Level Of Spurious Signals Research Articles

Reduction of Spurious Signal Upconversion in Frequency Multipliers

Usually many applications of radar transceivers and heterodyne frequency synthesizers assume a spurious signal power level below −60 dBc. In the case of modern synthesizers using direct digital synthesis (DDS) systems, the number of emerging spurious signal frequencies is very large, and spectral purity within −60 dBc can only be obtained in the relatively narrow tuning band of the DDS unit. For the purposes of widening this useful frequency range, the frequency multiplying operation is applied commonly. Then, during the process of frequency multiplication of the baseband signal containing inband spurious signals, the effect of the upconversion of spurious signals occurs. The paper contains an analysis of the undesirable effects of the conversion of spurious signal frequencies accompanying the process of frequency multiplication. A method of reducing the level of upconverted spurious signals is proposed. The numerical calculations and measurement results are provided. For the case of a frequency multiplier with a multiplying factor equal to N, the power ratio between the desired output signal and upconverted spurious signal drops by an additional 1/N2. It has been found that the application of the presented method during the design process of the frequency multiplier allows this ratio to be improved by 6 dB.

Centrifugal compressor surge detecting method based on wavelet analysis of unsteady pressure fluctuations in typical stages

Centrifugal compressors are complex energy equipment. Automotive control and protection system should meet the requirements: of operation reliability and durability. In turbocompressors there are at least two dangerous areas: surge and rotating stall. Antisurge protecting systems usually use parametric or feature methods. As a rule industrial system are parametric. The main disadvantages of anti-surge parametric systems are difficulties in mass flow measurements in natural gas pipeline compressor. The principal idea of feature method is based on the experimental fact: as a rule just before the onset of surge rotating or precursor stall established in compressor. In this case the problem consists in detecting of unsteady pressure or velocity fluctuations characteristic signals. Wavelet analysis is the best method for detecting onset of rotating stall in spite of high level of spurious signals (rotating wakes, turbulence, etc.). This method is compatible with state of the art DSP systems of industrial control. Examples of wavelet analysis application for detecting onset of rotating stall in typical stages centrifugal compressor are presented. Experimental investigations include unsteady pressure measurement and sophisticated data acquisition system. Wavelet transforms used biorthogonal wavelets in Mathlab systems.

Planck2013 results. III. LFI systematic uncertainties

We present the current estimate of instrumental and systematic effect uncertainties for the Planck-Low Frequency Instrument relevant to the first release of the Planck cosmological results. We give an overview of the main effects and of the tools and methods applied to assess residuals in maps and power spectra. We also present an overall budget of known systematic effect uncertainties, which are dominated by sidelobe straylight pick-up and imperfect calibration. However, even these two effects are at least two orders of magnitude weaker than the cosmic microwave background fluctuations as measured in terms of the angular temperature power spectrum. A residual signal above the noise level is present in the multipole range ℓ < 20, most notably at 30 GHz, and is probably caused by residual Galactic straylight contamination. Current analysis aims to further reduce the level of spurious signals in the data and to improve the systematic effects modelling, in particular with respect to straylight and calibration uncertainties.

On the calculation of the Morlet wavelet used for the analysis of the dynamics of electroencephalogram signal responses

The function describing the Morlet wavelet transform used for calculating spectrograms characterizing cerebrum brain electroencephalograms is analytically integrated. It is shown that the exact integration reduces the spurious signal level, a circumstance that facilitates obtaining ridges of wavelet spectrograms.

