Parasitic Amplitude Modulation Research Articles

Increased Spurious-Free Dynamic Range in Frequency Sweeping Interferometry by Suppression of Parasitic Amplitude Modulation

We propose an envelope detrending method to correct parasitic amplitude modulation (AM) in frequency sweeping interferometry (FSI). Parasitic AM arises from effects such as device spectral response and residual AM of laser. The periodic component in parasitic AM introduces spurious harmonics, which degrade the spurious-free dynamic range (SFDR) and interfere with information analysis. The proposed method employs a Hilbert transform to obtain a time-varying amplitude fluctuation curve from an auxiliary interferometer. This curve is then employed to detrend the parasitic AM in the main interferometric signal, thus restoring the point spread function and improving the SFDR. An optimal SFDR can be obtained when the optical path differences (OPDs) of the two interferometers are equal, while mismatched OPDs lead to a rapid degradation of the SFDR correction. Experimental results demonstrate the effectiveness of the proposed method and the optimal matching condition. A maximum SFDR of 90.2dB can be achieved when the OPD matching condition is satisfied. By applying the method to an optical frequency domain polarimeter, we realized a measured 81.1dB polarization extinction ratio. We believe that the proposed method can also help to improve the SFDR in other FSI systems.

Towards fast sensing along ultralong BOTDA: flatness enhancement by utilizing injection-locked dual-bandwidth probe wave.

Brillouin optical time-domain analysis (BOTDA) using distributed Brillouin amplification (DBA) only requires a milliwatt-level pump to achieve a sensing range beyond 100 km, which provides a powerful tool for temperature/strain sensing. However, similar to the majority of other long-range BOTDAs, the state-of-the-art reports require > 1000 times average, severely restricting the sensing speed. The blind area over tens of kilometers caused by the nonuniform Brillouin response and parasitic amplitude modulation (AM) are crucial factors affecting the signal-to-noise ratio (SNR). Here, a comprehensive performance optimization and substantial enhancement for BOTDA sensors was presented by the direct demodulation of an injection-locked dual-bandwidth probe wave. Injection locking (IL) can completely eliminate the impact of AM noise; dual-bandwidth probe enables self-adaptive pulse loss compensation, thereby intensifying the SNR flatness along the ultralong fiber, and direct probe demodulation can overcome nonlocal effects and allows ∼19.7 dB enhancement of probe input power. Therefore, using only 100 times average, ∼148.3 km sensing, and ∼5 m spatial resolution were achieved with < ∼0.8 MHz standard deviation of Brillouin frequency shift (BFS) over a broad range (∼131.7 km). The reduction in averages was more than 10 times that of the reported majority of long-range BOTDAs. Such performances were achieved without using time-consuming or post-processing techniques, such as optical pulse coding and image denoising. Because this approach is compatible with optical chirp chain technique without frequency sweeping, fast acquisition (0.3 s) was also realized, which has the potential for fast sensing at 3.3 Hz along a ∼150 km fiber.

Reliability Demodulation Algorithm Design for Phase Generated Carrier Signal

Phase-generated carrier (PGC) demodulation technology has been widely used in fiber-optic interferometric sensors system in recent years. Nonlinear errors caused by interference noise and parasitic amplitude modulation (AM) are key factors that affect the reliability of traditional PGC demodulation system based on arctangent (PGC-Arctan) algorithm. In order to enhance the reliable performance of PGC demodulation system under the conditions of low signal-to-noise ratio (SNR) and high parasitic AM interference, an ellipse fitting algorithm based on the Gauss–Newton iteration (GNI) is proposed, which is called the PGC-Arctan-GNI demodulation algorithm. The proposed algorithm uses the Euclidean distance to accurately estimate the geometric parameters of ellipse, which can correct the nonlinear distortion of the demodulated signal. In addition, the PGC demodulation system designed based on GNI algorithm has the advantages of strong antinoise ability, good antiparasitic AM ability, and widely dynamic range of the input signal amplitude. Experimental results show that, compared with the direct demodulation algorithm and the ellipse fitting algorithm based on the least squares (LS) method, the SINAD value of the proposed algorithm is always greater than 30 dB and higher than the other two algorithms under the condition of low SNR. Under the condition that the parasitic AM index <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula> changes from 0 to 0.5, the signal-to-noise and distortion ratio (SINAD) curve of the proposed algorithm is more stable than the curves of the other two algorithms, and the fluctuation range of the SINAD value does not exceed 3 dB. This algorithm also has a wider input signal amplitude range than the LS algorithm. Especially, when the amplitude of the measured signal is less than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pi$</tex-math></inline-formula> /2, the relative amplitude error of the GNI algorithm is less than 2% and significantly smaller than the LS algorithm. Finally, the demodulation results using real measured data in the actual system show that the frequency and amplitude of the demodulated signal are same as the original signal. At the same time, the SNIAD value of this system reaches 67.40 dB, the spurious-free dynamic range value reaches 68.82 dB, and the total harmonic distortion value reaches -68.40 dB@1 KHz. In summary, the proposed PGC-Arctan-GNI algorithm can eliminate system nonlinear errors caused by the parasitic AM, harmonics, and noise interference by estimating the ellipse parameters, thereby effectively enhancing the stability and reliability of the PGC demodulation system.

