Showing papers on "Chirp published in 1976"

Filter response of nonuniform almost-periodic structures

Abstract: The filter response of nonuniform, almost-periodic structures, such as corrugated optical waveguides, is investigated theoretically. The filter process, leading to reflection of a band of frequencies near the Bragg frequency, is treated as a contradirectional coupled-wave interaction and shown to obey a Riccati differential equation. The nonuniformity of the structure is represented by a tapering in the coupling strength (e.g., the depth of the corrugation) and by a chirp in the period of the structure. For small reflectivities, the filter response is a Fourier transform of the taper function. For large reflectivities, the Riccati equation was evaluated numerically and plots are given for the response of filters with linear and quadratic tapers and with linear and quadratic chirps.

Surface roughness measuring system

Abstract: Apparatus for obtaining significant height information of ocean waves, or peaks of rough terrain utilizing means for compressing the radar signal over different widths of the available chirp or Doppler bandwidths, and means for cross-correlating one of these images with each of the others, where the center frequencies of the images have a spacing ΔfrO. Upon plotting a fixed (e.g., zero) component of the cross-correlation values RE (ΔfrO) as the spacing is increased over some empirically determined range, such as 0 to 1 MHz, the system is calibrated. Thereafter to measure height with the system, a spacing value is selected and a cross-correlation value is determined between two intensity images at a selected frequency spacing, such as 0.1 MHz. The measured height is the slope of the cross-correlation value determined to the spacing value used. Both electronic and optical radar signal data compressors and cross-correlations are disclosed for implementation of the system.

Compression of 100‐psec laser pulses

Abstract: Optical pulse compression using diffraction gratings has been demonstrated previously for ultrashort pulses (<20 psec). It has not been previously demonstrated for longer pulses because the chirp bandwidth required for a reasonable grating spacing was unattainable. In this letter we report the compression of a 100‐psec pulse by a factor ⩾12, at a grating spacing of 23 m, and with an efficiency approaching the theoretical limits.

Gated pulse compression radar

Abstract: A pulse compression coherent radar for obtaining target information is described wherein a pulse compression waveform having a predetermined bandwidth is gated at a frequency greater than the predetermined bandwidth to form a sequence of spaced apart pulses which are transmitted towards a target. A sequence of target reflected pulses are received during predetermined time intervals. The target reflected pulses of the sequence are compressed and then filtered to extract predetermined frequency components of the compressed signal. Alternatively, the target reflected pulses of the sequence are first filtered to extract predetermined frequency components and then compressed. In either embodiment, the output signal contains target doppler frequency and phase, range, and amplitude signature information. The gated pulse compression waveform may be for example a chirp or linear FM signal, phase coded signal, or pseudo noise coded signal.

Generation and correlation of digitally controlled coherent frequency-hopped waveforms using surface acoustic wave devices

Abstract: A digitally controlled coherent frequency synthesizer is reported which uses two surface acoustic wave chirp filters to generate frequency-hopped waveforms. Attractive features are wide bandwidth and instantaneous switching between frequencies, which are selected by an external word generator. Experimental results show waveforms which hop every 2.5 µs within a 12.5-MHz band. Phase coherence between pulses is demonstrated by correlating the waveform in a surface acoustic wave plate convolver.

Rotating optical frequency chirp device

Abstract: A system for generating pulses of laser radiation which vary in frequency with time to achieve a "chirp" or frequency sweep. In a preferred embodiment, a variable-thickness, rapidly rotating disk of optically-dense material is placed in the optical loop of an injection-locked oscillator. Rotation of the disk in the optical loop of the injection-locked oscillator varies the loop optical path length, resulting in a frequency chirp. By appropriately choosing the shape of the optically dense material, a desired linear output chirp may be obtained.

Real-Time Fourier Analysis of Spread Spectrum Signals Using Surface-Wave-Implemented Chirp-Z Transformation (Short Papers)

Abstract: In many communication and RADAR applications it is desirable to determine the spectral content of signals in real time. A technique employing dispersive surface acoustic wave devices to implement the chirp-Z transform is described. The experimental results obtained for a number of commonly used signals, including the maximal-length pseudonoise sequences, are shown, and the agreement with theoretical prediction is discussed.

