Low Peak Factor Research Articles

Conceptual design of a novel megawatt portable nuclear power system by supercritical carbon dioxide brayton cycle coupled with heat pipe cooled reactor

In order to meet realistic requirements for flexible and reliable power supply in special scenarios such as remote areas and deep ocean exploration, a novel megawatt portable nuclear power system called SUPERHERO (SUPERcritical carbon dioxide cycle-HEat pipe ReactOr power system) is proposed in this paper. The system consists of a heat pipe-cooled reactor and a closed supercritical carbon dioxide (S–CO2) Brayton power cycle. Different cycle concepts were compared, and the simple recuperated cycle was adopted for compactness and simplicity of the system. The multi-objective optimization is performed to maximize cycle efficiency and minimize system volume, resulting in optimized parameters for a 1MWe system. By utilizing uranium nitride (UN) modular fuel elements and high heat transfer capacity potassium heat pipes, a compact reactor scheme with 3.13MWt is determined that can operate for 15 years without refueling. The neutronics and thermal analysis of the reactor are carried out, and the results show that the reactor has safety features such as a low peak factor, negative temperature feedback coefficient, sufficient reactivity compensation and control, and anti-failure of single heat pipe. The primary heat exchanger is also designed using the CFD method, and the heat exchange tubes combined with ribbed plates and self-supporting structures are used to enhance heat transfer and ensure structural stability. All these studies outline the general features of SUPERHERO, demonstrating that SUPERHERO has great development potential.

A Novel Class of Zero-Correlation Zone Sequence Set Having a Low Peak-Factor and a Flat Power Spectrum

A Novel Class of Zero-Correlation Zone Sequence Set Having a Low Peak-Factor and a Flat Power Spectrum

Extending Schroeder-phase masking: Influence of direction and shape of masker instantaneous frequency

Schroeder (1970) devised an algorithm to produce low peak factor signals. Schroeder signals with equal amplitude spectra, but reversed phase spectra, reveal large differences in masker effectiveness for listeners with normal hearing [Smith et al. (1986)]. Results reported here extend previous work to include detection of multiple bursts of the same frequency, and multiple bursts that increase or decrease in frequency. Signal frequencies were selected to correspond with harmonics in the maskers. Results indicate that changing the frequency of the signal amplifies the difference between the Schroeder-phase maskers, but the direction of the change does not. When a frequency modulated tone is substituted for the Schroeder maskers, masked threshold depends on the shape of the instantaneous frequency (IF) function, as well as the direction of change. For linear FM (with IF similar to the Schroeder maskers), masked thresholds are comparable to the Schroeder-phase maskers. However, for logarithmic FM, IF changes at a constant ERB/s and directional differences are much smaller. A modified channel model [Oh (2012)] shows substantial differences in the basilar membrane response to these different maskers. [Research supported by a grant from the Office of Naval Research #N000140911017.]

Dominant region for pitch at low fundamental frequencies (F0): The effect of fundamental frequency, phase and temporal structure

When a complex tone contains many harmonics, its pitch is usually determined by harmonics in a restricted frequency region, called the “dominant region,” which for fundamental frequencies (F0s) ≥ 100 Hz corresponds to low, resolved harmonics. We estimated the dominant region for tones with low F0, by measuring thresholds, F0DLs, for detecting a change in F0 of a group of harmonics embedded within harmonics with fixed F0. The spectral position of the shifted group was systematically varied. Components were added in either cosine or random phase. For F0s of 35 and 50 Hz, the position of the dominant region depended strongly on the relative phases of the components. When the envelope had a low peak factor, with multiple peaks per period (random phase), the dominant region fell at low harmonic numbers (for F0=50 Hz), or was not well defined (for F0=35 Hz). When the envelope had a high peak factor, with one peak per period (cosine phase), the dominant region fell at high harmonic numbers, where harmonics were unresolved. Generally, performance was better for cosine than for random phase. The results indicate that harmonics in the dominant region are not always resolved.

A Class of Two-Dimensional Signal Having a Flat Power Spectrum and a Low Peak Factor

This paper presents a new generative approach for generating two-dimensional signals having both a low peak factor (crest factor) and a flat power spectrum. The flat power spectrum provides zero auto-correlation, except at the zero shift. The proposed method is a generative scheme, not a search method, and produces a two-dimensional signal of size 2(2n1 + 1)2 × (2n2 + 1)2 for an arbitrary pair of positive integers n1 and n2 without any computer search. The peak factor of the proposed signal is equal to the peak factor of a single trigonometric function.

Synthesis of Two-Dimensional Pseudo White-Noise Signal With Low Peak Factor

This letter presents a new systematic approach for generating two-dimensional (2-D) signals having both a low peak factor (crest factor) and a flat power spectrum. The flat power spectrum provides zero auto-correlation, except for the zero shift. The proposed synthesis method is a systematic scheme, not a search method, and produces a 2-D signal of size (2n/sub 1/+1)(4n/sub 1/+1)/spl times/(2n/sub 2/+1)/sup 2/ for an arbitrary pair of integers n/sub 1/ and n/sub 2/. The peak factor of the proposed signal is equal to /spl radic/1+1/(4n/sub 1/+1) times the peak factor of a single trigonometric function.