Design features of negative feedback and power smoothing filter of the DC / DC converter with pulse loads for transmitter-receiver modules of active electronically scanned arrays

The fundamental difference of the voltage converter of the secondary power supply source for the transmitter-receiver modules of active electronically scanned arrays with impulse load and variable duty cycle from traditional voltage converters that constantly consume power from the primary network is shown. Methods for improving the quality of transient processes in voltage converter with constant power consumption during start-up by soft start are not applicable to improve the quality of transients in periodic current pulses caused by a pulsed load. Since any ripples, noises, instabilities in the power supply of the transmitter-receiver modules of active electronically scanned arrays operating in a linear mode lead to parasitic amplitude modulation of the emitted signal and to a deterioration in the quality of selection and target tracking, very strict requirements are imposed on the quality of the transmitter-receiver modules supply voltage (voltage ripple kп ≤ 0,5 %, the amount of overshoot of the output voltage sU ≤ 2 %, etc.). The article shows that such a quality of the output voltage with a pulsed load of the transmitter-receiver modules of active electronically scanned arrays can be obtained using a lower-type voltage converter, with a double-circuit negative feedback for the output voltage and current of the inductor, with a storage capacitor and with a power smoothing filter with the characteristics of Chebyshev and Butterworth. The work investigated the influence of the negative feedback depth on the output voltage and current of the smoothing filter choke, the value of the storage capacitor capacity, the type and parameters of the smoothing filter, and recommendations were given to select their optimal values, at which the required quality of the supply voltage for the transmitter-receiver modules of active electronically scanned arrays was ensured, taking into account the minimization of their dimensions.

Multi-Resonant Frequency Shift Keying: A Novel and Efficient Modulation Scheme for Magnetic Communication

Magnetic communication is a promising technique in harsh environments, such as seawater. To enable efficient communication, transmitting and receiving coils are conventionally operated as part of resonant circuits. However, this results in narrow bandwidths and the associated disadvantages. Thus, most digital modulation schemes cannot be used efficiently on these systems. In this contribution, a mixed hardware/software approach for efficient frequency shift keying is presented. With this energy-efficient modulation concept, the full available bandwidth of a receiver can be utilized for data transmission, while all advantages of using a resonant circuit on the transmitter side can be retained. Towards this goal, a transmitter-side resonant circuit is designed, which supports discrete switching between multiple resonant frequencies. The data symbols are mapped onto these unique resonant frequencies. This prevents the parasitic amplitude modulation associated with conventional frequency shift keying applied to narrow bandwidth circuits, resulting in maximum transmitted magnitude and minimum symbol distortion without need for any further measures. Furthermore, the frequency spacing and/or the cardinality of the symbol alphabet can be increased considerably. At the receiver side, the classical pick-up coil is replaced by a high-sensitivity broadband magnetic field sensor. Numerical results of the mixed hardware/software scheme are provided, and are verified by an experimental setup. This setup is constructed using principles from the field of power electronics and demonstrates, how this modulation concept can be implemented with relatively simple and highly energy efficient electronics and without the use of linear amplifiers in particular.

