Direct Demodulation Research Articles

Direct demodulation method and its application to hard X-ray imaging

Direct demodulation method and its application to hard X-ray imaging

Reconstruction of objects by direct demodulation

High resolution reconstruction of complicated objects from incomplete and noisy data can be achieved by solving modulation equations iteratively under physical constraints. This direct demodulation method is a powerful technique for dealing with inverse problem in general case. Spectral and image restorations and computerized tomography are only particular cases.of general demodulation. It is possible to reconstruct an object in higher dimensional space from observations by a simple lower dimensional instrument through direct demodulation. Our simulations show that wide field and high resolution images of space hard X-rays and soft gamma-rays can be obtained by a collimated non-position-sensitive detector without coded aperture masks.

Simultaneous measurements of in-plane and out-of-plane surface displacements of shell vibrations at several points by an array of miniature laser probe heads

Experiments are being conducted to measure directly by laser Doppler interferometry the in-plane and out-of-plane components of the surface motion of a vibrating shell simultaneously at several surface points. The proposed design requires three Bragg cells to generate three coherent optical beams with different carrier frequencies. Three 1×4 fiber optic connectors are used to transmit each beam to four surface points. At each point, both the out-of-plane component and one of the in-plane components of the displacement vector are measured. The light scattered by the vibrating surface is Doppler shifted and collected into a large core multimode fiber situated close to the surface and terminated by a photodiode. The signals are acquired and digitized for direct phase demodulation on a computer by means of Hilbert transforms. [Work supported by ONR.]

Direct demodulation of 140 Mb/s FSK signals in an injection-locked multiquantum-well DFB laser

The authors report the transmission of frequency shift keying (FSK) signals using an injection-locked multiquantum-well distributed-feedback (DFB) laser that acts simultaneously as an optical frequency discriminator and a photodetector. 140 Mb/s FSK signals are directly demodulated by detecting changes in the forward voltage of an injection-locked GaInAs/GaInAsP/InP MQW DFB laser. A sensitivity of -25.9 dBm was measured at a bit-error-rate of 10/sup -9/. Bias-current-dependent FM demodulation efficiency is also reported together with the optical-power-dependent linewidth enhancement factor. >

Analysis of receiver using injection-locked semiconductor laser for direct demodulation of PSK optical signals

A receiver for PSK optical signals is analysed. The demodulation is performed directly by launching the received signal into an injection-locked semiconductor laser. The locked laser converts phase modulation into current modulation, which is sent to the data detection system after some signal processing. The signal to noise ratio of the receiver can reach 4599π2 for a 100Mbit/s rate and a 100 kHz emitter linewidth, where π is the phase deviation of the modulator.

A Digital Phase Delay Compensation Beam-Forming Scheme for Ultrasonic Imaging

This paper describes an efficient digital implementation scheme of the phase delay compensation beam-former (PBF) for acoustic imaging using arrays, which is based on direct quadrature demodulation through sampling for narrowband signals. The scheme eliminates the need for analog quadrature demodulators employed in conventional digital PBFs and enables the sampling frequency to be reduced lower than the Nyquist rate. The lower bound of sampling frequency, within which images can be reconstructed without unacceptable degradation, has been experimentally studied by Bmode imaging in air using a microphone array.

Wavelength-discriminating photodetector for lightwave systems

We demonstrate that a dual-wavelength photodetector can be used to determine simultaneously the wavelength and the power level of a lightwave signal. The signal wavelength is determined by monitoring the ratio of the two outputs from this photodetector at the crossover of its responsivity curves; the signal level is determined from the sum of the two outputs. Possible applications for this type of photodetector include mode-control of single-frequency injection lasers, direct demodulation for lightwave systems utilising frequency-shift-keyed transmission, wavelength-division multiplexing and optical logic.

Technique for direct phase demodulation in heterodyne interferometry

FM demodulation techniques have been applied to an existing heterodyne plasma interferometer. The direct phase readout so obtained is useful both for ambient density and for forward-scattering measurements.

Range predictions for a CO_2 laser communication system

Calculations are presented to permit range predictions of a CO(2) laser communication system when attenuation statistics are available. Both TV and digital transmission are considered. In the former case, an AM-FM modulation scheme is used. Direct demodulation is assumed. Examples are given in the use of the results to obtain range probabilities based on reported propagation statistics.

760 Mbit/s Serial MSK Microwave Modem

Serial modulation and demodulation provide a significant simplification in the hardware realization of high data rate MSK systems. The serial MSK (SMSK) technique results in theoretical performance charateristics identical to conventional MSK. A 750 kbit/s low data rate system and a 760 Mbit/s high data rate system were constructed to evaluate the feasibility of the design concepts. The low data rate system had an overall BER performance degradation of 1.3 dB at a 10-6BER. The 760 Mbit/s system had a corresponding degradation of 2.0 dB and used direct modulation and demodulation at the 14.7 GHz carrier.

