Nonlinear Distortion Reduction Research Articles

Low-Voltage Continuous-Time Filter Based on a CMOS Transconductor with Enhanced Linearity

Summary The proposed design of a low-voltage continuous time filter is based on a CMOS transconductor with enhanced linearity. The compensation principle is used for the reduction of transconductor non-linear distortions. The discussed transconductor consists of two transconductors connected in parallel. The input transistors of the first transconductor are working in the triode region, while the input transistors of the further one are in the saturation. A fifth-order 1 MHz low-pass Bessel filter is synthesized and simulated using transconductors. The supply voltage is equal to +2.5 V. A tuning system of the filter is also simulated and discussed. A comparison shows that the discussed filter provides a higher linearity (from 4 to 9 dB) than the known circuits with the exception of filters based on the amplifier with degeneration. But it is noted that the last approach is difficult to use for the low-voltage application, because the voltage drop on the degeneration resistors limits the possible decrease of the voltage supply.

Nonlinear distortion reduction in transverse mode stabilized oxide-confined VCSELs

Fundamental mode operation has been achieved in a large area oxide-confined vertical-cavity surface-emitting laser (VCSEL) by etching a shallow surface relief in the top mirror. The nonlinear distortion during analog modulation has been measured and compared to the distortion in a planar multimode VCSEL. The distortion was quantified by conventional two-tone measurements from which the spurious free dynamic range was determined. Several decibels improvement in dynamic range was observed in the low frequency range (0.1-2 GHz) for the single-mode VCSEL. This is primarily due to a higher relaxation frequency and an absence of mode competition and mode partition noise in the single-mode VCSEL, indicating the advantages of using single-mode VCSELs for high-performance analog modulation.

Reduction of intrachannel nonlinear distortion in 40-Gb/s-based WDM transmission over standard fiber

Intrachannel cross-phase modulation and four-wave mixing in high-bit-rate WDM transmission systems employing standard single-mode fiber are investigated. The effects of imperfect third-order dispersion compensation are included in the study and analytical expressions giving optimum values of dispersion precompensation minimizing the distortion due to the intrachannel nonlinear effects are derived.

Reduction of nonlinear distortion in MQW semiconductor optical amplifier using light injection and its application in multichannel M-QAM signal transmission systems

By using an external light-injected multiple-quantum-well (MQW) semiconductor optical amplifier (SOA), we have demonstrated a 100-km optical fiber link that transports 77 channels of 64-QAM signals. This is equivalent to a system capacity of 2.3 Gb/s, while using a laser bandwidth of only 550 MHz. Under a requirement of carrier to noise and nonlinear distortion ratio of 30 dB per channel, the 1310-nm gain-peaked SOA input dynamic range was increased from 0 to 9 dB due to an injected light with 8.8 dBm and a wavelength of 1284 nm.

Reduction of nonlinear distortion in directly modulated semiconductor lasers by coherent light injection

A theoretical investigation of the second- and third-order intermodulation distortions (IMDs) in an injection-locked semiconductor laser under small-signal modulation is presented. The results show that a substantial reduction of the laser nonlinearity can be obtained, depending on both the injection level and frequency detuning between the master and slave lasers. The intensity modulation frequency response is also reported and shows that the injection-locked laser may also have a significantly improved behavior over the same free-running laser, revealed mostly in the reduced resonance peak and the broadening of the modulation bandwidth available.

Smart amplifier for loudspeaker motional feedback derived from linearization of a nonlinear motion responsive signal

A motional feedback loudspeaker system provides acceptable bass performance even with inexpensive speakers by the reduction of nonlinear distortion with the use of feedback derived from the back emf signal generated by the motion of the voice-coil within the magnet field. A novel linearizing circuit corrects the nonlinearity of the back emf signal due to the falloff of the BL factor as a function of cone displacement. The linearizing circuit is a recursive feedback loop comprising a multiplier for multiplying the back emf signal by a corrective function which is the inverse of the BL nonlinearity so as to provide a velocity signal linearly proportional to the cone velocity, an integrator for integrating the velocity signal to generate a displacement signal linearly proportional to the cone displacement, and a correction generator comprising a nonlinear curve shaper responsive to the displacement signal to generate said corrective function.

Evaluation of MESFET nonlinear intermodulation distortion reduction by channel-doping control

This paper is intended to evaluate the linearity that can be provided by general-purpose MESFETs. By a simple physics-based analysis and a practical amplifier design, it will be shown how educated device and bias-point selection can approximate intermodulation distortion (IMD) performance of some normal channel-doping profiles, for which previous theories would not be able to predict good IMD performance, to the one expected from MESFET devices with specially tailored doping profiles.

Frequency planning for nonlinear distortion reduction in wideband transmission

A frequency planning scheme for nonlinear distortion reduction in video transmission systems is presented. The scheme is suitable for wideband application because it is not limited by electronic circuits. For this frequency planning algorithm, experimental data showed a third order distortion reduction of 14 to 16 dB.