Short-Pulse Target Characteristics

Abstract: A survey of radar scattering principles related to the estimation of short-pulse (or, more generally, high-resolution) characteristics of radar targets is presented The discussion is presented in the following sections: 1 Introduction; 2 Scattering centers on conducting targets; 3 Techniques for approximating scattering-center contributions; 4 Computation of short-pulse responses of radar targets; 5 Analytical approximations for specific scattering centers; 6 Examples of short-pulse target responses; 7 Relationships between FM/CW, chirp, and short-pulse radars; 8 Special considerations for short-pulse imagery; 9 Summary and conclusions; 10 Acknowledgments; 11 References

Carrier-compatible chirp-z transform device

Abstract: A carrier-compatible device for computing the discrete Fourier transform of an input signal, using the chirp-Z transform (CZT) algorithm, comprising means for connecting to a real and imaginary part of an input signal g k . A pulse generator generates a sequence of very short pulses. A surface acoustic wave (SAW) chirp generator, whose input is connected to the output of the pulse generator, generates cosine chirp signals and sine chirp signals. Four input mixers have as their two inputs a real or imaginary part of the signal g k and a sine or cosine chirp signal from the SAW chirp generator. First and second summers have as their two inputs the outputs from two of the input mixers. A SAW chirp filter, whose two inputs are the outputs of the summers, filters out the higher components from the input signal and passes the lower components. Third and fourth summers are connected to the SAW chirp filter, whose two inputs are components from the SAW chirp filter. First and second delay lines, whose inputs are connected to the output of the sine or cosine SAW chirp generator, delay their input signals an amount of time such that their output signals are coincident in time with the output signals from the summers. Four output mixers have as their inputs the output from the first or second delay lines and the output of the third or fourth summer. Fifth and sixth summers have as their two inputs the positive or negative components from two of the four output mixers. First and second low-pass filters have as their input the output of the fifth or sixth summer, and their output comprising the real or imaginary part of a complex number G k at zero frequency.

Frequency-hopped-waveform synthesis with a surface-acoustic-wave tapped delay line

Abstract: A coherent frequency-hopped-waveform synthesiser, based on the detection of contradirected chirp signals within a surface-acoustic-wave tapped delay line, is reported. Attractive features are programmable bandwidth and hop duration through control of external chirps, plus the capability to synthesise many simultaneous coherent outputs.

Deflection of atomic beams with isotope separation by optical resonance radiation using stimulated emission and the A.C. Stark effect

Abstract: Improved atomic beam deflection and improved isotope separation, even in vapors, is proposed by substituting the A.C. Stark effect for the baseband chirp of the pushing beam in the prior proposal by I. Nebenzahl et al, Applied Physics Letters, Vol. 25, page 327 (September 1974). The efficiency inherent in re-using the photons as in the Nebenzahl et al proposal is retained; but the external frequency chirpers are avoided. The entire process is performed by two pulses of monochromatic coherent light, thereby avoiding the complication of amplifying frequency-modulated light pulses. The A.C. Stark effect is provided by the second beam of coherent monochromatic light, which is sufficiently intense to chirp the energy levels of the atoms or isotopes of the atomic beam or vapor. Although, in general, the A.C. Stark effect will alter the isotope shift somewhat, it is not eliminated. In fact, the appropriate choice of frequencies of the pushing and chirping beams may even relax the requirements with respect to the isotope absorption line shift for effective separation. That is, it may make the isotope absorption lines more easily resolvable.

Examples of manufactured dispersive delay lines using acoustic surface waves

Abstract: Two kinds of dispersive delay lines operating in radars at the present time are described as examples of manufactured acoustic surface-wave components. These lines were designed some yearsago for a non-linear chirp impulse response and they are used as pulse expander and weighting matched pulse compressor. These dispersive delay lines consist of metallic interdigital transducers deposited on piezoelectric quartz substrates.The first kind of line, which was produced in quantity, (about 180 units) has a compression ratio equal to 23.5. The relative sidelobe level of the compressed pulse is −26 dB. The delay varies by 4 μs in a 7 MHz frequency range, the centre frequency being 30 MHz.A sampled transducer with variable finger pair spacing is used for the second kind of line in order to limit the number of fingers. The delay change is 25 μs for a 1 MHz frequency range centred at 30 MHz. The compression ratio and the relative sidelobe level of the compressed pulse are 19.2 and −24 dB respectively.

Application of SAW Chirp Transform Filter in Spread Spectrum Communication Systems

Abstract: Narrowband jamming of a spread spectrum signal can be filtered out in the frequency domain using acoustic surface wave convolvers as real time Fourier transformers. This is accomplished by employing modulated chirp signals as inputs. The particular frequency range in which the jammer occurs can be removed from the Fourier transformed signal by using a d.c. pulse and a balanced mixer. These concepts are discussed and experimental results demonstrating the use of the device as a Fourier transformer (with 110 MHz center frequency, 25 MHz bandwidth, and 14 ?sec interaction range and 220 MHz center frequency, 50 MHz bandwidth and 14 ?sec interaction range) are presented. The performance of the spread spectrum receiver using either pseudonoise or Barker codes and different jammers is discussed.

Quadratic residues: Application to chirp filters and discrete Fourier transforms

Abstract: A complete solution is given to the problem of finding the number of distinct quadratic residues for a composite modulus. Two specific applications of this result are described. The first one concerns the efficient implementation of chirp filters. It is shown that by an optimum choice of the number of taps, the number of multiplications required to realize a transversal chirp filter can be greatly reduced. Secondly, an algorithm for the computation of DFT, based on chirp filtering, is discussed. It has the potential of being faster than the FFT in certain cases and, in addition, requires less storage for the sine-cosine values.