Direct demodulation of a 10.6µm FM signal with an NH<inf>2</inf>D stark cell

A new technique for direct demodulation of a 10.6-μm FM signal using a Stark absorption cell is described. A Stark tunable absorption line in NH 2 D is used to track the time-varying frequency of the incoming FM laser signal using a closed servo loop. The demodulated FM signal is proportional to the feedback voltage applied to the Stark cell. Demodulation at frequencies up to 5 kHz has been demonstrated, the rate being limited only by the bandwidth of the available servo electronics.

Direct frequency demodulation with frequency-locked Gunn oscillators

The use of a frequency-locked Gunn oscillator as a frequency demodulator is described. A detection sensitivity of −40 to −30 dBm is obtainable with detected signals in the millivolt range.

Stereo and Color Signal Demodulation with Integrated Circuit Techniques

This paper outlines general considerations of silicon monolithic integrated circuit design. It then reviews principles of gate demodulation with special emphasis on stereo and color signal demodulations. The ideas presented are merged to form the basis for discussion of the design and development of two monolithic integrated circuits. An FM broadcast stereo demodulator is described which encompasses the functions of subcarrier regeneration as well as additional functions of inter-station mute, weak signal demodulator defeat, indicator lamp driver, and inter-stereo-station mute. A versatile monolithic color demodulator is described which is capable of either direct or difference signal color demodulation. In addition, this IC has provisions for blanking and the option of brightness/luminance control.

A Method of Directly Demodulating Color Signals

Color TV signal evolution is examined from a sampling encoder (modulator) viewpoint. This logically leads to an analysis of decoding (demodulating) the NTSC color signal with sampling techniques. In the process of enumerating and solving the problems that arise by attempting to demodulate by time - interruption of the signal, basic concepts relating to direct demodulation of the NTSC signal are derived.

Direct Demodulation and Frequency Conversion of Microwave-Modulated Light in a CdSe Bulk Photoconductor

Direct Demodulation and Frequency Conversion of Microwave-Modulated Light in a CdSe Bulk Photoconductor

The traveling-wave phototube part II: Experimental analysis

This paper reports a detailed experimental investigation of the traveling-wave phototube (TWP) as a broad-band demodulator of microwave-modulated light. As Part II of a theoretical and experimental analysis of the interaction between a density-modulated photoelectron beam and a slow-wave circuit, it examines the output power, bandwidth and voltage modewidth of the traveling-wave phototube as a function of photocurrent, beam voltage and interaction length. Three types of experiments are described: 1) direct demodulation of light, amplitude-modulated at 3 Gc by a cavity-type modulator, 2) measurement of the microwave shot noise from photoelectron beams initiated by light from both coherent and incoherent sources and 3) comparison of the previous two measurements with the microwave shot noise of a thermionic electron beam. Comparisons of the results of these experiments with the predictions of the theory of Part I are drawn, showing good agreement. In particular, a number of new effects which are predicted by the detailed analysis of Part I, and which had not been previously observed or predicted are reported.

Microwave signal-to-noise performance of CdSe bulk photoconductive detectors

The microwave signal and noise response of CdSe bulk photoconductive detectors have been measured and compared with that of a high-speed junction photodiode. The signal measurements were made with CW intensity modulated light at 3000 Mc. Excellent quantitative agreement was obtained between the simple photoconductivity theory of direct light demodulation and experiment. At low optical intensities, the observed noise can be accounted for within the framework of existing theories of generation-recombination (GR) noise when the effects of electron retrapping are accounted for. At high optical intensities, nonlinear deviations are observed which can be accounted for only qualitatively. The results demonstrate clearly that such a photoconductor, when used as a simple quantum counter, will always be thermal rather than GR noise limited. Measurements show that the photoconductor as it is usually used in a nonoptimum configuration is some 50 db less sensitive than a good photodiode.

Solid-state modulation and direct demodulation of gas laser light at a microwave frequency

Solid-state modulation and direct demodulation of gas laser light at a microwave frequency

Highly-selective transmission-measuring equipment for communication circuits

A circuit is described which permits a highly-selective transmission-measuring equipment to be designed to meet a specified accuracy of measurement; it is suggested that ±0.25 db is quite readily obtainable. The basic principles of the method are (a) the direct demodulation of the test signal by means of an identical frequency obtained from an oscillator synchronized to the test tone, (b) discrimination against unwanted line signals obtained by means of low-pass filters, (c) the elimination of the effect of the phase difference between the test and demodulating tones by Barber's two-path method, and (d) the use of an envelope-modulated test signal.

Highly-selective transmission-measuring equipment for communication circuits

A circuit is described which permits a highly-selective transmission-measuring equipment to be designed to meet a specified accuracy of measurement; it is suggested that ±0.25 db is quite readily obtainable. The basic principles of the method are (a) the direct demodulation of the test signal by means of an identical frequency obtained from an oscillator synchronized to the test tone, (b) discrimination against unwanted line signals obtained by means of low-pass filters, (c) the elimination of the effect of the phase difference between the test and demodulating tones by Barber's two-path method, and (d) the use of an envelope-modulated test signal.