Showing papers on "Chirp published in 1973"

On the Utility of Chirp Modulation for Digital Signaling

Abstract: The issue of signal selection in binary data transmission is presented. The question of the relative utility of linear frequency sweeping (LFS or chirp), compared to PSK and FSK, in terms of error probability and spectrum usage, is discussed. The transmission media considered are the coherent, partially coherent, Rayleigh, and Rician channel models. Theoretically, LFS has unconditionally superior characteristics in the partially coherent and fading cases, for certain ranges of channel conditions. This is due to the more negative values of cross-coherence parameters possible with the LFS signal set over the FSK signal set. For the fading channel, theoretical supremacy of LFS over FSK depends upon the specular-to-Rayleigh signal power ratio and the adjustability of in-phase cross coherence, with a constraint upon quadrature phase cross coherence. From a practical standpoint, coherent reception of the LFS signal set has severe limitations. These are manifested primarily in two aspects: the need for phase synchronization of a chirp signal set, and the fact that the optimum value of cross coherence is highly sensitive to synchronization channel signal-to-noise ratio (SNR), and/or spectral-to-Rayleigh signal power ratio. The latter would require that modulation characteristics track the channel conditions in order to achieve the supremacy in performance theoretically predicted by optimization of the cross-coherence parameter in LFS.

Fresnel zone plate imaging of gamma rays; theory

Abstract: The use of a Fresnel zone plate as a coded aperture for imaging incoherent radiation such as gamma rays has been previously reported. The coded image is in many respects similar to a hologram and can be decoded or reconstructed with a coherent optical system. In this paper, the general theory of coded-aperture imaging is presented, first for an arbitrary code and then for an on-axis zone plate, an off-axis zone plate, and a one-dimensional zone plate (or linear chirp). With the on-axis plate, a matched imaging condition is suggested as a guide to optimizing image contrast. With the off-axis zone plate and the linear chirp, it is necessary to use a half-tone screen to spatially heterodyne the object spectrum into the passband of the aperture. In all three cases, expressions for the resolution, depth of field, field of view, and relative efficiency are derived. A simplified noise analysis is presented, and some practical system constraints are discussed.

Signal Processing by Parametric Interactions in Delay-Line Devices

Abstract: A new type of signal-processing device which employs the parametric interaction between surface or volume acoustic waves passing in opposite directions through an acoustic delay line is described. These devices are capable of giving the real-time convolution of two modulated signals, and the time inversion of an arbitrary signal. As one signal acts as the reference for the other, a virtually infinite range of electronically variable signal-processing functions, such as recognition of digital codes, pulse compression of an FM chirp, and the realization of a fast Fourier transform, is possible. Early devices which employed the nonlinearity of the acoustic medium tended to give outputs of the order of 60 dB less than the input signal. Recent developments, in which a semiconductor placed near the piezoelectric medium interacts with the acoustic wave, have yielded results where the loss from input to output is of the order of 30 dB.

Method and apparatus to maintain constant phase between reference and output signals

Abstract: A control system for an electro-hydraulically driven vibrator for the purpose of maintaining a constant phase relation between the output signal of the vibrator, passing into the earth, and a reference signal being used to drive the vibrator. The system operates on a digital basis and provides two digital sweep signal generators, each of which provides a train of time spaced voltage pulses of selected magnitude, in accordance with a corresponding train of timespaced time-words signals. One of these is a reference sweep signal generator and has a continuous application of time-words. The second is designed so that the sequence of time-words signals can be modified, that is, time-words are advanced or retarded, thereby effectively shifting the phase of the second, or driver signal generator, with respect to the first, or reference signal generator. The driver signal is used to control an electro-hydraulic vibrator. Feedback signals are provided from the output of the vibrator. These are compared with the reference signal and the result used to provide a feedback error signal, which controls the sequence of time-word signals which are sent to the driver generator. Thus, the two trains of output pulses, from the two generators, are relatively shifted in instantaneous phase, in accordance with the feedback error signal, by the shifting of the time-word sequence to the driver generator.