High-precision nuclear-quadrupole-resonance thermometer for aerohydrodynamic research

In hydroaerodynamics, there are problems that require increased accuracy of temperature measurement. An example is the measurement of excess temperature at the periphery of turbulent jets. Unlimited temperature increase on the axis of the jet to increase the accuracy of measurement is impractical due to non-automodeliness by the Archimedes number. To do this, it is possible to use the phenomenon of nuclear quadrupole resonance. The device for exact measurement of temperature in the range from 0° C to +200 °C is offered. Using the LabView graphic programming package, a virtual model of an automatic thermometer based on nuclear quadrupole resonance in КClO3 was built. The temperature sensor is a superregenerative spectrometer with automatic monitoring of the center of the absorption line of the thermometric substance, consisting of the actual superregenerator, bandpass filter, comparator, DC amplifier and modulating voltage generator. The circuit that switches the supergenerator to the mode of continuous generation and output of information on the display contains a single vibrator, a buffer stage, a microcontroller and an indicator. From the output of the superregenerator, the absorption signal is fed to the bandpass filter, and then to the comparator. After the comparator, a voltage equal to the input voltage difference is amplified by a DC amplifier. This signal controls the resonant absorption frequency through the feedback circuit. The frequency of the sensor polling is determined by a single vibrator, the oscillations of which are synchronized with the clock generator. Measuring uncertainty in the range above changes from 0.02 K to 0.002 K It has been shown that either a manual frequency measurement method or a regenerative mode must be used to reduce the measurement uncertainty up to 0.002 K in the range. This improved automatic device is more complex, because of requirements of special measures to eliminate parasitic amplitude modulation. To increase the temperature range and precision, it is possible to find other thermometric substance, such as Сu2O crystals.

ANALOG PHASE SHIFTERS OF THE DECIMETER RANGE FOR INCREASED POWER LEVEL

The results of designing a phase control device in the decimeter wavelength range in the form of a tunable bandpass filter are presented. Tuning such a filter by 1/2 of the relative bandwidth, not exceeding 40 - 50%, provides a frequency-independent controlled phase shift. The minimization of parasitic amplitude modulation, along with an increase in the high-frequency power transmitted by the phase shifter, is achieved by optimizing the switching factor of the varicap into the quasi-polynomial bandpass filter circuit.

Оброблення результатів вимірювання фази періодичних сигналів автогенераторних перетворювачів фізичних величин

The problem of phase measuring is a promising scientific and technical direction, but the general tendency of its solution is the inextricable development of the methodology of information processing and software and hardware part of the control and measuring equipment used in such fields as radiophysics, radio navigation, precision measurement of roughness, size and defects, surfaces and crystals, detecting, including noisy FM signals. At present, the following methods for measuring the phase difference of signals, such as phase compensation, phase difference transformation in time intervals, correlation methods, Fourier series transformation method with subsequent phase separation, as well as digital methods, the most common of which is the time gate method and method of a discrete number of pulses of a measuring signal. Most of the described methods are designed to measure the phase of harmonic and quasi-harmonic signals. However, in real-world measurement, the measuring channel is a source of noise; the output measurement signal is distorted and requires filtration, which requires the use of complex filter systems, or application software that implements a signal filtering algorithm. This problem can also occur due to the relaxation mode of the measuring self-generator and violation of the power supply. In order to simplify the hardware and reduce the number of mathematical operations that increase the measurement time and reduce the sensitivity of the measurement, a numerical method for calculating the phase of the measurement signal of self-generator converters of physical quantities in the background of the action of the additive noise of the signal propagation channel, including parasitic amplitude modulation, is demonstrated. , thus eliminating the need to consider the spectral composition of the signal and the cumbersome calculations of the Fourier transform.

Residual phase noise suppression for Pound-Drever-Hall cavity stabilization with an electro-optic modulator

We propose a simple method to suppress residual frequency/phase noise in Pound-Drever-Hall (PDH) cavity stabilization setups. This method makes use of the electro-optic modulator that is commonly used in PDH setups, and hence, only requires the addition of a simple electronic feedback circuit. Here, we apply this method to stabilize a commercially available continuous wave (cw) laser to a high-finesse cavity, and we demonstrate additional phase noise suppression of ~30 dB and more from DC to 100 kHz, compared to the use of a fast-piezo actuator alone. This method can be applied to almost all PDH systems to achieve feedback bandwidths in the multi-MHz range. Furthermore, this method does not require additional actuators, such as acousto-optic modulators, and it does not introduce parasitic amplitude modulation to the first order. The limitations and drawbacks of this approach are also discussed.

