Showing papers on "Chirp published in 1982"

Optical pulse compression based on enhanced frequency chirping

Abstract: Through numerical simulations, we show that, under relatively general conditions, passage of an intense picosecond pulse through a single‐mode optical fiber can cause the pulse to become strongly frequency broadened with a positive chirp (linear frequency sweep) describing essentially all of the energy of the output pulse. Also, because the optical fiber supports only a single transverse mode, the entire output beam profile has the same frequency modulation. These two features allow for unprecedented optical pulse compression.

An investigation into the origin of frequency sweeping in a hybrid TEA CO2 laser

Abstract: An analysis of the sources of the intrapulse frequency modulation in the hybrid TEA CO2 laser is presented. Two principal effects predominate: the falling frequency at the start of the pulse is shown to be due to plasma effects, while the increasing frequency during the remainder of the pulse arises from a laser induced refractive index perturbation. The significance of these results to the design of high stability pulsed lasers is discussed.

Digital filtering system

Abstract: A digital filtering system is provided for acquiring and processing signals using a digital filter for signal separation and signal enhancement. A digital correlator is provided for generating high resolution output data in response to low resolution input data processed with low resolution computational circuits. In one embodiment, a real-time time-domain correlator is provided with single-bit resolution computational elements to implement the correlation filtering operation. Use of the high speed real-time correlator of the present invention permits further enhancement of signals with the capability of compositing-after-correlation and with the capability of correlation using a plurality of correlation operators. Particular advantages are achieved with the use of the real-time correlator in a geophysical exploration system embodiment and in a communication embodiment. Systems applications of the digital filter includes a communications modem for modulating and demodulating chirp signals to enhance data communication and compositing-after-correlation in a geophysical exploration system. Detailed circuitry is provided to implement such systems including an improved chirp signal generator, a multi-chirp signal generator, a chirp modulator, and a correlation demodulator.

Effect of Fresnel ripples on sidelobe suppression in low time-bandwidth product linear FM pulse compression

Abstract: The paper discusses the effects of Fresnel ripples on the suppression of peak time sidelobes in low time-bandwidth (TB) product linear-FM pulse compression systems employing internally Hamming weighted compression filters. Two methods for the reduction of these effects are investigated. The first, amplitude tapering of the rectangular FM pulse, yields significantly diminished sidelobes for TB products less than about 100 and is of particular interest for spectrum analysis. The second, applicable to short risetime high-power radar transmitters, is cubic phase predistortion of the rectangular transmitted pulse. This procedure is found to be very effective for TB products less than about 50 (e.g. about 10 dB improvement for TB=10 compared with usual chirp). Both techniques force the fall-off of far sidelobes. Computed results on the achievable sidelobe suppression are presented as a function of the TB product and compared with conventional Hamming weighting. Finally, it is shown that the superior Doppler performance of linear FM is not affected by the discussed waveform modifications.

RAC-Filters with Position Weighted Metallic Strip Arrays

Abstract: Reflective array compressors with metal gratings are attractive devices because they can be fabricated in a single layer photolithographic process. However, known weighting techniques for'achieving the desired time response suffer either fran sensitivity to fabrication tolerances or fran complex reflection behaviour. We use a new amplitude weighting scheme whereby only the positions of metallic strips in the array are shifted. These shifts are caused by an additional phase modulation of the desired waveform with an oscillating function whereby the number of oscillations exceeds the time-bandwidth product. The envelope of the oscillating function depends on the required weighting. This weighting technique is free of additional secondary effects, insensitive to technological linewidth-deviations and accurate to calculate and to design. Fabricated matched filter sets with a time-bandwidth product of 12 and nonlinear frequency modulation (FM) attain -41 dB sidelobes in the compressed pulse.

Method and apparatus for ΔK synthetic aperture radar measurement of ocean current

Abstract: A synthetic aperture radar (10) is employed for Δk measurement of ocean current from a spacecraft (11) without the need for a narrow beam and long observation times. The SAR signal is compressed (12) to provide image data for different sections of the chirp bandwidth, equivalent to frequencies f1 (ta,t), f2 (ta,t) . . . fn (ta,t), and a common area for the separate image fields is selected (14). The image for the selected area at each frequency is deconvolved (16) to obtain the image signals for the different frequencies (f1, f2 . . . fn) and the same area. A product of pairs of signals is formed (18, 20), Fourier transformed (22) and squared (24). The spectrum thus obtained from different areas for the same pair of frequencies fjk, fj+n,k are added (26) to provide an improved signal to noise ratio. The shift of the peak from the center of the spectrum is measured and compared (28) to the expected shift due to the phase velocity of the Bragg scattering wave. Any difference is a measure of current velocity vc (Δk).

