Showing papers on "Demodulation 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.

Spread spectrum apparatus for cellular mobile communication systems

Abstract: An apparatus is disclosed for a cellular mobile communication system utilizing a spread spectrum. A plurality of fixed base stations is connected with a central processor so that calls originating in any cell can terminate in any other cell. The available frequency spectrum is divided such that one-half is used for mobile-to-base station transmission and the other one-half is used for base station-to-mobile unit transmission. A power control/mobile locator system and method is utilized in all mobile units to compensate for changes in signal level and to maintain the average received signal level at the base station constant regardless of the location of the mobile unit within the cell. In addition, each mobile unit has a unique set of time-frequency coded waveforms that are used for both transmission and reception. The waveforms have large time-bandwidth products so that interfering signals are well suppressed, and which provide a large set of signals so that each user can be assigned a unique set of waveforms. Message modulation is accomplished by digitizing the speech and encoding the resulting binary sequences into the set of waveforms available to the user, while detection is accomplished by a demodulation system and method in which the phases of each received waveform are compared with those of the preceding waveform.

Ultrasonic imaging system

Abstract: A real time ultrasound imaging system provided with a fixed focus and including coherent demodulation of echo signals followed by time delay and coherent summation of the demodulated signals is described. As the range from which echoes are being received increases, coherent demodulation is maintained. Demodulation and time delay accuracies are relatively easy to maintain over a large field of view and a long range. The system has high resolution and high sensitivity.

Demodulator unit for spread spectrum apparatus utilized in a cellular mobile communication system

Abstract: A demodulator unit is disclosed for spread spectrum apparatus utilized in a cellular mobile communication system. The cellular mobile communication system includes a plurality of fixed base stations connected with a central processor so that calls originating in any cell can terminate in any other cell. Each mobile unit has a unique set of time-frequency coded waveforms that are used for both transmission and reception. Message modulation is accomplished by digitizing the speech and encoding the resulting binary sequences into the set of waveforms available to the user, while detection is accomplished by the demodulator unit in which the phases of each received waveform are compared with those of the preceding waveform. The demodulator unit includes a tapped delay line which can have mixers before and after the delay line and can be a narrowband unit with delays and mixers.

Baseband differentially phase encoded radio signal detector

Abstract: A radio signal is applied to an in phase detector channel and a quadrature phase detector channel. Each detector channel includes, in cascade arrangement; an in phase downconverter means for downconverting the received radio signal to a baseband signal; a sampling means for sampling the baseband signal at a predetermined rate; and a matched filter means matched to the symbol waveform and wherein the sample pulses are stored over a predetermined period of time that corresponds to two data symbols. The sum of the pulses for the first data symbol and the sum of the pulses for the second data symbol are compared to determine if there has been a phase reversal between the two data symbols. The outputs of the in phase and quadrature phase channels are summed, which results in demodulation of the encoded data.

Introduction to communications engineering

Abstract: Communication System Models Carrier Modulation Carrier Transmission Carrier Reception Carrier Demodulation Baseband Waveforms, Subcarriers, and Multiplexing Binary Digital Systems Block Waveform Digital Systems Frequency Acquisition and Synchronization Satellite Communications Fiber Optic Communications Appendix A: Fourier Transforms and Identities Appendix B: Random Variables and Random Processes Appendix C: Orthogonal Expansions of Signals Index

Digital radio paging communication system

Abstract: A digital paging communication system including a transmitter and a plurality of receivers is disclosed. The transmitter generates a preamble digital signal, a calling digital signal and an end mark digital signal in a predetermined sequence. Each receiver demodulates a received signal into the preamble digital signal, calling digital signal and end mark digital signal. These signals are separately detected in synchronism with a recovered clock signal. Power is supplied to the radio frequency, intermediate frequency and demodulator sections of the receiver intermittently until such time as the calling signal of the receiver is detected, after which power is supplied continuously until an end mark signal is detected. The intermittent operation is a battery saving feature.