Influence of Acousto-Optic Resonances in Electro-Optical Modulator on Fiber Optic Gyro Performance and Method for Its Compensation

The electro-optical modulator is one of the most important parts of the fiber optic gyroscope. Unfortunately, the integral implementation of the modulator has a number of undesirable effects such as parasitic amplitude modulation, phase drift, and acousto-optical resonance effects. Influence of the first two effects on the gyroscope operation has been described in a number of works, but at the moment, there are no published papers aimed at investigating the relationship between the presence of acousto-optic resonances in modulators and changes in the performance of a fiber optic gyroscope. There are several design solutions that provide damping of undesired acoustic waves in electro-optical modulators. However, the use of these solutions requires a method to estimate their effectiveness. The main purpose of the article is to describe such a method and to evaluate the correctness of its application. We also offer an additional way to reverse the effect of the mentioned resonant effects on the characteristics of the fiber gyroscope.

Improved Type3-PLL to Mitigate Parasitic Amplitude Modulation Effects Caused by Time-Varying Plasma Sheath

Time-varying plasma sheath likely causes parasitic amplitude and phase modulation effects on communication signals. It can increase the carrier tracking error of tracking telemetry and command systems for hypersonic vehicles flying in near space, and it may lead to loss of lock under serious conditions. This paper comprehensively analyzes the influence mechanism of the amplitude modulation effects on the loss threshold of Type3-phase-locked loop (PLL) in phase-locked receivers. Analysis result shows that amplitude fading in time-varying plasma sheaths is similar to the Gaussian distribution unrelated to the fluctuant densities. The standard deviation of amplitude fading increases with the fluctuant intensity. The 8% fluctuant intensity of electron density begins to have a significant impact on the lock probability of Type3-PLL. On the basis of the analysis results and the amplitude modulation effects, Type3-PLL with a prefiltering (PF-Type3-PLL) method is proposed to mitigate the amplitude modulation effects. PF is placed before the discriminators of Type3-PLL to restrict the amplitude variation within a reasonable range. The tracking loop PF-Type3-PLL is tested with spatial–temporal fluctuant plasma sheath model and compared with the traditional Type3-PLL. Tracking results show that PF-Type3-PLL is more robust than the traditional Type3-PLL. The phase error jitter is significantly reduced when the fluctuant intensity is 8%, and the loss threshold is reduced by 4 dB. The robustness is also enhanced under different loop noise bandwidths (5–15 Hz).

Distance Measurement by a Frequency-Modulated Rangefinder Using the Difference Frequency Spectrum of the Signal, Taking Into Account Frequency Dispersion in the Antenna-Waveguide Path

We studied the effect of frequency dispersion in an antenna-waveguide tract on the properties of the difference frequency signal when measuring distances in free space. The dispersion leads to an increase in the measurement error and reduced noise immunity. It has been shown that the simultaneous impact of frequency dispersion and parasitic amplitude modulation further increases the error. We propose and investigate variants of the Fourier transform that reduce said error, and give quantitative estimates of the achieved performance results.

Generation of signals with prescribed spectral characteristics

The main causes of undesirable changes in signal frequency-amplitude characteristics at the output of modern generators are stated in the paper. A functional scheme of the waveform generator is presented. The generator is constructed with the use of home-produced hardware components. A simplified scheme of the on-board transmitting device for the small spacecraft Aist-2 is taken as an example. The operation and interaction of units of the scheme, as well as the principle of generation of signals with various types of modulation are described. The device developed is analyzed. The units that make the most undesirable changes in the frequency-amplitude characteristic of the output signal are identified. A method of combatting bandpass flatness of frequency-amplitude characteristics is described. The spectra of the output signal prior to and after frequency correction are presented. The process of origination of parasitic amplitude modulation in a digital computation synthesizer in the case of generating signals with frequencies close to the Nyquist rate is explained. A method of combatting parasitic amplitude modulation caused by the use of a digital attenuator in the digital computation synthesizer is also discussed. It is shown that the application of the methods proposed makes it possible to develop a special waveform generator that can ensure the required spectral characteristics of the signals.

Noise Response and Stability in Injection Locked Semiconductor Ring Laser

Stability and e ects of optical injection locking in semiconductor ring laser is modeled in detail. It is veri ed that the injection locking in slave semiconductor ring laser depends on detuning frequency and external optical injection ratio between the master laser and the slave semiconductor ring laser. The stability of injection was locked using the resonance frequency and damping factor. The parasitic phase modulation response due to amplitude modulation (chirp response) is derived and simulated. Similarly parasitic amplitude modulation due to phase modulation response is also investigated.