High-frequency characterization and modeling of paraelectric∕superconductor resonators for high power-handling microwave passive devices

The theoretical conditions required to observe reduced nonlinearities in passive microwave devices made of a paraelectric (PE) thin layer and a high-temperature superconductor (HTS) thin film are presented. In a transmission line (TL) combining both materials, the nonlinear permittivity εr(Vrf) of the PE generates a nonlinear capacitive contribution Cd(Vrf), whereas the nonlinear penetration depth λ(Irf) of the HTS produces a nonlinear inductive contribution Ld(Irf). These contributions act in opposite ways. Under determined conditions that will be defined, they may fully compensate or at least lead to the reduction of the spurious signal level. The HTS nonlinearities may be lowered by employing very thin layers of PE. In this case, the high dielectric loss tangent (tanδ) of PE should not be such an issue. Based on experimental results obtained with superconducting YBa2Cu3O7−δ resonators integrating different thicknesses of paraelectric SrTiO3 (STO) deposited on LaAlO3 substrates, the loss level could then be considered in the simulation of the proposed compensation effect. The results show that the reduction of the unloaded quality factor (Q0) as the thickness of STO increases also impacts the level of spurious signals. Then, an analytic expression for the Q0 of a resonant TL that integrates a PE and a HTS thin layer is proposed. It specifies the conditions for reducing the total nonlinear response. These results could then be applied to realize high power handling HTS passive microwave devices.

Generation of two-mode optical signals with broadband frequency tunability and low spurious signal level

For continuous millimeter and terahertz-wave applications, a two-mode optical signal generation technique that uses two arrayed waveguide gratings and two optical switch units is presented. In addition to easy and fast operation, this scheme offers broadband frequency tunability and high signal purity with a low spurious mode level. Mode spacing, which corresponds to the frequency of the generated MM/THz-wave signal after photomixing, was successfully swept in the range of 200 ~ 550 GHz and the optical spurious mode suppression ratio higher than 25 dBc was achieved. In addition, spurious modes characteristics were investigated by using second harmonic generation (SHG) autocorrelation methods for several frequencies.

A Frequency Shift Technique for Pattern Correction in Hologram-Based CATRs

The measurement accuracy of a compact antenna test range (CATR) is limited by the level of spurious signals. At millimeter and submillimeter wavelengths the spurious signals are caused by range reflections and deformations of the range collimating element. Several antenna pattern and range evaluation techniques that can be used to mitigate the effects of these nonidealities have been published. At submillimeter wavelengths these techniques are typically time consuming or they are not able to correct the antenna pattern close to the main beam. The frequency shift technique does not have these limitations in a hologram based CATR. In this paper, the applicability of the frequency shift technique is analytically studied for a hologram based CATR. The technique is verified by measurements. The accuracy provided by the frequency shift technique is compared to that of the antenna pattern comparison (APC) technique

A feed scanning based APC technique for compact antenna test ranges

The measurement accuracy of a compact antenna test range (CATR) depends on the level of spurious signals. To improve the measurement accuracy, several error compensation methods have been developed, but most of them are not feasible at submillimeter wavelengths. This paper introduces an error compensation technique for compact antenna test ranges, which is especially suitable at submillimeter wavelengths. The method is based on antenna pattern comparison (APC). In the original APC technique the antenna pattern is recorded several times at different positions of the quiet-zone field, and the corrected pattern is obtained by averaging the measured patterns. In the proposed method, the lightweight transmitter is moved instead of moving the heavy combination of the antenna under test (AUT) and the rotation stage. The feasibility of the method is studied and the method is tested with measurements in a hologram based compact antenna test range at 310 GHz. The accuracy provided by the proposed method is compared to the accuracy provided by the conventional APC. The accuracies provided by both methods are practically equal.

Reducing Spurious Output of Balanced Modulators by Dynamic Matching of I, Q Quadrature Paths

This paper presents a technique for reducing spurious output of balanced modulators used in transmitters and arbitrary waveform generators. Two-step upconversion is a convenient way to produce a desired single-sideband (SSB) radio-frequency (RF) signal-baseband quadrature I and Q signals (which are analog outputs of direct digital frequency synthesizers) are upconverted by mixers and local oscillators (LOs)-but mismatches between the DACs in I and Q paths cause spurious output. We propose a method of dynamically matching the I and Q paths by multiplexing two DACs between I and Q paths in a pseudo-random manner. MATLAB simulation shows that multiplexing the two DACs spreads the spurious output, caused by mismatches between the two DACs, in the frequency domain, and reduces the peak level of spurious signals.