Feedback nonlinear equalization of modulated data signals

Abstract: A receiver for a quadrature amplitude modulated data signal impaired by linear and nonlinear distortion, phase jitter and additive noise includes circuitry which compensates for these impairments. In particular, the receiver includes a processor (FIG. 1 44; FIG. 2, 44') which subtracts a feedback nonlinear signal (FIG. 1, D(n); FIG. 2, D'(n)) from each sample of the received signal, either prior or subsequent to demodulation, providing compensation for nonlinear intersymbol interference. The feedback nonlinear signal subtracted from each sample is comprised of a weighted sum of products of individual data decisions and/or the complex conjugates of data decisions, each such product, in turn, being multiplied by a predetermined harmonic of the carrier frequency. In an illustrative embodiment, compensation for second- and third-order intersymbol interference is provided by including two- and three-multiplicand weighted products in the feedback nonlinear signal. Weighting coefficients for each product are adaptively updated in a decision-directed manner.

Single sideband signal demodulator

Abstract: A single sideband signal can be demodulated by using sampled data techniques. An intermediate frequency (IF) radio signal is split into two branches and is sampled by two sample and hold devices; each sample and hold device operates at a rate that is not only a submultiple of the carrier frequency, but is also at least twice the information bandwidth. Prior to, or during sampling of one branch (the quadrature branch), a Hilbert's transform is performed upon the IF signal which results in the output of the two sample and hold devices being in phase quadrature. A phase shift of 90°, using a baseband finite impulse response Hilbert's transform filter, is performed upon the output signal of the quadrature branch's sample and hold device. The sum of the inphase sampled signal and the filtered quadrature phase sampled signal produces the demodulated lower sideband while the difference between the two produces the demodulated upper sideband. There are two basic examples shown of single sideband demodulation through the use of sampled data techniques; in addition, there are several examples of implementing the Hilbert's Transformer.

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

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.

A Microprocessor-Based PSK Modem for Packet Transmission Over Satellite Channels

Abstract: This paper describes a microprocessor-based modem developed for use in a packet switching network over satellite channels. This digital modem can process both BPSK and QPSK packets with near optimum error rate performance over channels with marginal signal energy-to-noise density ratio. Of principal concern is the development of the discrete time algorithms which form the basis of the microinstruction program. The processing of a packet can be decomposed into two basic stages: (1) the detection of the packet preamble and the rapid estimation of signal parameters (symbol timing and carrier frequency offset) for the specific packet, and (2) the demodulation of the data portion of the packet including the tracking of both symbol timing and carrier offset phase, phase ambiguity resolution and data symbol estimation. The paper concludes with details on the performance of the modem under operational conditions and a brief discussion of implementation of the processor.

System for removing interference distortion in the demodulated signal of a frequency-modulated signal

Abstract: A system for removing interference distortion in the demodulated signal of a frequency-modulated signal comprises a demodulator for demodulating an input frequency-modulated signal Ca(t) which has been subjected to interference by another frequency-modulated signal, and thereby obtaining a demodulated signal eo (t), a first analog multiplier for obtaining a signal in which the input frequency-modulated signal is squared, a low-pass filter supplied with the squared signal and detecting an envelope of the input frequency-modulated signal in square characteristics, and thereby producing as output a squared envelope signal {Env(t)}2, a second analog multiplier supplied with the demodulated signal eo (t) from the demodulator and with the squared envelope signal {Env(t)}2 from the low-pass filter, and thereby producing as output a signal eoa (t) of the product of both signals, a DC blocking capacitor for suppressing DC component of said squared envelope signal {Env(t)}2, and thereby obtaining only AC component, a first operation circuit supplied with the product signal eoa (t), a delay circuit for delaying an output signal of the first operation circuit by a specific delay time, a second operation circuit for adding the output signal of the first operation circuit and the output signal of the second operation circuit, and a third analog multiplier for carrying out multiplication of the AC component signal {Env(t)}2 and the output signal of the second operation circuit, and thereby supplying the resulting output signal h(t) to the first operation circuit. The first operation circuit substantially carries out subtraction of the signal eoa (t) and the signal h(t). From the first operation circuit is derived the demodulated signal in which the interference distortion has been substantially cancelled and thus removed.

