Showing papers on "Modulation published in 1980"

Solid State Radio Engineering

Abstract: Radio Communication Systems. Electrical Noise. Resonant Circuits and Impedance Transformation. Small--Signal High--Frequency Amplifiers. Sinewave Oscillators. Phase--Locked Loops. Mixers. Modulation. Amplitude Modulation Receivers. FM and PM Receivers. Television Receivers. Linear Power Amplifiers. Tuned Power Amplifiers. High--Efficiency Power Amplifiers. CW, FN, and AM Transmitters. Single--Sideband Transmitters. Index.

Carrier and Bit Synchronization in Data Communication--A Tutorial Review

Abstract: This paper examines the problems of carrier phase estimation and symbol timing estimation for carrier-type synchronous digital data signals, with tutorial objectives foremost. Carrier phase recovery for suppressed-carrier versions of double sideband (DSB), vestigial sideband (VSB), and quadrature amplitude modulation (QAM) signal formats is considered first. Then the problem of symbol timing recovery for a baseband pulse-amplitude modulation (PAM) signal is examined. Timing recovery circuits based on elementary statistical properties are discussed as well as timing recovery based on maximum-likelihood estimation theory. A relatively simple approach to evaluation of timing recovery circuit performance in terms of rms jitter of the timing parameters is presented.

Half duplex integral vocoder modem system

Abstract: Disclosed is a system whereby in a transmit mode analog speech is sampled d converted digitally to 12 bits of accuracy and then fed into a fast Fourier transform (FFT) processor which analyzes the speech into spectral and pitch parameters. These parameters are then quantized into a data stream which acts as an input to a differential phase shift keying modulator. The modulator constructs a multi-tone modem output signal from a 25 tone stack which is converted to an analog signal which is fed into a communications channel. In the receive mode a modem input signal from the communications channel is fed through the same analog to digital converter which was used for input speech. A data stream is now provided which is coupled back into the same digital FFT processor which now operates to provide pitch spectral coefficients which are then separated and used by a synthesizer to reconstruct a speech waveform. The speech waveform is applied to a digital analog converter which is the same converter used to produce the line signal when the processor operates in the transmit mode. The FFT processor implements a single FFT algorithm which is used for both vocoder and modem processing in both the transmit and receive modes.

Receiver performance evaluation of various digital optical modulation-demodulation systems in the 0.5-10 µm wavelength region

Abstract: Error rate characteristics of various digital optical modulation-demodulation schemes are studied. The main concern is whether we can improve receiving power levels to achieve a prescribed error rate by employing a coherent optical transmission system in place of the presently available amplitude-shift-keyed (ASK) baseband direct detection system. The receiving power level reduction in various modulation-demodulation schemes is calculated by taking into account the optical carrier wavelength, data rate, photodetector performance, local oscillator power level, and number of levels in multilevel codes. The phase-shift-keyed (PSK) homodyne detection system requires the least receiving power. The improvement in the receiving power level compared to the conventional ASK baseband direct detection system is expected to be 16-22 dB at the carrier wavelength of \lambda_{c} = 0.5-3 \mu m, 31-36 dB at \lambda_{c} = 3-5 \mu m, and 35-40 dB at \lambda_{c} = 5-10 \mu m.

High-speed operation of LiNbO 3 electro-optic interferometric waveguide modulators

Abstract: High-speed optical intensity modulation is reported for the first time using single-mode electro-optic interferometric waveguide modulators formed from Ti-diffused waveguides in LiNbO3. For cw 0.633-μm optical inputs, modulation at rates up to 1.4 GHz has been achieved by driving the interferometer through multiple-π phase shifts. Envelope modulation of a 275-MHz optical pulse train from a cw mode-locked frequency-doubled Nd:YAG laser with a 68.9-MHz drive signal has also been demonstrated.