Applications of lasers and fiber-optic systems in control of the formation of microwave signals and their distribution in antenna arrays

This review is based on the non-Soviet literature and deals with the applications of lasers and fiber-optic systems in phased microwave antenna arrays. An analysis is made of the promising variants of fiber-optic systems for the distribution of microwave signals and the requirements which must be satisfied by components of such systems are formulated. An account is given of the advantages of using fiber-optic beam-forming systems for antenna arrays, of fiber-optic devices for processing microwave signals, and of laser control of the generation of microwave oscillations. A review is given of ways for improving optical and optoelectronic components so as to achieve fast modulation of the intensity of laser radiation as well as reduction of noise and nonlinear distortions in a communication channel.

Equalization problems in a digital FM receiver

In some recent papers a new all-digital intermediate frequency unit for the application in commercial broadcast FM receivers was introduced. Serious problems of an efficient hardware solution are the dynamic requirements of the input analog-to-digital converters, which can be solved by narrow-band analog pre-filtering, only. In this paper a digital nonrecursive equalization network is described, which can be used to compensate the non-ideal characteristics of the analog pre-filters. Furthermore, it will be shown that this equalization technique can be applied to multipath echo cancellation, as well. Some examples will be given to illustrate the reduction of nonlinear distortions after demodulation, if digital equalization is carried out.

Superluminescent damping of relaxation resonance in the modulation response of GaAs lasers

It is demonstrated experimentally that the intrinsic modulation response of injection lasers can be modified by reducing mirror reflectivities, which leads to suppression of relaxation oscillation resonance and a reduction of nonlinear distortions up to multi-GHz frequencies. A totally flat response with a 3-dB bandwidth of 5 GHz was obtained using antireflection coated buried heterostructure lasers fabricated on a semi-insulating substrate. Harmonic distortions were below 40 dB within the entire 3-dB bandwidth. These results are in accord with theoretical predictions based on an analysis which include the effects of superluminescence in the laser cavity.

Reduction of modal noise by using reduced spot excitation

The paper describes a computational model to estimate the reduction of nonlinear distortions due to a fiber-fiber joint by properly focusing the optical beam from a single transverse-mode semiconductor laser source to the input end of the first fiber. Mode mixing effects are included in the model. Material dispersion is found to make a negligible contribution in comparison with a typical source coherence time. In a typical situation, where the fiber joint coupling efficiency eta is better than 0.7, all harmonic distortion components produced in the joint could be reduced by ~10 dB by halving the input beam spot radius from the value equal to the fiber core radius.

Predistorsión Digital mediante Señales Enventanadas Tipo Chirp para la Linealización de Amplificadores de Potencia

Digital PreDistortion (DPD) is a well-known method to reduce nonlinear distortion in power amplifiers (PA). In a LTE uplink transmission, the modulation schemes are adaptive. Thus, the DPD function will have to be re-calibrated once the modulation changes. This fact increases the DPD computational cost and the required memory. If the DPD is not re-trained its linearity performance will decrease. This effect can be reduced using a suitable training signal. We propose several novel non-stationary calibration signals based on a windowed-chirp waveform. The envelope of these sequences is bounded by different windows used in signal processing such as Bartlett, Blackman, Hamming or Welch. Bartlett and Blackman windows generate calibration sequences with higher peak to average power ratio values than Welch. Welch envelope creates a signal with a more uniform probability density function (pdf) than the Hamming-window, whose pdf is similar to a Rayleigh distribution. The linearization strategy is based on capturing the described sequences at the PA input and output to extract the predistortion parameters. Once the predistorter functions are computed, we apply them on various LTE-transmissions and perform linearity measurements in terms of the adjacent channel leakage ratio to compare with the standard requirements. In all cases the maximum nonlinear distortion reduction is accomplished with the Chirp-Bartlett sequence (up to 37dBc when transmitting a QPSK-LTE-signal). Thus, good DPD performance is achieved when using the proposed Bartlett-Chirp in the DPD-training-stage. This avoids generating specific DPDs for each modulation scheme, saving computational cost, required memory, and increasing the system efficiency.

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Nonlinear distortion reduction by complementary distortion

Nonlinear distortion produced in a given circuit can be reduced by pre- or postdistorting the signal applied to or from the circuit. Such complementary distortion cannot reduce the original distortion to zero in practice because of distortion of distortion, but it can result in greatly reduced output distortion over a limited amplitude range. General results for the design of pre- or postdistortion circuits are given, and the mathematical results are illustrated by comparing the total harmonic distortions obtained with pre- and postdistortion corrections of increasing complexity applied to a simple nonlinear circuit.

The application of power series to the solution of non-linear circuit problems

It is shown that Carson's power series solution of a triode circuit may be extended to the case in which the triode is associated with a linear network of any form. The reduction of non-linear distortion in an amplifier by negative feedback is discussed. The conditions of stability for a triode circuit are derived, and it is shown that the solution remains convergent even when the circuit is regenerative, failing completely only at the point of critical regeneration. When the circuit is unstable and an e.m.f. near the natural frequency is applied, the solution represents an unstable oscillation, but, when the applied frequency is not near resonance, the solution will represent a stable forced oscillation if the amplitude is sufficiently large.