Threshold-less collision detection and bit arbitration in a CFSK data system

Abstract: In a chirped-frequency-shift-keyed data system, in which a plurality of stations each contain a data transmitter and a data receiver constantly monitoring a common system communications medium even while the companion transmitter is transmitting digital data, the CFSK signal is received during each of a successive plurality of synchronous bit time intervals with a first amplitude. The received signal is: squared and filtered in a first channel; multiplied with a locally generated signal of a second amplitude and associated with a first binary chirp condition, and then filtered, in a second channel; and multiplied with another locally generated signal of the second amplitude and associated with the remaining binary chirp condition, and then filtered, in a third channel. The channel outputs are functions of the signal amplitudes and also of the number of system transmitters simultaneously transmitting one or the other of the binary data states. Comparison of the energy contents of the various channels determines if a collision has occurred; and if a collision is detected, further comparisons arbitrate the collision to the bit state of the larger number of transmitters then active. The collision detection and bit arbitration is carried out without reference to a threshold value.

Compound surface acoustic wave matched filters

Abstract: Compound surface acoustic wave matched filters comprise chirp-biphase and biphase code multiplex transducer pairs on piezoelectric substrates. Both alternatives offer significantly improved processing gain when compared with conventional biphase filters. The advantage is realized by convolving the correlation peak of the chirp or biphase input transducer on the response of the conventional biphase output transducer, thus achieving a large processing gain with a minimum number of transducer electrodes.

The encoded sweep technique for Vibroseis

Abstract: In many areas correlation noise has become a serious problem because of the high dynamic range of more than 60 dB between the strong first arrivals and the weak reflection events arriving several seconds later. Several efforts have been made to reduce correlation noise either by tapering the sweep signals or by deconvolution. The effect of tapering is limited by the strong amplitude distortion of the reflected signals; deconvolution cannot cope with such a large dynamic range in the presence of noise. The sweep signal encoding technique using complementary codes was first proposed for Vibroseis® by Bernhardt (1977). The coding allows a complete correlation noise suppression after a predetermined time interval which is completely independent of the amplitude characteristic of the subsurface and the recording arrangement. Binary and quaternary codes have been investigated. Moreover, the quaternary encoded sweep allows the advantages of the encoding technique to be combined with the preferences of the Combis...

Wideband radio receiver

Abstract: A wideband radio receiver utilizes a plurality of frequency domain samplers to process an input signal. Each of the frequency domain samplers includes a sweep frequency oscillator and a mixer for combining the input signal with the sweep signal. The output of the mixer is passed through a frequency weighting filter to a dispersive delay line. The output of the dispersive delay line is digitized to produce I and Q channel digital outputs. Digital outputs from each of the frequency domain samplers are compensated for time and amplitude and then stored in a random access memory. A signal detection processor examines the outputs of the frequency domain samplers to detect the presence of a signal within the input signal. When a particular signal is detected, a data access circuit is commanded to read the stored digital outputs which correspond to the intercept bandwidth cell for the detected signal. The data thus read from the random access memory comprises the digital I and Q outputs of the wideband radio receiver.

System for the transmission of digital signal pulses over the lines of an A.C. distribution network

Abstract: In this system, chirp-frequency-modulated mains-synchronised signal pulses are transmitted by the frequency-division multiplex process from a teleaction transmitter (21) in a substation (2) connected to a low-voltage network (16) to a teleaction receiver (22) in a central station (1). The input of this teleaction receiver (22) is connected via a coupling transformer (10) to a medium-voltage level feeding the low-voltage network (16). The frequency deviations of chirp signal pulses with adjacent frequencies are partially overlapping. The starting and concluding edges of the amplitude envelope of the chirp signal pulses have a predetermined, limited steepness in accordance with a cosine function. The substation (2) also contains a control circuit (24), a processing circuit (25), a resetting circuit (29) and a ripple-control receiver (19), and the central station (1) contains a teleaction analyser (23), a ripple-control transmitter (18) and a ripple-control centre (17).