Optimum transionospheric pulse transmission

Abstract: The synthesis of optimum signals for transionospheric propagation along a vertical path is considered in this paper. It is seen that optimum band-limited signals having complicated amplitude and frequency modulation results in good pulse compression over a wide range of ionospheric conditions. The transmission of linearly swept carrier (chirp) pulses having rectangular and Gaussian envelopes is then considered with the result that appreciable compression is also seen to occur for these signals. Finally, an example is given of the statistical effects that variations in the total ionospheric electron content have on enhanced pulse transmission.

Intra-pulse MTI system with range ambiguity suppression

Abstract: A dual-pulse coherent MTI system having a time interval between pulses as short as zero. The pulses are "chirped" in opposite sense, the latter constituting a unique coding for eliminating range ambiguity problems. Video return signals are received and applied to two parallel channels each containing pulse compression and limiting circuits. The pulse compression circuits are matched uniquely to the positive chirp slope pulse in one channel and to the negative chirp slope pulse in the other channel. The channel corresponding to the earlier of the two pulses is subjected to a fixed delay of one pulse width before the outputs of the channels are differenced to produce a net MTI signal. This is a continuation of application Ser. No. 159,751, filed July 6, 1971, now abandoned.

Ranging and Data Transmission Using Digital Encoded FM-"Chirp" Surface Acoustic Wave Filters

Abstract: A digital encoded multislope chirp modem/demodem unit has been implemented using two 3-port surface acoustic wave filters. Each filter operating at a 30-MHz center frequency provides either positive or negative slopes as digital 1's or 0's with a time-bandwidth product of 280, an unweighed bandwidth of 5.6 MHz, and a time dispersion of 50 /spl mu/S.The modems were used to calibrate and compare the operational performance of the conventions multitone CW ranging system with a multislope chirp ranging system. Range measurements and range-rate observations were made to a synchronous satellite with both systems using a ground communication satellite terminal. Both ranging techniques provided accuracies well within the predictable satellite range of 20 000 nmi; the multitone system provided a theoretical range resolution of 1 m, and the chirp system 0.4 m. Data transmission was also accomplished, using 12-bit binary code at a 1.25-Mbit rate. The significant advantage to be noted with the chirp system is the ability to obtain continuous range data from the satellite repeater simultaneously while other modes of information are being transmitted, and to combine the ranging and data-link transmission on a time-order basis using digital encoded chirp sequences. The chirp system was found the more desirable technique, since it provides a more direct range measurement, with minimal calibration requirements, and provided greater processing gain with relative ease and reliability. The data transmission at low data rates provided little deterioration in theoretical compressive gain. However, at the higher data rate a greater loss was encountered due to power sharing of the overlapping chirp coded carriers in the limiting satellite.

Stabilized Linear FM Generator

Abstract: A technique for generating high-linearity chirp (linear FM) signals is presented. It uses a delay line and mixer to sense the slope of the output of a swept oscillator, and the difference from desired slope is fed back to correct the oscillator frequency. The technique also improves the coherence of the oscillator. An analysis is performed of the amount of linearization that can be achieved for three possible loop configurations. Experimental results are presented of tests run on a backward wave oscillator (BWO) operating over a bandwidth of 240 MHz with time duration of 120 ?s. The results indicate a linearity corresponding to a phase error (from parabolic) of 30 degrees (peak) and coherence within 5 degrees.

Measurement of the spectrum of a helical TEA CO2 laser

Abstract: Measurements of the chirp of individual TEA CO2 laser pulses are reported. Rates of chirping during the peak and the tail are attributed to two different mechanisms. Spectrum analyzer results corroborate the time‐resolved measurements.

Parallel operation of seismic vibrators without phase control

Abstract: A control system for a seismic vibrator, in which the reference, or sweep signal, drives the vibrators directly without the need to control the vibrator so that the seismic signal generated in the earth will be inphase with the reference signal. Instead, the output of one or more sensors on the vibrator are used to generate a "transmitted" signal which is representative of the seismic signal generated in the earth. The transmitted signal is used to phase-shift the reference signal to provide a counterpart signal which is inphase with the transmitted signal. The counterpart signal is used to correlate with the received seismic signal. A plurality of vibrators may be used. They may be grouped together at one point, or they may be spaced apart at independent points. The vibrators may use the same or different reference signals, and may start simultaneously or at delayed times. An improved sensor for mounting on the vibrator is described to provide information as to the absolute displacement of the base plate during vibrator operation.