Measuring fluorescence decay times by phase-shift and modulation techniques using the high harmonic content of pulsed light sources

Abstract: In the present work we discuss the possibility to measure fluorescence lifetimes by coupling pulsed excitation with phase-shift and amplitude-modulation detection techniques We show that, because of the high harmonic content of narrow light pulses, the sample may be considered as simultaneously excited with a set of modulation frequencies up to the GHz region with noticeable power By measuring then the phase-shift and modulation ratio of fluorescence with respect to the exciting light in the high-frequency region, impressive time resolutions may be achieved On the other hand, since we dispose of a wide range of modulation frequencies, the problem of the multiexponential decay may be easily handled and has an exact analytical solution, provided the signal-to-noise ratio is good enough We anticipate the possibility to introduce cross-correlation methods in order to perform the experiment in the very-low-frequency region A short discussion about the notion of time resolution for this kind of measurement is also included

Amplitude modulator using variable width rectangular pulse generator

Abstract: A method and apparatus for amplitude modulating a carrier signal with an audio or information input signal utilizes an arcsine generator to cause the envelope of the carrier to follow the input signal. The invention stems from the realization that if rectangular pulses of bipolar amplitude are symmetrically centered about 0 and π radians of the carrier signal, the harmonic content can be expressed as a function of the angle of the rectangular pulses. This concept is realized by producing two opposite phase carrier signals which are compared with the modulation input signal to form two opposite phase rectangular pulse signals whose pulse widths are a function of the modulation input signal. Alternate phase pulses are extracted from these two rectangular pulse signals by two NOR gates, and the extracted pulses are combined and filtered to obtain an amplitude modulated output signal.

Apparatus for writing digital information in a disc-shaped optically readable record carrier

Abstract: An apparatus for recording optically detectable information in a record carrier (1) provided with information areas (9) arranged in accordance with a spiral or concentric track pattern, which areas (9) alternate with synchronization areas (8) in each of which the address of the associated information area is recorded in an optically detectable manner. Recording is effected with the aid of first modulated laser beam (16a), whilst a second beam (16b) is projected after the first beam for reading the recording. The track pattern has previously been provided with a periodic track modulation of a frequency at which the power spectrum of the information to be recorded substantially exhibits a zero point. The apparatus comprises a first and a second filter for filtering a signal corresponding to the track modulation out of the first or second laser beam reflected or transmitted by the record carrier, whilst in series with the second filter there is included a delay network (51), from which a synchronization signal is available. Furthermore, there is provided a phase comparator (52) which during reading of the synchronization areas determines the phase difference between the signals obtained by means of the first and the second filter and which controls the delay network (51) in such a way that the output signal of said delay network is in phase with the output signal of the first filter.

Optically readable record carrier with track variations which provide clock and tracking signals and apparatus for recording and/or reproducing data from such a record carrier

Abstract: Disclosed is a record carrier for storage of digital data which is provided with a first periodic track variation having a period corresponding to a frequency at which the power spectrum of the digital signals is at a zero level. The first track variation produces a corresponding modulation of the radiation when scanned by a beam so that a clock signal can be derived from the detected radiation for synchronizing readout or recording of the data. The record carrier is further provided with a second periodic track variation in a direction transverse to the track direction having a period of the same order of magnitude, and preferably equal to the period of the first periodic track modulation. The second track variation produces a corresponding modulation of the beam which is detected and is used to generate to a tracking signal, the clock signal being used as phase reference signal for demodulation of the tracking signal. If the periods of the two track variations are equal, low-pass filtering automatically yields the tracking signal.

Some implications of the cutoff-rate criterion for coded direct-detection optical communication systems

Abstract: The cutoff rate is derived for a digital communication system employing an optical carrier and direct detection. The coordinated design of the encoder, optical modulator, and demodulator is then studied using the cutoff rate as a performance measure rather than the more commonly employed error probability. Modulator design is studied when transmitted optical signals are subject simultaneously to average-energy and peak-value constraints. Pulse-position modulation is shown to maximize the cutoff rate when the average-energy constraint predominates, and the best signals when the peak-value constraint predominates are identified in terms of Hadamard matrices. A time-sharing of these signals maximizes the cutoff rate when neither constraint dominates the other. Problems of efficient energy utilization, choice of input and output alphabet dimension, and the effect of random detector gain are addressed.

Control Strategies for PWM Drives

Abstract: The majority of pulsewidth modulation (PWM) drives operate in three distinct modes: pure PWM at the lower speeds, pure six-step at the higher speeds, and quasi-PWM at the intermediate speeds. The pure PWM stage can be operated with a constant frequency, unsynchronized carrier. The transitional quasi-PWM stage, because of the operation of the minimum dwell time protection system, must be synchronized. The phase of the carrier relative to the signal then becomes important. Under these circumstances it is advisable that the carrier to signal frequency ratio be an odd multiple of three and the phase of the carrier be such that, at a signal zero, the carrier and signal have slopes of opposite sign.