Low-Peak Pseudo-White-Noise Arrays

This letter presents a new approach to the construction of arrays having both a low peak-factor and a flat power spectrum. The proposed construction uses a systematic scheme and not any search methods. The size of the proposed array is 4n/sub 1//sup 2//spl times/2n/sub 2//sup 2/ for an arbitrary pair of positive integers n/sub 1/ and n/sub 2/. The proposed array can be used in applications such as position detection patterns, diffraction patterns for the calibration of laser beam, and so on.

A comparison of white test signals used in active sound cancellation

Several different test signals with flat power spectrum, such as white noise, Schroeder-phased waveform, and Galois noise sequences were generated and used to adaptively model the error path in an active sound cancellation (ASC) controller. The standard filtered-X LMS (FXLMS) algorithm was used with a Finite Impulse Response (FIR) model of the error path and an FIR controller structure. Both adaptive FIR filters were operated during the active sound cancellation of 80 and 125 Hz tones. The residual modeling error for different test signal mean power spectral densities (PSD) was determined. It was discovered that the residual error decreased with an increase in the mean PSD, until a minimum residual error was achieved. When the mean PSD was increased beyond this value, no further decrease in residual error occurred. Each test signal was distinguished by the minimum mean PSD that achieved the minimum residual error. All three test signals achieved approximately the same residual error at approximately the same mean PSD for both the 80 and 125 Hz tone. Therefore, the best test signal was selected as the signal with the lowest peak factor, in this case the Schroeder-phased waveform. Given a low peak factor, power can be injected into the test waveform without significantly increasing the size of the time domain envelope.

Asymptotic limits in peak envelope power reduction by redundant coding in orthogonal frequency-division multiplex modulation

This paper uses some computational results to show that the peak envelope power (PEP) of orthogonal frequency-division multiplex (OFDM) waveforms can be reduced very substantially by coding to select only those messages with low peak factor as valid codewords in the scheme. It is shown that the asymptotic limit of the PEP is between two and three, and that only four bits of redundancy are necessary to limit the PEP to within 10% of this optimum value as the number of carriers becomes large.

The distribution of peak factor in QPSK Multi-Carrier modulation

This paper presents the results of a preliminary investigation into the distribution of peak factors in the QPSK message space for different numbers of carriers with the aim of determining those messages which exhibit a low peak factor relative to a given threshold. A group structure is suggested which associatescosets of 16 messages intoequivalence classes having the same peak power. Algorithms for finding the members of cosets and equivalence classes are given. These messages are then rank ordered by peak power to show how a suitable code could be derived. It appears that allocating just one or two bits of redundancy to such a code will be sufficient to achieve substantial reductions in peak factor.

More on low peak-factor periodic functions

A low value of peak factor (PF), the ratio of the range to the rms value of a periodic function, is desirable for radar, sonar, communications, and test signals. A previous paper [L. E. Hargrove, J. Acoust. Soc. Am. 92, 2312 (1992)] described low PF functions constructed from 1≤M≤101 harmonically related and equal-amplitude components with signs derived from spectra components of sinusoidally frequency-modulated sinusoids. In that paper, the flat and bandlimited spectra of the low PF periodic functions consisted of integer multiples n, 1≤n≤M, of a fundamental frequency f1. In the present paper, similar low PF functions consisting of M equal-amplitude components with frequencies which are higher multiples n, 1<p≤n≤(p+M−1), of a frequency f1 are described. A somewhat different set of values for PF obtain for the higher multiple cases. Some comparisons will be made with low PF periodic functions similarly derived from linearly frequency-modulated sinusoids. [Work performed in the ONR Scientific Officer Research Program.]

Reference phases of low peak factor multiharmonic signals

Use is made of the reference phase of low peak factor multiharmonic signals in identification experiments. It is shown that the reference phase can be used to suppress the transient of a large time constant during excitation, allowing significant reductions in experimental time. It is also shown that reference phasing can eliminate the output drift inherent when testing type 2 systems.

An algorithm for synthesis of low-peak-factor periodic functions

Peak factor (PF) is the ratio of the range to the rms value of a periodic function. A low PF is desirable in applications such as radar, sonar, communications techniques, and the design of test signals. The PF of a frequency-modulated (fm) periodic function is low because the peak-to-peak amplitude remains fixed and the rms value changes very little for all values of the modulation index. But in some applications an ‘‘fm-like’’ function is undesirable, and for many applications a low PF periodic function consisting of a specified number of equal-amplitude and harmonically related frequency components is sought. Here, results from an algorithm that uses phases of some spectral components of a frequency-modulated sinusoid to construct low PF functions with M harmonically related and equal-amplitude components are presented for the range 1≤M≤101. These functions are less fm-like than frequency-truncated fm functions. [Work performed in the ONR Scientific Officer Research Program.]

Chirp design for acoustical system identification.