Introduction to Chirp Concepts with a Cheap Chirp Radar

Abstract: The elements of pulse compression of a chirp in a dispersive delay are reviewed and chirp and pulse modes are contrasted. A simple microwave Doppler-shift apparatus is modified info a chirp system to permit distance measurement in a laboratory experiment that is rich in fundamental concepts of physical parameters which are related by the Fourier transform, and which involve the convergence of ideas from several disciplines.

Electronically Controllable Time Delay

Abstract: An electronically controllable, time delay implementation is described for composite TV video signals using quartz substrate, surface acoustic-wave delay lines with CHIRP coded interdigital transducers Potential applications to RADAR, data links, and CATV systems are indicated

Time/bandwidth interchange system

Abstract: There is disclosed for use in a pulsed sensor system, such as radar or the like, a method and apparatus for time/bandwidth interchange in which the target response chirp pulses are first offset in frequency linearly with range; the offset response pulses are then delayed as a function of frequency; and, finally, the delayed, offset response pulses are rechirped into narrow band chirp pulses suitable for processing. There are also disclosed a number of methods and apparatuses for adapting the new time/bandwidth interchange system to various problem situations which arise in radar applications.

Development and preliminary test of a 1000-Hertz pulse compression seismic reflection profiling system

Abstract: A continuous seismic reflection profiling system was developed to achieve acoustic penetration into sea floor sediments between that achieved by air guns and by 3500-Hz systems in all water depths at ship cruising speeds. The system utilizes a multiple-bit correlator to compress a 1000-Hz chirp signal of low amplitude and long duration into a signal of high amplitude and short duration, thereby significantly increasing the signal-to-noise ratio without loss of time resolution. The results of a field test demonstrate the feasibility of such a system.

Partitioned and modified chirp Z-transform: Signal processing technique for simultaneous multifrequency evaluation of the surface reradiated spectrum in slowly varying environments

Abstract: The partitioned and modified chirp Z-transform (PAM-CZT) is a signal processing technique for computing the Z-transform of a sequence at equally spaced samples on a sector of the unit circle in the Z-plane. Unlike the computationally cumbersome classical technique (4), the PAM-CZT utilizes the fast Fourier transform (FFT) that allows real time implementation with state of the art minicomputers. Another advantage of the PAM-CZT is its partitioning capability; long sequences can be processed in short-time-ordered sequences, thereby providing spectral analysis to any frequency resolution by allowing large time records. This signal processing technique will aid in the analysis of acoustically propagated noises in the ocean. Classical experiments utilize a monochromatic probe set at different frequencies for each of several orthogonal time records. However, the quasi-stationarity of the ocean inhibits reaching conclusions as to the frequency dependence of the surface reradiated spectrum' because the measurements are corrupted by the slowly varying characteristics of the ocean. An alternative approach is to simultaneously probe the ocean surface with several sinusoids and perform a PAM-CZT analysis of the results.

Compression and shaping of pulses from high power solid state lasers

Abstract: Pulse compression is discussed as one possible means to overcome the pulse energy limitations imposed by self focusing in high power solid laser systems. Linear and nonlinear processing techniques are compared. In linear systems, a short oscillator pulse is expanded, chirped, and shaped entirely by linear dispersive and filter elements. It is then re-compressed after being amplified. In nonlinear systems, the oscillator pulse is long, and the chirp is created by self phase modulation in a Kerr liquid. A comparison is made between the Gires-Tournois interferometer and the grating pair for use in high energy pulse compression.

Temporal-Spatial Compression Antennas.