Variable-angle, multiple channel amplitude modulation system

Abstract: A compatible AM stereo system employing a modified quadrature modulation scheme where the gain in the quadrature-phased channel, and thus the phase angle between the L and R modulated phase components of the composite modulated signal, is dynamically varied in accordance with changing modulation conditions The average phase angle between the L and R modulated phase components is much greater than was previously possible in fixed gain systems, thus signal-to-noise ratio (SNR) in stereophonic receivers is improved In the transmitter (FIGS 3, 7A and 7B), a matrix circuit (40, 188) adds and subtracts L and R audio signals to produce sum (L+R) and difference (L-R) signals An analog divider (54; 194, 200) adjusts the gain of the (L-R) signal in accordance with a gain control signal A Q The (L+R) and gain adjusted (L-R) signals are transmitted on two quadrature-phased carriers by a quadrature AM (QAM) transmitter (50, 160) A distortion estimator (56, 190, FIGS 5A and 5B) calculates the amount of distortion which the envelope of the composite modulated signal will produce in a conventional monophonic receiver and dynamically adjusts the gain control signal A Q to maximize gain without exceeding predetermined distortion constraints A circuit (60, 210, FIGS 9 and 10) generates a pilot signal and modulates it with the dynamically varying gain control This pilot signal is added to, and thus transmitted with, the gain adjusted (L-R) signal In one transmitter embodiment (FIGS 7A and 7B), a circuit (156, FIG 8) is provided for compressing low amplitude audio signals so as to further improve SNR A signal indicative of the amounts of compression is also modulated onto the pilot signal, and thus transmitted to the receiver A filter network (154) provides a gap in the low frequency portion of the (L-R) signal into which the pilot signal may be inserted, and processes the (L+R) and (L-R) signals so that no loss in bass occurs as a result of this filtering Stereophonic receivers (FIGS 11-13) are disclosed which demodulate the pilot signal and control the gain of the recovered (L+R) and (L-R) signals in accordance therewith so as to track the varying phase angle and signal compression of the composite modulated signal, and thus optimally recover the L and R audio signals in their original form The system may easily be converted into an improved independent sideband (ISB) system by the insertion of appropriate phase delays in the (L+R) and (L-R) signal paths

Method and apparatus for digitally implementing a linked compressor-expander telecommunications system

Abstract: A linked compressor-expander (LINCOMPEX) circuit for use in telecommunications is implemented digitally. A speech compressor circuit in a modulator and a speech expander circuit in a demodulator controllably attenuate given syllables of a speech signal by values determined in response to binary input signals which collectively represent the amplitude of a given symbol of the speech signal. A control tone signal is generated by the modulator, and the demodulator is responsive to the control tone signal for operating the voltage attenuator circuit of the digital expander to thereby reconstruct the speech signal. Calibrate circuitry is provided for periodically detecting a test frequency of the control tone generator during a brief period prior to transmission of syllables of the speech signal. Also included is a data multiplexing circuit for transmitting and receiving data between speech syllables to optimize transmitter loading.

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.

Detector for position of pilot's helmet - uses opto-electronic system giving line of sight for arming system

Abstract: A semi-active opto-electric detector system is used to ensure that the angular position of the pilot's helmet in a combat aircraft is known. This is necessary in order to determine the pilot's line of sight for an arming system. The line of sight (DV) of a pilot is determined from reflecting point elements (M1, 2, 3) on the helmet (SM). A generator (12) produces 3 different modulating signals for optical modulators in the reflecting elements. The modulated signals pass via an optical system to a photodetector (7) behind a plane matrix (6). The signals are preamplified (15) prior to demodulation (16) where the binary coded modulating signals are recovered. The signals are then amplified (17) prior to input to a calculator circuit (20). This determines the spatial position of the reflecting points and consequently the pilots line of sight.