Digital Preshaping of Ultrasonic Signals: Equipment and Applications

Abstract: This p aper describes an a rrangement that uses AD and DA conversion and a computer in order to preshape a transmitted ultrasonic wave. Normally the ultrasonic transducer is excited by a very short e lectrical pulse. Using a DAC, the ultrasonic transducer can be excited by any desired waveform e g the digitally determined inverse filter, chirp signals etc. The desired excitation waveform is stored in a high speed memory. It is shifted out via the DAC to an amplifier and thence to the transducer. The ADC, which is triggered by the DAC equipment, converts one sample per sweep into 12 bits. Therefore, actual sample rate is low. But the virtual sample rate limited by the sample and hold circuit can reach 15 MHz. The ADC and DAC are connected to a microcomputer system that determines the desired excitation waveform. Various waveforms can be stored and it is then possible to change the excitation waveforms interactively during clinical examinations.

Subsurface Acoustic Microscopy Using Pulse Compression Techniques

Abstract: One of the key problems in imaging below the surface of a solid object arises from the large surface reflection, which tends to mask the much smaller object signal. Using sufficiently broadband transducer systems, it is possible to separate these returns - but at the price of a reduced detection sensitivity. As in the comparable problem familiar in radar, the situation can be greatly improved by resorting to long, coded pulses. We have recently shown that a chirp system centered on 60 MHz, implemented using SAW filters, can give significant improvement in the imaging of subsurface objects. Further examples, particularly in the investigation of metalemetal diffusion bonds will be presented. At 60 MHz, the differential loss through the microscope, over the 25 MHz bandwidth used is not a major problem. In seeking to work over large bandwidths, centered on 750 MHz, it is, however, vital fully to include this effect, which can involve an undesirable frequency weighting of several tens of dBs. The problem has been examined by the use of a full computer simulation. It is concluded that, whilst the advantage which can be derived at higher frequencies is not as great as at low frequencies, an improvement of signal to noise ratio can still be anticipated. Experiments on a pulse compression 1 GHz instrument are currently being prepared.

Electrical signal generation

Abstract: For digitally generating a chirp signal. a ROM 5 stores digital numbers representing successive values of the chirp, and a counter 8 is driven by pulses derived from a timing and clock unit 6 so as to cause the ROM 5 to output the digital numbers in turn. A digital to analogue converter 12 converts the signals into an analogue output which is mixed with a Carrier signal derived from a local oscillator 18. A band pass filter 20 selects the upper or lower sideband to produce the required chirp output on line 22. Also disclosed is a modification to the arrangement to enable the bandwidth of the chirp to be increased.

The potential application of analogue matched and adaptive filters in spread-spectrum communications

Abstract: Spread-spectrum communication techniques use a common wideband channel to obtain subscriber accessing in secure, jam resistant links. Phase shift keying (p.s.k.) and frequency hopping (f.h.) under the control of a pseudo-noise (p.n.) code are favoured signal waveforms. The paper reviews the performance of surface acoustic wave and charge-coupled device fixed coded and electronically programmable analogue matched filters for the detection of short (13 to 1023 chip) 1 kHz to 20 MHz chip (clock) rate p.n.-p.s.k. coded signals. The extension of these matched filters to accommodate the longer codes 10000 chips) and integration times (several milliseconds) used in current spread-spectrum systems is reviewed, and their importance in reducing synchronization acquisition is highlighted. Surface acoustic wave device approaches to synthesize and detect the alternative f.h. spread-spectrum waveforms are also discussed. The realizations of digitally-controlled coherent frequency synthesizers using s.a.w. chirp filters and bandpass filterbanks are highlighted. Finally the paper identifies the problem in many spread-spectrum systems, where the input-signal-to-interference ratio is so low that code matched filtering alone is not sufficient to detect the transmitted data, and it investigates the use of adaptive signal processing techniques for further suppression of wide and narrowband interference.

Phase Compensation of Linear FM Slanted Transducers by Use of Metallized Stripes

Abstract: First order slanted array correlator transducers have been constructed in the past and are the basis for many pulse compression systems. Phase errors in these d evices when used in a pulse compression application cause degradation of compressed pulse time sidelobe levels. The possibility of compensating these phase errors over large bandwidths exists due to the geometry of such devices. R eported in this paper is a method of phase compensation which utilizes the time delay created by the propagation of a surface a coustic wave under a metallized stripe. In slanted transducer devices the propaation paths for each frequency in the device bandwidth are distributed spatially. By placing a metallized stripe between the transducers in the acoustic path, phase compensation can be achieved. Using this method several devices have been compensated and phase error reduction of 90% peak has been observed causing a corresponding reduction in time sidelobes of 5 dB. The analysis of this method is presented along with some test results.