Fast acting phase shifting apparatus for use in digital sampling systems

Abstract: A method and apparatus is disclosed for controlling the sampling of a composite analog color video signal so that the analog signal is sampled at precise locations relative to the phase of the color subcarrier of the color video signal in the absence of modulation. The invention thereby enables a digitized signal having a constant phase relationship relative to the unmodulated subcarrier phase to be provided which can be advantageously used in magnetic recording and reproducing apparatus. Since the constancy of the location of the samples relative to the phase of the color burst is maintained prior to recording, there is no phase error that would pose a problem during reproducing of the signal. The apparatus stores a set of reference samples of a selected number of cycles of the color burst interval of the color video signal and generates an output clock signal that is phase synchronized with the reference samples, subsequently stores sets of comparison samples of color burst cycles from subsequent horizontal video lines using the generated clock signal and detects any error in the phase location thereof. The apparatus adjusts the phase of the generated output clock signal in response to any detected error signal and stores a new set of reference samples in the event that the error detected is beyond predetermined limits.

Wave modulation in a nonlinear dispersive medium

Abstract: A model describing the simultaneous amplitude and phase modulation of a carrier wave propagating in a nonlinear dispersive medium is developed in terms of nonlinear wave‐wave interactions between the sidebands and a low frequency wave. It is also shown that the asymmetric distribution of sidebands is determined by the wavenumber dependence of the coupling coefficient. Digital complex demodulation techniques are used to study modulated waves in a weakly ionized plasma and the experimental results support the analytical model.

Fiber optic system for transmission of video signals by pulse-frequency-modulation

Abstract: A fiber optic system utilizing pulse-frequency modulation (PFM) for transmission of video information signals substantially enhances the signal-to-noise ratio of the transmitted video signals. FM processing gain is realized together with system noise reduction which results from operation of the fiber optic components in a pulsed mode. The amplitude of a video signal modulates the repetition frequency of a stream of narrow light pulses emitted by a laser diode. The pulses are transmitted through a low loss, wideband optical fiber to an avalanche photodiode biased at an optimum gain point. Further improvement is possible when preemphasis processing of the video signal is performed prior to transmission.

Frequency‐modulated shot noise limited stimulated Raman gain spectroscopy

Abstract: We demonstrate for the first time an electro‐optic frequency‐modulated stimulated Raman gain laser system which achieves shot noise limited detection. The frequency modulation technique reduces the otherwise large thermal background produced by absorbing substrates by a factor of 104–105, thereby permitting the full use of the ultrahigh sensitivity which is sufficeint to observe the vibrational spectroscopy of monolayers.

Method and means of minimizing simulcast distortion in a receiver when using a same-frequency repeater

Abstract: Simulcast distortion in a receiver when using a same-frequency repeater is minimized by applying double modulation to a signal that is broadcast to the same-frequency repeater. A voice signal is applied without delay as AM on a carrier to a modulation index of up to 30%, and the same signal is also applied as FM on the same carrier with a time delay equal to the time delay in the same-frequency repeater. The same-frequency repeater detects the AM and applies the detected signal as FM on a carrier that is rebroadcast. Received FM signals are thus delayed by substantially the same amount whether they are received from the original broadcast or the same-frequency repeater.

Polyphase coded mixer

Abstract: A radar fuzing system for a guided missile is shown to include means for impressing a polyphase coded modulation on a transmitted signal and delayed replicas of such modulation on a bank of correlator/mixers, each one of the latter including dual gate field effect transistors as the active elements.

Theory of frequency modulation detection for low modulation frequencies.

Abstract: There is general agreement that the frequency difference limen measured in a two‐sine‐tone frequency discrimination experiment is smaller than that measured in a frequency modulation (FM) experiment. We present a model of frequency modulation detection for low modulation frequencies, within the framework of signal detection theory, which accounts well for the observed difference between frequency discrimination experiments and FM detection experiments. The FM detection model also predicts psychometric functions for detection of FM with different modulation waveforms. FM detection experiments with square, sine, trapezoid, and triangle FM are in reasonable agreement with the model predictions.

