignment of the received signal and reference in each convolver. The modem achieves coarse synchronization to a precision of one PN chip using only four preamble bits with an effective correlation loss only 1.5 dB from ideal. Alignment of the coded bits within the convolver interaction region for subsequent DPSK data detection is achieved by shifting the reference time base. Fine synchronization of the receiver for a data sampling precision of one-eighth of a PN chip is completed at the end of the 13-bit preamble. Message data modulation is DPSK and spreading modulation is MSK. Receiver performance in the presence of wideband interference has been measured to be within 1.5 dB of theoretical predictions.

Phase-insensitive hologram readout technique

Abstract: A hologram-containing storage medium is rapidly scanned or swept by a sinusoidal spatial fringe pattern, the spatial frequency of which varies with time. This may be achieved by the use of a suitable light beam deflection system and frequency shift arrangement disposed up-stream of a focussing lens. The deflection system and frequency shift arrangement may incorporate a pair of light beams, such as those obtained from a laser, a source of coherent monochromatic light. A single deflector-frequency shifter, such as an acousto-optic element and an electro-mechanical deflector may be disposed in the path of one of the beams, with a prescribed carrier modulation frequency applied to shift the frequency of that one beam relative to that of the other beams. The modulated beam is deflected and is then directed along with the unmodulated beam through the focussing lens so that the beams combine with each other to form a read-out interference pattern on the hologram. Alternatively, a respective frequency shifter, such as an acousto-optic modulator may be disposed in the path of each of the pair of beams and a separate modulation frequency signal applied to each modulator. The frequency difference between the two signal sources represents the temporal frequency carrier component of the interference pattern. The beams are also deflected relative to one another and the light which is then diffracted by the hologram impinges upon a single photo-detector, whereby a signal representative of the original data, together with the imparted modulation components, is derived. The temporal and spatial modulation components introduced in the signal beam by the frequency-shift, deflection system are then removed, to obtain only the original data. In accordance with the invention, a constant scan frequency may be used or the frequency shift of a light beam may be caused to vary with time (e.g. ω(t)=kt) so as to effectively create a travelling optical chirp. In each case, the interference pattern created on the hologram for reading out the data will contain a translating sinusoidal fringe system with a time varying spatial frequency, and the output of the photo-detector represents the integral of the product of hologram transmissivity and the illumination intensity pattern, which may then be demodulated to obtain the original data.

Direct modulation of InGaAsP/InP double heterostructure lasers

Abstract: Direct modulation of an InGaAsP/InP double heterostructure laser was investigated experimentally. Sinusoidal modulation up to 2.5 GHz was achieved with almost constant modulation efficiency. Pulse responses showed that the damped relaxation oscillation in light output was well suppressed.

Automatic modulation control apparatus

Abstract: The present invention concerns circuitry for comparing the output power of an amplitude modulated RF transmitter with the supply power to the RF transmitter and using feedback from a comparator to a variable gain modulation amplifier to maintain the level of modulation constant. The scheme is illustrated for four different basic types of RF amplitude modulated transmitters.

Method and apparatus for bandwidth reduction

Abstract: The bandwidth of human speech signals, 125 to 3000 Hz, is reduced to approximately 125 to 1500 Hz by folding or shifting the high-frequency consonant-band portion, 1000 to 3000 Hz, into the low-frequency vowel-band portion, 125 to 1000 Hz. After transmission, the reduced speech signal is expanded to reconstruct an approximation of the original speech signal. One embodiment features folding (modulator carrier frequency is 3000 Hz) and modulator feedthrough. Another embodiment features shifting (carrier is 1800 Hz) and no modulator feedthrough. Both embodiments boost lower vowel frequencies prior to modulation, and use no switching circuitry.