A repeated linear chirp (choip) is a broadband waveform with the lowest possible peak to rms amplitude ratio (peak factor) also having the property that the frequency band for its power concentration (power band) can be specified. A low peak factor is desirable to maximize signal power while avoiding clipping or saturation effects. The capability to concentrate power is desirable to maximize the signal to noise ratio. Choips, having both properties, make reduced measurement time possible in practical system identification relative to impulse and maximum length sequence waveforms. This is particularly useful with time-varying systems. Choips for use with analog-digital data acquisition systems can be designed easily and quickly. They can be designed to minimize leakage and aliasing. For application to a typical acoustical system, over 99% of a choip's power can be concentrated in a specified power band.

New approach to designing low peak factor, discrete‐time waveforms

Periodic waveforms can be designed to have the smallest possible peak (or crest) factors while having their power concentrated in a desired frequency band. A well‐known approach is first to specify the magnitudes of the desired waveform harmonics, then to specify or search for a set of phase angles [M. R. Schroeder, Number Theory in Science and Communication (Springer‐Verlag, New York, 1984); J. Pumplin, “Low‐noise noise,” J. Acoust. Soc. Am. 78, 100–104 (1985)]. This approach requires a significant amount of computer time to search for sets of phases that provide waveforms with near‐minimum peak factors. Another approach is to specify the waveform as a bandlimited, periodic chirp, x(n) = sin[a(n + n0)]2, 0⩽n <N. This approach has the advantage that the waveform has the smallest peak factor possible while allowing some control of the waveform spectrum. It has the disadvantage that the spectrum is distorted by leakage. The design approach, results, and some recent efforts to reduce leakage will be discussed. [Work supported in part by National Research Council of Canada and Lanikai Foundation.]

Comparison of test signals for aircraft frequency domain identification

The validity of applying frequency domain techniques to the identification of aircraft system dynamics is investigated using a simplified linear analytic model of a helicopter. A closed-loop identification of the helicopter model, with the longitudinal and lateral motions decoupled, is described. Measurement noise effects are included in the simulations. Input signals are applied to excite the system modes. Frequency response estimates are obtained by spectral analysis, and the coherence function is evaluated to indicate the likely accuracy. The transfer function coefficients are obtained by a nonlinear least-squares curve-fitting technique. With prior knowledge of the system order, the advantages of using a low peak-factor harmonic test signal for identification instead of the more usual frequency sweeps are demonstrated.

Low peak factor waveforms for efficient system analysis using a fast Fourier transform

Low peak factor waveforms [M. R. Schroeder, Number Theory in Science and Communication (Springer, New York, 1984)] can be designed so that multiple harmonics in the waveform appear to have the ideal spectrum of a true Fourier transform when the waveform is analyzed using a fast Fourier transform (FFT). If the harmonics in the waveform are restricted to those having an integral number of cycles in the FFT data record length, there is no leakage, and the magnitudes and phases of the Fourier coefficients are identical to those of the harmonics in the waveform. Low peak factor waveforms drastically reduce peak power requirements. For example, the peak power requirement for a waveform having 63 harmonics can be reduced by a factor of about 30. Waveforms can be designed having harmonics with constant, octave, and close approximations to one‐third octave frequency differences. This approach can provide an efficient alternative to other methods in current use for measuring system transfer functions. [Work supported in part by the National Research Council of Canada and the Lanikai Foundation.]

Developments in frequency-response determination using Schroeder-phased harmonic signals

A multi-frequency identification scheme employing Schroeder-specified relative phasing to achieve low-peakfactor excitation signals has been devised. These so-calledSchroeder-phased harmonic signals have been proposed as a convenient multi-frequency signal for frequencyresponse testing offering high-power with low peak-factor and a flexible choice of test spectrum. In this paper the use of these signals is extended to obtain frequencyresponse information from multi-variable and sampleddatasystems. Additionally there is a detailed discussion of the computational processes relevant to the use of these signals when the system output signals are contaminated significantly with noise.

Phase independence in low‐frequency pulse detection

The detectability of 10‐Hz pulse trains was measured for four pulse waveforms having identical Fourier series amplitude specta. One pulse was a simulated helicopter rotor pulse, the second was a polarity reversal of the rotor pulse, and the other two were random phase waveforms selected for low peak factor. The amplitude spectral envelope of the pulses had a broad maximum at 100 Hz. The detectibility was measured in a two interval forced choice task with 1‐s signals in continuous white noise. The results for two experienced and two naive subjects showed no detectibility differences between the pulses either across subjects or within single subjects, even though the sounds are clearly discriminable at suprathreshold levels and the waveform of the random phase pulses is similar to that of the masking noise. If the observed maximum detectibility difference between the pulses had attained 2 dB, it would have been significant at the 0.01 level. [Work supported by a grant to the Stanford‐NASA Ames Joint Institute for Aeronautics and Acoustics from the Army Air Mobility Research and Development Laboratory, Ames Directorate.]

Application of Schroeder-Phased Harmonic Signals to Practical Identification

A multi-frequency identification scheme employing low peak-factor test signals is described. These signals may be designed using a simple formula originally due to Schroeder and may, additionally, be tailored to suit, most conveniently, signal processing based on the Fast Fourier Transform algorithm. The use of method in practice is shown by presenting results from a hydrodynamic experiment.