Passive Superconducting Microwave Circuits for 2-20 GHz Bandwidth Analog Signal Processing

Abstract: A new technology for making analog signal-processing devices such as linear-FM chirp filters with time-bandwidth products up to 1000 is being developed using niobium stripline on sapphire. Preliminary results of delay lines, resonators, and a 25-ns, 2-GHz chirp filter will be presented.

Narrow-aperture chirp transducers for SAW convolvers

Abstract: Narrow-aperture SAW beams can be generated efficiently using chirp transducers, thus avoiding the need for beam compression in convolvers The transducers can be designed such as to compensate for phase errors due to dispersive SAW propagation Experimental confirmation was obtained using convolvers with 120 MHz bandwidth and 16 μs interaction length, and a bilinearity factor of -65 dBm was achieved

Optical pulse chirp modification in degenerate four‐wave mixing at finite pump powers

Abstract: Exact solutions of the linearized transient envelope equations for degenerate four‐wave mixing in a finite‐length cell show that reflection coefficients near unity severely restrict the conditions under which chirp reversal occurs, and can lead to nulls in the amplitude of the reflected and transmitted fields.

Linear Sweep Synthesis

Abstract: During the last 10 years, many new techniques for sound synthesis have been developed. Among them, some of the most popular are frequency modulation (FM) (Chowning 1973) and nonlinear distortion, or waveshaping (Arfib 1979; LeBrun 1979). The method presented here is well known as chirp filtering in many fields such as radar (Cook 1960), optics (Papoulis 1968), and Fourier Transform theory (Oppenheim 1969; Bluestein 1970). The basic idea is very simple. Let us take as an example a sinusoidal sound c(t) whose frequency is swept between two values fl and f2 in a linear way (Fig. 1). At each moment the amplitude of that sound is multiplied by another modulating sound r(t) (Fig. 2). The period of r(t) is identical to the time of one sweep of c(t). The result of that process is shown in Fig. 3 in the time domain and in Fig. 4 in the frequency domain. We can see from these figures that the shapes are very similar with the exception that the spectrum is curved because of the logarithmic plot. The musical application of this process lies in the possibility of designing any spectral shape F(o) and realizing it through an envelope function r(t). Some properties of this method, called linear sweep, or linear sweep FM, are as follows.

Performance Of The Integrated Optic Spectrum Analyzer

Abstract: The integrated optic spectrum analyzer is a device which utilizes the Bragg interaction between a SAW and a guided optical beam to perform spectral analysis. Hughes has developed the first fully integrated IOSA which uses a butt-coupled GaAlAs semiconductor laser, a LiNb03 waveguide substrate, a 400 MHz bandwidth SAW transducer and a silicon detector/CCD array chip. The completed device exhibited a 3 dB bandwidth of 380 MHz with 10%/watt deflection efficiency at ), = 0.82 μm. The RF resolution was found to be 8 MHz and the dynamic range was greaterPthan 20 dB. Data is also discussed on a modified chirp transducer which exhibits a 450 MHz acousto-optic bandwidth with a center frequency of 1.0 GHz.*© (1982) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Femtosecond Continuum Generation

Abstract: We obtain white light continuum pulses with durations as short as 80 fsec. The broad spectral range (0.19μ–1.6μ) minimal chirp, stable repetitive character, and availability of powers extending to the gigawatt range suggest these pulses will provide a powerful new tool for femtosecond spectroscopy. We obtain these pulses by using short (65 fsec) pulses from our colliding pulse mode locked laser [1] which have been amplified to gigawatt powers in an amplifier designed specifically for short pulse amplification [2]. The short duration and high intensity of these amplified pulses permit us to generate continuum pulses in a short length of nonlinear medium with the consequence that pulse distortion by group velocity dispersion is minimal. We thus generate pulses which are temporally short and which also have almost negligible chirp over broad spectral regions. Such a feature is essential for femtosecond time resolution in most practical applications. This approach is also of special interest in that the temporal distribution of the continuum is determined primarily by the generation mechanism rather than by group velocity dispersion in the generating or analyzing media. We can thus present convincing evidence, e.g., that self phase modulation plays a prominent role in the continuum generation process.