FM Signal demodulator with defect detection

Abstract: In the playback of a video disc record employing an FM carrier recording format, a defect compensation system is provided which substitutes delayed video signals for current video signals when a defect is encountered. A phase locked loop including a phase detector and a voltage controlled oscillator is employed in FM detection circuits. A defect detector compares the phase between the oscillator output signal and a signal corresponding to the recovered signal. When the phase difference between these two signals exceeds a certain value, a defect signal is generated to disable the normal signal path of the recovered signal so that the delayed version of the video may be substituted for the current video for the duration of the occurrence of the defect.

In null steering apparatus a reference to spread spectrum signals

Abstract: A transmitter for transmitting a desired signal composed of a carrier modulated by a pseudorandom noise code and periodic bursts of data and a receiver including null steering apparatus wherein the desired signal is applied to a demodulator for removing the PN code, or collapsing the spectrum of the signal, and the collapsed signal is demodulated to provide the data and the carrier, after which the carrier is remodulated with the PN code to provide a reference signal between bursts of data for the null steering apparatus to form a lobe in the antenna pattern in the direction of reception of the desired signal.

QPSK Demodulator with two-step quadrupler and/or time-multiplexing quadrupling

Abstract: A QPSK demodulator apparatus utilizing a pair of doubling units in tandem but separated by a bandpass filter to remove any undesired cross products and to eliminate possible noise signal that may be applied to the second doubler. The use of two doubling units to provide a times 4 quadrupling allows the use of heterodyning in order to operate at a lower frequency.

Demodulator circuit for chopper amplifier

Abstract: In a demodulator circuit for a chopper amplifier including a rectifier and a smoothing filter having a capacitor, an impedance conversion circuit is connected to the output side terminal of the capacitor to ensure holding of the stored charges of the capacitor at each negative half-cycle of the amplitude-modulated AC signal applied to the demodulator circuit. A discharge circuit including a switching element is further provided for the capacitor and the discharge circuit is rendered conductive at each positive half-cycle of the amplitude-modulated AC signal which is also applied to a control terminal of the switching element.

Compensated capacitive transducer demodulator circuit

Abstract: A compensation technique and network circuitry for a quad-diode demodulator and capacitive transducer combination is disclosed. The compensation method includes varying the amplitude of an alternating carrier frequency oppositely to the changes produced in that amplitude by the compensable errors in the demodulator and transducer. In one preferred embodiment ratiometric compensation and temperature compensation for the demodulator and any capacitive transducer is produced. In a second embodiment linearization and temperature compensation for the demodulator and a quartz capacitive transducer is provided.

Method and apparatus for generating timing phase error signals in psk demodulators

Abstract: D-21,734 METHOD AND APPARATUS FOR GENERATING TIMING PHASE ERROR SIGNALS IN PSK DEMODULATORS By Robert J. Tracey Stevan D. Bradley William F. Hartley ABSTRACT OF DISCLOSURE A circuit arrangement for combining a measure of the signal phase error for a received data signal in a PSK demodulator with a measure of the di-rection of rotation of the receive data signal phasor between adjacent sample times for producing a timing phase error signal for controlling the phase of a local clock timing signal in the demodulator. In a demodulator producing a digital word defining differences between the phases of decoded phasors at adja-cent sample times, a binary bit Dk of the digital word may define the direction of rotation of the received signal phasor between the adjacent sample times. Sample values of the signal phase error signal in the demodulator are quantized into single binary bits Ek indicating the sense of the signal phase error at sample times. In one circuit arrangement, binary bits Ek and Dk are combined in an exclusive-OR gate for producing a binary timing phase error bit Mk. In a demodulator where phase differences are consecutively numbered clockwise in straight binary, the output of the exclusive-OR gate is inverted for producing binary timing phase error bits Mk. In another circuit arrangement, binary bits Ak and Bk indicating the sense of the in-phase and quadrature-phase signal com-ponents for decoded phasors at a number of sample times are logically combined with signal phase error bits Ek for producing binary timing phase error bits Mk at sample times.