Scattering matrix approach to STW resonators.

The scattering matrix method was used for the analysis of surface transverse wave (STW) resonators on quartz. An expression for the transfer function of the resonators with different numbers of electrodes in the reflectors was derived. It was found that, for a proper ratio of these numbers, the spurious signal level below the resonance frequency can be lowered. The STW resonator for the frequency near 1090 MHz was designed, fabricated, and measured. By matching the measured and calculated transfer functions, the velocity, the electromechanical coupling coefficient, and the reflection coefficient of one aluminium strip of the STW on the 36 degrees Y-cut quartz were determined. The insertion loss about 7 dB, loaded quality coefficient near 4200, and the spurious signal level about 5 dB lower compared with the resonance one were obtained for the resonator.

Reference Level Stability of the Canadian Superconducting Gravimeter Installation

Reference Level Stability of the Canadian Superconducting Gravimeter Installation

Spurious signals in direct digital frequency synthesizers due to the phase truncation

Direct digital frequency synthesizers (DDS or DDFS) are widely used in modern communications and measurement devices. Their advantages are small size and power consumption together with excellent frequency stability, high frequency resolution, and short switching times. The difficulties are rather low output frequencies (500 MHz at the present state of the art) and a large set of the spurious signals very often above the -80 dB level. One source of spurious signals in DDS is the use of smaller number, W, of the most significant bits (MSB) applied for the output sine wave reconstruction from all R bits stored in the accumulator. The result is a phase modulation of the output signal. The problem was first solved in a rather complicated way with the result that the level of the largest spurious signal is about -6 W dB below the carrier with an increase of 3.9 dB in some instances. A simpler solution of the problem of spurious signal level due to the phase truncation in DDS was found earlier. However, no attention was paid to the validity of the corrections suggested. In this paper we will be concerned with this problem and investigate the validity and correctness of these generally cited results and provide a simple way for finding positions, levels, and numbers of these spurious signals generated by truncation to W bits of the phase information stored in the DDS accumulator memory of R bits (W<R).

Analysis of effect of fabrication errors on performance of surface-acoustic-wave m-sequence correlators

The paper describes in detail the statistical analysis used in a study of the degradation produced by imperfections in surface-acoustic-wave correlators. We consider the effect of errors in either one or both of the generator/detector pair when the input signal is either a single or a periodic m-sequence. A theoretical model, which incorporates taper, band limitation, random amplitude variations and spurious signal level, accounts for a large measure of the experimentally observed degradation of performance.

Television Pickup Tubes Using Low-Velocity Electron-Beam Scanning

Television pickup tubes which use a low-velocity electron beam for scanning a photosensitive target offer the possibility of avoiding some of the limitations of devices which use a high-velocity beam. Some of the advantages of the low-velocity beam are: (1) low level of spurious signals, (2) high maximum signal output, and (3) high efficiency of conversion of light into signal. The major problems encountered are: (1) ways to keep the beam in focus and (2) ways to obtain undistorted scanning of the photosensitive target. One solution to the difficulties of low-velocity-beam scanning involves the use of a uniform axial magnetic field to guide and focus the beam near the target. Because of this magneticfield, the usual type of deflection plates or deflection coils cannot be used to advantage. Two types of scanning which may be used are: (1) the release by a flying light spot of a beam of photoelectrons from successive points on a photocathode already immersed in the magnetic field and (2) the displacement of a beam from a thermionic cathode by a pair of defiection plates or coils operating in conjunction with the axial magnetic field and having an aperture at least as wide as the target to be scanned. Developmental television pickup tubes using each of these types of scanning with low-velocity electrons were tested and found to give the low spurious signal, high signal output, and high efficiency which had been expected.