Apparatus for digital demodulation or modulation of television chrominance signals

Abstract: If an N.T.S.C. signal is to be demodulated using a line-locked sampling rate such as 800 times the line frequency, the ratio between the sampling period and the subcarrier period has the awkward value of 455/1600. Line-locked sampling is nevertheless desirable, e.g. in digital standards conversion. The subcarrier digital signals for digital demodulation are derived with the correct frequency from the sampling rate clock pulses CP by an adder and accumulator into which the number 582 is added modulo-2048 (the register is an 11-bit register) to generate an 11-bit number which represents the subcarrier phase angle at each sampling pulse and which addresses a ROM providing sin and cos values representing subcarrier samples. 582/2048 is not exactly equal to 455/1600 but 2048 is a desirable denominator as it is a power of 2 and implies a ROM of suitable size. ##EQU1## is exactly equal to 455/1600. Another adder and accumulator counts modulo-40960 by increments of 16384 and when this adder overflows, an extra 1 is added in through the carry-in to the first adder. Modulo 40960 is set up by altering 10256 to 40960 when the overflow occurs. Alternative numerical values are disclosed which apply to a PAL system with sampling rate 816 times line rate. Phase lock to the color burst is established by examination of the V output of the main demodulators during the color burst. V=sine θ where θ is the phase error. This quantity is applied via a negative feedback loop to effect fine adjustment of the numbers 40960 and 16384. Similar provisions for PAL are disclosed as is the generation of a PAL switch signal of the correct phase. The apparatus can also be employed as a modulator.

Pulsed, pseudo random position fixing radio navigation method and system and the like

Abstract: This disclosure is concerned with a technique and apparatus for reducing CW and other inband interference in a broad bandwidth RF pulse navigation system to inobtrusive noise through the use of pulse-position random modulation, including, where desired, superimposed random phase modulation, with signal-random modulation code cross-correlation in reception.

Carrier recovery network for QPSK modems employing synchronized oscillators

Abstract: A carrier recovery network for QPSK modems employs a synchronous oscillator which may be used as a frequency multiplier, divider and tracking bandpass filter A preferred embodiment of the carrier recovery network includes a multiply-by-four circuit to remove QPSK data modulation, and a synchronous oscillator tuned to one-fourth the input frequency to thereby act as a frequency divider and tracking bandpass filter to provide a recovered carrier signal

Simultaneous feedback control of bias and modulation currents for injection lasers

Abstract: A feedback circuit is described which regulates the light output from an injection laser through simultaneous independent control of bias current and modulation current. The circuit is capable of preserving a large extinction ratio and it is independent of the duty cycle of the modulating signal. Experiments were performed on simulated `lasers? and the results are discussed.

Secure facsimile transmission system using time-delay modulation

Abstract: Method and apparatus for secure transmission of analog signals, particularly facsimile signals. The analog signal is first converted to a frequency-modulated signal, if not already one, and is then converted to a sequence of digital words representing the instantaneous frequency thereof, by sampling at a fixed rate. Next the digital words representing the sample values are written in sequence into a buffer memory, at the constant sampling rate. The digital words are then read out of the memory at a different, pseudo-randomly variable rate and converted back to an analog FM signal for transmission. The transmitted signal is, thus, a time-delay-modulated version of the original FM signal. Optionally, the digital word values may be transformed before being converted back to analog frequency form, so that the modulation content of the transmitted signal is scrambled, as well. Further security may be obtained by reading out from the memory in a pattern which differs from the write-in address pattern. The read-out rate from the memory is varied by changing the clocking rate of the read-out operation at pseudo-random times, to select the read-out rate from a plurality of available rates. The available rates include at least one which is faster that the write-in rate and one which is slower than the write-in rate. To avoid underflow or overflow of data at the memory, detection of an incipient underflow or overflow condition causes the read-out rate to be changed, respectively, from a faster-than-write-in rate to a slower-than-write-in rate, and vice versa; when the fast/slow nature of the rate is changed, the selection of the new rate is made pseudo-randomly from among the available choices. Also, the fast/slow nature of the rate may be reversed pseudo-randomly, either at random times or at preselected points.

Simultaneous multi-beam optical modulation system

Abstract: The simultaneous multi-beam optical modulation system comprises the steps of producing a dummy signal in such a manner that the sum of a plurality of signals and the dummy signal is kept constant, modulating the amplitudes of assigned carriers by the dummy signal and the plurality of the signals so as to produce a modulated output and applying the modulated output to an acoustic optical element.

Stark-effect-modulated phase-fluctuation optical heterodyne interferometer for trace-gas analysis

Abstract: A narrow-bandwidth (cw) phase-fluctuation laser-heterodyne interferometer for trace-gas detection is described. Performance was evaluated using both Stark-effect modulation and conventional beam chopping. Sensitivities of 10−8 cm−1 were obtained in both cases. However, when Stark-effect modulation was employed, the usual background signal caused by interfering absorbing species and windows could be subtracted from the overall signal. In this mode, a detection limit of 5 parts in 109 (ppb) NH3 in air was obtained.