Frequency synthesis via the discrete chirp and prime sequence ROM's

Abstract: Surface-acoustic-wave (SAW) sampled-data filters can be utilized as serial access READ-ONLY memories to directly implement at carrier frequencies a coherent fast-frequency-hop synthesizer in the VHF and UHF ranges. An example of frequency synthesis using surface acoustic wave discrete chirp filters is shown. A frequency synthesis scheme using a SAW prime-sequence filter is described.

Relation between i.f. and baseband chirp filtering

Abstract: Extracting the modulus output for a complex signal convolved with a complex chirp impulse response requires four real filters at baseband plus components for summing, differencing, squaring and square rooting. Modulating the signal onto a carrier allows the equivalent processing to be performed to a good approximation by a single i.f. filter.

Spectral analysis using CCDs

Abstract: Charge-coupled devices (CCDs) are ideally suited for performing sampled-data transversal filtering operations in the analog domain. Two algorithms have been identified for performing spectral analysis in which the bulk of the computation can be performed in a CCD transversal filter; the chirp z-transform and the prime transform. CCD implementation of both these transform algorithms is presented together with performance data and applications.

Real-time network analyser using dual analogue chirp transform

Abstract: A dual implementation of analogue chirp transform is applied, for the first time, to the realisation of a network analyser capable of providing both amplitude and phase characteristics of the network transfer function. A chirp waveform excites the network and multiplies its response prior to processing in a surface-acoustic-wave chirp filter. This simple configuration ensures maximum energisation of the network and can allow bandwidths larger than the s.a.w. filter bandwidth to be analysed.

Nanosecond pulse propagation in a high power iodine laser amplifier

Abstract: Pulse propagation in a photochemical iodine laser amplifier is investigated both experimentally and by solving the Maxwell-Schrodinger equations by computer. Pulse shortening and pulse reshaping are observed in agreement with theory. For pulse durations approaching the dephasing time, it is found that a further decrease in duration by gain saturation is hard to obtain and requires high power levels or high initial inversion. The numerical analysis includes reservoir effects due to tightly coupled hyperfine levels in the iodine groundstate and phase modulation (chirping) of the input pulse. Comparison of the iodine parameters with those of Nd and CO2 lasers shows that for nanosecond pulses pulse shortening occurs more readily in the latter two systems in agreement with experiments.

Long‐range echo sounding with a chirp source

Abstract: An experiment has been performed to determine the applicability of coherent signal processing techniques to a long-range low-frequency side-looking bathymetric survey system. A linear FM sweep of 280-Hz center frequency, 10-Hz bandwidth, and 20-s duration was produced by a source at depths between 120 and 150 m towed parallel to the Blake escarpment at a range of about 250 km. Reflected signals were received by a line array towed at depths ranging from 765 to 825 m. The received signals were passed through a digital pulse compressor and averaged. Groups of the received signals were easily identified as reflections from the Blake escarpment. The experiment has shown that coherent signal processing techniques are applicable to low-frequency side-looking bathymetric survey systems.

Graphical Interpretation of Chirp Echoes from Complex Targets

Abstract: Echoes from complex targets due to chirp waveforms show different time responses for different chirp rates, because of the ambiguity of range and radial velocity information. A graphical method is described which enables the derivation of the time response for an arbitrary chirp rate when the scattering cross-section distribution in the range and radial velocity plane is known for complex targets. The graphical method can also be used to generate an approximately realistic scattering cross-section distribution when the time responses for two different chirp rates are given. An example is shown which applies to the computer simulation of the radar echo from a waking reentry space vehicle.

An Analysis of Modulation Techniques for the Simultaneous Measurement of Range and Reflectance Information by an Airborne Laser Scanner.

Abstract: : The performance of an airborne laser terrain mapper, which measures both slant range and reflectance, depends on the method used to modulate the output power of the laser. This study analyzes four possible modulation techniques for a direct-detection scanner which utilizes a semiconductor laser. The first technique is sinusoidal modulation of the laser output. Range performance is found to improve as the frequency of the modulating sinusoid is increased. The second technique modulates the laser output with a subcarrier which is a periodic FM chirp pulse. Range performance improves as the frequency deviation of the chirp is increased. However, this also increases the required detector bandwidth. The third technique modulates the laser with a subcarrier which is phase-modulated by a periodic pseudonoise (PN) code.

Digital sweep circuit for generating sawtooth wave form

Abstract: A combination electronic device provides a digital sweep circuit adapted to generate a sweep signal which increases in frequency at constant intervals over a selected range of frequencies and over a selected period of time.