Multiplexing of multiple loop sidelobe cancellers

Abstract: A sidelobe canceller in which the undesired signals in the main antenna channel are cancelled at RF using signals from multiple auxiliary antennas, where each auxiliary antenna signal has been weighted in a vector modulator while at RF. The weighting is determined in a single wideband IF loop by multiplexing the multiple auxiliary antenna signals through the same auxiliary receiver, correlator and demodulator. The demodulated output signal from each auxiliary antenna is sampled and retained as a weighting to adjust the vector modulator for the corresponding auxiliary antenna. A timing and control circuit coordinates the multiplexing and sampling functions.

Soft Decision Demodulation for PCM Encoded Speech Signals

Abstract: The effect of digital errors in PCM encoded speech signals transmitted over a noisy channel is reduced by using soft decision demodulation at the receiver. The reliability information supplied by the soft decision demodulator is used to point out likely transmission errors, especially in the most significant PCM bits. When a likely transmission error is identified, the corresponding PCM word is rejected by the receiver and replaced by a predictor estimate or an interpolation estimate if delayed decisions are used. We have analyzed soft decision demodulation schemes for standard PCM encoded speech signals transmitted over the Gaussian channel with coherent PSK (phase shift keying). A signal to noise ratio gain in E_{b}/N_{0} of the order of 1-2 dB is Obtained at low input signal levels. The gain depends on the performance of the predictor or, alternatively, the interpolator. No modifications of the transmitter are required to obtain this improvement. The suggested soft decision schemes are optional at the receiver. The comparisons are made with hard decision demodulation.

Multiple sound reception device

Abstract: PURPOSE:To secure the fixed holding for the aural output sent from the speaker regardless of the absence or presence of the additional signal be switching the amplification factors of the compound output signals in coupling to the switching action whether or not the additional signal is delivered from the output of the multiple demodulation circuit. CONSTITUTION:The right and left aural signals of the stereo signals are delivered to output terminals 7 and 8 of multiple demodulation circuit 2, and at the same time the main or secondary aural signal of the hetero-broadcast is delivered. The output signal of circuit 2 is applied to additional sound switch circuit 16 via amplification switch circuits 14 and 15; the output of cirucit 16 is applied to compound circuits 20 and 21 via delay circuit 17 and defeat circuit 18; and the output of circuit 18 is compounded to the output of circuits 14 and 15 in the opposite polarity. And the output of circuits 16, 17 and 18 are switched by the output of broadcast mode detection circuit 3, and then the additional signal is added to deliver the aural signals suited to each broadcast mode. Coupling to this action and with the output of additional switch circuit 22, circuits 14, 15 and 18 are switched to switch the amplification factors of the compound signals. Thus the level of the aural output can be held constant.

Receptive condition automatic selection device for FM receiver

Abstract: A receptive condition automatic selection device for an FM receiver comprises a band width changing circuit for changing a band width of an intermediate-frequency amplifier of the receiver, a first detector for detecting a noise level contained in a demodulation signal from a demodulator, a second detector for detecting a tuning condition of the receiver by the demodulation signal and a bistable circuit connected to said first and second detectors for producing a control signal for the band width changing circuit. The band width of the intermediate-frequency amplifier can be automatically changed to either a narrow band or a wide band in accordance with receptive conditions of the FM receiver.

Receiver having a phase-locked loop

Abstract: A receiver incorporating a phase-locked loop to demodulate a signal comprising a plurality of distinct baseband components is disclosed The loop filter circuit includes an adjustable resistive element for varying the loop damping factor, thereby providing optimum separation of the baseband components In addition, the loop filter circuit provides a bandpass high-frequency noise output to a noise-activated muting circuit