Fiber optic transmission line stabilization apparatus and method

Abstract: A fiber optic transmission line stabilizer for providing a phase-stabilized signal at a receiving end of a fiber optic transmission line (26) with respect to a reference signal at a transmitting end of the fiber optic transmission line (26) so that the phase-stabilized signal will have a predetermined phase relationship with respect to the reference signal regardless of changes in the length or dispersion characteristics of the line (26) More particularly, a reference signal of RF frequency modulates a 085 micrometer wavelength optical transmitter (20) The output of the optical transmitter (20) passes through a first optical filter (24) and a voltage-controller phase shifter (22), the output of the phase shifter (22) being provided to the fiber optic transmission line (26) At the receiving end of the fiber optic transmission line (26), the signal is demodulated, the demodulated signal being utilized to modulate a 106 micrometer optical transmitter (34) The output signal from the 106 micrometer optical transmitter (34) is provided to the same fiber optic transmission line (26) and passes through the voltage-controlled phase shifter (22) to a phase error detector (36) The phase of the modulation of the 106 micrometer wavelength signal is compared to the phase of the reference signal by the phase error detector (36), the detector (36) providing a phase control signal related to the phase difference This control signal is provided to the voltage controlled phase shifter (22) which alters the phase of both optical signals passing therethrough until a predetermined phase relationship between modulation on the 106 micrometer signal and the reference signal is obtained

A generalized formulation of minimum shift keying modulation

Abstract: The modulator structure consisting of a single-input two-output sequential transducer whose output sequences control the modulated signal in the manner s(t) = x(i)p(t) + y(i)q(t) is considered. Here, iT is less than or equal to t, which is less than iT + T, where T is the modulation bit duration and where x(i) and y(i) take values in /+1, -1, 0/. The behavior of a sequential transducer is characterized by means of a 'trellis'. It is shown that, provided only that the 'carriers' p(t) and q(t) have the same energy in every bit interval, the optimum demodulator for the additive white Gaussian noise channel need process the received signal over only two bit intervals in making each data bit decision. It is demonstrated that particular choices of the sequential transducer yield differential minimum shift keying and differential staggered quadriphase shift keying. Possible generalizations of this approach are discussed.

Two-dimensional dpcm image transmission over fading channels.

Abstract: A combined source-channel coding approach is described for the encoding, transmission, and remote reconstruction of image data. The transmission medium considered is that of a fading dispersive communications channel. Both the Rician fading and Rayleigh fading channel models are considered. The image source encoder employs two-dimensional (2-D) differential pulse code modulation (DPCM). This is a relatively efficient encoding scheme in the absence of channel errors. In the presence of fading, however, the performance degrades rapidly. By providing error control protection to those encoded bits which contribute most significantly to image reconstruction, it is possible to minimize this degradation without sacrificing transmission bandwidth. Several modulation techniques are employed in evaluation of system performance including noncoherent multiple frequency shift-keyed (MFSK) modulation. Analytical results are provided for assumed 2-D autoregressive image models, while simulation results are described for real-world images.

Injection modulation in coupled laser oscillators

Abstract: Injection modulation is the effect observed when laser oscillation is perturbed by an injected signal below the threshold of locking. In this regime the oscillation becomes a wave modulated in frequency as well as in amplitude. The modulation envelope has a characteristic waveform which depends on the amplitude and phase of the injected signal. Starting from the Lamb's equations for a dual-mode oscillator, we develop a theory of the injection modulation and calculate the waveforms in amplitude and frequency. The treatment applies both to the external injection into a laser and to the case of mutual coupling between two modes. Experimental results for a dual mode He-Ne laser are found to be in good agreement with the theory. It is pointed out how the injection modulation effects are appreciable even at very weak levels of injected amplitudes, e.g., down to 10-5with respect to the oscillation field amplitude.

Control circuit for the drive current of a laser

Abstract: Control circuit for controlling a laser drive current. In known control circuits certain levels of the optical output signal are kept constant. This renders it difficult to satisfy certain requirements during the life of the laser as regards the linearity of the modulation of the optical output signal. In the described control circuit the intermodulation of two signals present in the modulation signal with substantially constant amplitudes is measured, an error signal which controls a component of the drive current being derived therefrom.