Continuous-Wave THz Homodyne Spectroscopy and Imaging System With Electro-Optical Phase Modulation for High Dynamic Range

We present a terahertz (THz) continuous-wave coherent homodyne spectroscopy and imaging system based on THz phase control using electrooptic (EO) phase modulation for fast measurement without mechanical delay movement. After describing the degradation effects of detection signal by the optical laser noise and parasitic amplitude modulation, we propose the push-pull EO phase modulation with optical delay control for dynamic range enhancement. Using wideband unitraveling-carrier photodiode and InGaAs photoconductive antenna, spectroscopy up to a 1.5-THz frequency range and imaging are demonstrated.

High performance laser linewidth broadening for stimulated Brillouin suppression with zero parasitic amplitude modulation

To enable optimum network transmission, the ideal is to launch as high laser power as possible into the optical fiber, to overcome the effects of fiber attenuation and maintain an acceptable signal to noise ratio. Launching high powers into a fiber however, results in unwanted nonlinear effects. Stimulated Brillouin scattering (SBS) is one of the nonlinear effects which reflects a significant proportion of the transmitted optical power back to the transmitter, degrading the system severely. This paper reports the development of a digitally selected supermode distributed Bragg reflector monolithic laser chip which can provide significant linewidth broadening using a pure frequency modulation technique by application of a dither current. By modifying a small segment of the laser chip material refractive index, it produces a modulation of the longitudinal mode and hence laser frequency; the monolithic laser chip has reduced the effects of SBS significantly with very little parasitic amplitude modulation.

Integrated ferroelectric millimeter-wave phase shifters with a periodic structure

Integrated phase shifters with a periodic structure that includes BaSrTiO3 ferroelectric varactors parallel-and series-connected to the transmission line are investigated theoretically and experimentally. The phase shifters are designed for the frequency range 26–32 GHz. The dispersion characteristics and insertion losses in the transmission line of the phase shifters are analyzed with regard to the dependence of the capacitance and loss tangent of the varactors on control voltage and temperature. It is shown that parasitic amplitude modulation in the phase shifters can be suppressed if the connection scheme of the varactors takes into account the voltage dependence of the their loss tangent. In the phase shifters with series-connected varactors, the temperature dependence of the phase shift is much weaker than in those with parallel-connected varactors.

Updatable phase filters for correlation with incoherent light

The possibility for exploiting an electronically addressed liquid crystal spatial light modulator as a phase filter, for optical correlators using incoherent light, is investigated. In such correlators, the phase filter must be specially designed with an iterative algorithm which takes into account certain degradations of the modulators. The degradations were considered in some detail. For the experiments, an electronically addressed liquid crystal spatial light modulator, derived from a commercially available video projector was adapted for use as the phase filter. The phase and amplitude modulation properties of this modulator were experimentally measured and full – 2 π dynamic range for phase modulation was obtained, although accompanied by some parasitic amplitude modulation. This liquid crystal spatial light modulator was incorporated into an incoherent on-axis light correlator, to demonstrate successful updatable, real-time correlations.

A baseband processor for IS-54 cellular telephony

This paper describes the design of a baseband processor for IS-54 North American cellular telephony standard. The effect of diverse circuit impairments on the error vector in digital mode and on the parasitic amplitude modulation in analog mode are analyzed. An analog offset compensation scheme, which takes advantage of the TDMA operation, is presented. The device incorporates a Manchester data decoder for data transmission in analog mode. The messages can be sent via two interfaces to the DSP or /spl mu/-processor. The architecture of the digital signal processing chain is discussed. The device is fabricated in a 0.9 /spl mu/ CMOS technology with an area of 40 mm/sup 2/.

An analog binary phase modulator suitable for secondary services broadcast

The modulation characteristics of a simple binary phase modulator with a compensated parasitic amplitude modulation is evaluated analytically and numerically. The modulation characteristic slope, giving the modulator circuit sensitivity is obtained analytically. The evaluation is supported by a small personal computer program package written in BASIC, allowing simple and direct use of the phase deviation versus modulation voltage characteristics for Q as a parameter. >