Ultrasonic image pickup device

Abstract: PURPOSE:To enable the transmission and reception to be performed at the same time even through central frequencies are close to each other, by modulating vibrating units which have different frequencies and a plurality of electricity-sound transducer elements, transmitting ultrasonic signals, and separating the received signals by a matching filter. CONSTITUTION:The output of an up-chirp wave oscillator 100 whose frequency is fo drives the vibrator 106 via a driver 102 and a switch 104. The ultrasonic wave T1 is sent to the inside of a body to be checked A, and reflected by an internal organ C. The reflected signal R1 is received by the vibrator 106. The received signal is outputted from the matching filter 110 through the switch 104, and receiving circuit 108. The output of a down-chirp wave oscillator 101 whose frequency is f1 is transmitted and received in the same way. The switching of transmission and reception are performed by the control of a timing circuit 112 at the same time. Therefore, e.g., cross talk N2 of the wave f1 is inputted to the receiving circuit 108, but it is attenuated by the matching filter 110 since the direction of the chirp modulation is different. Even though the frequencies f0 and f1 are close to each other, interference is not caused, and this is suitable for the image pickup of the inside of a human body.

Adjustable optimal filter e.g. for acoustic surveillance system - uses two signal transformation networks operating in synchronism

Abstract: The filter has the full filter transmission range divided into a number of partial ranges, for each of which the transmission factor can be individually adjusted. The filter has an initial stage in which the real or complex input signal (f(t) is multiplied with a given exponential function before feeding the real and imaginary components separately to a CHIRP-filter. The real and imaginary spectral components supplied by the CHIRP-filter are fed to a third stage where they are multiplied with adjustable complex transmission factors for the respective partial transmission ranges. The third stage is followed by a second CHIRP filter and a final multiplier stage with the same algorithm as the first CHIRP filter and the initial multiplier stage.

Narrow band sweep controller

Abstract: PURPOSE:To make the observation of a steep frequency response easy, by a narrow band sweep for a desired frequency part and its vicinity within a frequency sweep band of a sweep signal through slow sweep speed. CONSTITUTION:A narrow band sweep controller 16 is connected between a triangle wave generator 10 generating a triangle signal and a sweep control circuit 12. A sweep control circuit 12 receiving a triangle wave signal generated from said device 16 through a part of modification incorporates a DC voltage source, and a DC voltage from said voltage source and a partly modified triangle wave signal are respectively summed with suitable magnitude to generate a sweep control signal. A sweep signal generator 14 consists of a voltage controlled oscillator and generates a frequency sweep signal having a frequency in response to a voltage value of the received sweep control signal. Thus, a part with greater slope of the sweep control signal has a large frequency sweep speed as broad band sweep and a part with small slope is slow in the frequency sweep speed as narrow band sweep.

Suppressing circuit for narrow band disturbing signal

Abstract: PURPOSE:To make the suppression of disturbing signals possible when chirp signal and the disturbing signals are superposed with time by subjecting reception input signals of a radar or the like to frequency analysis, and limiting their amplitude on a frequency axis then subjecting the same to pulse compression. CONSTITUTION:An input signal is converted to an intermediate frequency band by a mixer 11, and is made into a Fourier transform waveform by a dispersion type delay line 12, whereby narrow band disturbing signal components are made into large amplitude pulses of sharp peaks and a chirp component S is made into a low amplitude square wave of wide pulse width. The converted waveform is processed with a wide band amplifier 13 and an amplitude limiting circuit 14, and the component J is considerably limited of power. Only the narrow band disturbing signal is suppressed by a compressor 15 forming a matched filter corresponding to the Fourier transformation of the component S in spite of the case in which the component J is superposed with the component S without separation with time, and the chirp signal is easily detected without receiving the influence of said signal.

Wave V Latency and Chirp (Linear Frequency Ramp): Repetition Rate

Abstract: Chirps or linear frequency ramps served as stimuli for eliciting brainstem electric responses (BER). Chirps were presented in a range from 1.2 to 8.7 kHz, both falling and rising, with interstimulus intervals (ISIs) of 10, 20, 50 and 100 ms. All parameters influenced the BER. The latency of wave Vdecreased with increasing mean frequency of the chirps and ISIs. Wave latency is affected by the interaction of chirp frequency, direction of sweep and ISI in a complex fashion.

Frequency variation of intra-cavity laser radiation

Abstract: A simple analysis is presented to show that 'chirping' of a laser is accompanied by a frequency change of the existing intra-cavity radiation so that it remains tuned to the cavity mode frequency. This result simplifies modelling of the stimulated emission process, and justifies the commonly-applied simple description of even a rapidly-chirping laser's output in terms of cavity modes.