Abstract: : SSION), SPIRAL ANTENNAS, LOG PERIODIC ANTENNAS, POLARIZATION, NETWORKS, TRANSFER FUNCTIONSThe idea of using an antenna as a pulse compression network is presented employing the natural dispersion inherent in some broadband antennas. It is shown that the power densities resulting from such antennas are increased by the time-bandwidth product of the antenna over its 'CW' power density. Experimental measurements have been performed on two (2) antennas to determine their transfer functions. Both the circular-polarized cavity-backed spiral and the linearly- polarized pyramid log periodic antennas have excellent chirp transfer functions and the results are given. (Author)

Continental Rayleigh Wave Dispersion and the Estimation of Chirpfilter Detectors

Abstract: : Rayleigh wave data were recorded by a single vertical high-gain seismograph located in north Texas It was found that Rayleigh waves propagating in a continental path over the pole to this station exhibited very stable dispersion characteristics, particularly in the period range 35-75 secs: at shorter periods an average dispersion curve could be estimated A composite dispersion curve was used to develop 'chirp' (or matched) filters, the form of which depended on the distance between the seismograph and the event Such filters are shown to be very efficient in improving the signal-to-noise ratio of Rayleigh waves emanating from events in the Sino-Soviet region and can be used as a means of separating 'mixed' Rayleigh waves For the path over the pole (or 'polar waveguide') the detection threshold is estimated to be at M(s)=35

Microwave Filters and Delay Lines

Abstract: : The significance of this research and development to the Air Force derives from the need for small, lightweight solid state integrated circuit devices suitable for signal processing applications at Microwave frequencies. The work covered by this report was directed specifically at such devices operating at X band (8.0 to 12.4 GHz). The performance improvement was accomplished by improving the quality of epitaxial YIG films, increasing the knowledge of magnetostatic wave propagation in dielectric layered structures, and utilizing high energy product samarium cobalt magnets. During the period covered by this contract several devices were built and evaluated. These were: 1. A nondispersive delay line at 8.425 GHz with a delay of 124 nsec, an insertion loss of 20 dB, and a bandwidth of 45 MHz; 2. A dispersive delay line operating at 9.1 GHz with a 300 MHz linear dispersion of 175 nsec of differential delay. This device compressed a linear chirp by a factor of 30:1 and weighted the adjacent sides lobes to -20 dB; 3. A two tap delay line having a nondispersive delay of 50 nsec and 80 nsec. The success of these devices in terms of size and operating frequency represents a considerable increase in the state of the art. For the first time, the rf signal designer has the opportunity to perform signal processing at radar signal frequencies, with devices which are compatible with Gunn and IMPATT solid state signal sources.

Application of a hybrid computer to sweep frequency data processing

Abstract: A hybrid computer program is presented which can process as many as 10 channels of sweep frequency data simultaneously. The program needs only the sine sweep signal used to drive the system, and its correponding quadrature component, to process the data. It can handle a maximum frequency range of 0.5 to 500 hertz. Magnitude and phase are calculated at logarithmically spaced points covering the frequency range of interest. When the sweep is completed, these results are stored in digital form. Thus, a tabular listing and/or a plot of any processed data channel or the transfer function relating any two of them is immediately available.

Large Pulse Compression Ratio Obtained with Nonlinear Interaction of Bulk Acoustic Waves in LiNbO/sub 3/

Abstract: We report here on increased bandwidth in bulk wave parametric convolvers, and demonstrate a new approach to cancellation of the inherent signal distortion in time inverters. A 6 ns compressed pulse has been generated in the convolver by autoconvolution of a 7 /spl mu/s V-FM chirp signal (center frequency of 1.2 GHz), using the nonlinear interaction of longitudinal waves in LiNbO/sub 3/. Time inversion of microwave signals has been demonstrated with uniform output for signal lengths up to 5 /spl mu/s, by compensating for the signal distortion resulting from delay line attenuation.

Multiple Pulse Chirping in Self-Induced Transparency

Abstract: The interest in phase modulation of short optical pulses has continually increased in the last few years[1]. In this note we will restrict ourselves to the first order theory of steady-state pulses. That is to say, we will work in the common “slowly varying” approximation according to which the envelope and the phase of the pulse vary very little within an optical cycle[la,c,d]. This approach leads to the prediction that the single optical pulses (solitons) propagating in linear absorbers must show no phase modulation[2]. It predicts also the possible existence of a variety of non-chirped as well as chirped pulse trains[3]. Here we will present a unified view of this problem.