Marius Neag

Compact Variable Gain Amplifier for a Multistandard WLAN/WiMAX/LTE Receiver

This paper presents a novel single-stage VGA architecture that employs two Gm cells, a voltage-controlled current attenuator, resistors and capacitors. The gain can be changed in three large steps by using digital controls, and continuously within these steps. The VGA bandwidth and output-related IP3 and 1dBCP are independent of the gain setting; the bandwidth can be programmed through a digitally-controlled capacitor array placed at its output. The proposed architecture was employed to realize the VGA for a WLAN/WiMAX/LTE radio receiver. Die area and power consumption were reduced by implementing the two Gm cells with one instantiation of a high-linearity Gm-core and scaled outputs; also, the current attenuator was implemented with a simple differential current steering circuit; finally, the load resistors were also used to sense the output common-mode level. The VGA was fabricated in 0.15 um standard CMOS process. Measurement results show the gain varying between -5 dB to 30 dB and the max bandwidth surpasses 60 MHz; 11.14 nV/√{Hz} input referred noise; O1dBCP of 8.6 dBm while taking 4.2 mA from a 1.8 V supply; it settles within 20 ns after a min-max step-change of the gain; it occupies 0.05 mm2.

Blind Frequency-Selective I/Q Mismatch Compensation Using Subband Processing

This brief proposes a new approach to frequency-selective in-phase (I)/quadrature (Q) mismatch compensation, based on subband processing: The spectrum of the signal received by the digital baseband processor-a combination of wanted and image signals-is split into subbands so that, within each subband, the I/Q mismatch can be considered constant with respect to frequency. Therefore, standard compensation algorithms, devised for frequency-invariant I/Q mismatch, can be applied to compensate the signal within each subband; finally, the outputs of each subband processing path are recombined to yield the wanted signal. This procedure is demonstrated by a case study that describes an I/Q mismatch compensation algorithm particularly well suited for the proposed method and proves that, at least for certain I/Q imbalances, the efficiency of compensation can be significantly improved even if only a small number of subbands are employed. The resulting frequency-selective I/Q mismatch compensation solution is suitable for wideband multistandard receivers.

Comparison of different experimental methods for the assessment of the room’s acoustics

The paper presents the acoustics analysis of three different enclosed spaces. These spaces (rooms) have different geometrical shapes and sizes and serve for different purposes. The early decay time, reverberation time, clarity and center time are evaluated with Dirac, WinMLS, Aurora and Caracad software using simple, low-cost equipment. The listed acoustic parameters were determined using linear sine sweep and impulsive sources. Comparisons between experimental measurements, simulations and analytic results were done. The room impulse response measurement proved to be the most reliable method to evaluate the properties of different rooms even if the measurements are perturbed by non-idealities of the low-cost equipment.

Application-oriented robustness optimization based on metamodels

Real-life systems should perform within tight limits even when affected by uncontrollable factors such as temperature and power supply variations. This paper outlines a methodology for robustness optimization based on the Design of Experiments and metamodelling concepts. The main idea is to derive metamodels of the mean response and its dispersion depending only on control factors; thus, the optimization can tackle more effectively the tasks of bringing the mean response to within a set range and minimize the response statistical dispersion. The proposed method was used to optimize the sizing of the external circuitry of a low dropout voltage regulator used in automotive applications; several parameters were improved, especially w.r.t. reducing their dispersion with temperature by up to 28.5%. The method makes no assumptions on the uncontrollable factors and can work with a large variety of metamodels employed; also, it can be easily extended to multiple-response optimization.

Matlab modeling and analysis of the signal path in Zero-IF DVB-T/H radio receivers

This paper presents a Matlab model for the signal path within a DVB-T receiver, along with a new method for analyzing the effects various block nonidealities have on the receiver, when considering unwanted signals such as the adjacent channels and out-of-band blockers. The models for amplifier-type blocks cover not only the basic parameters - gain, bandwidth - but also the noise - though the noise factor - and the nonlinearity - through the compression and intercept points. Additional features are provided for blocks with more specific functions: gain imbalance and quadrature error are added in for the mixer model, frequency characteristics for channel filters models, quantization error for the analog-to-digital converter. The model also comprises an OFDM signal generator and the DSP block. The proposed analysis method is based on a direct, sample-by-sample, comparison between a reference signal - obtained by using a reference version of the receiver, for which some aspects are idealized - and the “real” signal, obtained by using the complete receiver model and considering all signals, wanted and unwanted.

Sub-band adaptive filtering for acoustic echo cancellation

Sub-band filtering is employed in various system identification applications. It reduces the computational complexity and adaptation time. The paper focuses on sub-band filtering for acoustic echo cancellation. A Simulink model for 2, 4, 8 sub-bands was created and simulated. The echo is created by applying a vocal test to a first Schroeder reverberator. The performance of the full- and sub-band echo canceller is measured in terms of error signal and echo-return loss enhancement.

Computer-aided network function approximation for multicriteria filter design

The paper presents the steps in computer-aided design of analog low and high-pass filters. The design starts with a classical filter, and then is multicriteria optimized to meet the behavioral and implementation requirements. The computer-aided tool uses Mathematica for symbolic computation of the objective function and its gradient, MATLAB for the optimization process.

A unitary analysis of voltage mode and current feedback op-amp circuits

This paper presents a unitary analysis method for voltage mode (VM) and transimpedance-current feedback (CFB) op-amp circuits. This methods uses a unified model for both op-amps; thus it permits a direct comparison for the circuit parameters. The point of the presented method is the separation of the signal graph of the analyzed circuit in two different paths: one that controls the gain and another that determines the circuit bandwidth. Thus one obtains a direct sight on the circuit elements that control this parameters. We extend the analysis method on differential current feedback (DCFB) op-amps circuits. Symbols for CFB and DCFB were introduced to simplify the circuit descriptions. As an application we analyze some high precision and large bandwidth amplifiers.

IIR filter synthesis based on Real-Coded Genetic Algorithm for human audiogram emulation

This paper presents a method for optimized synthesis of IIR filters with user-defined magnitude response, based on a real-coded genetic algorithm. The application envisaged in this paper is the design of filters for hearing aids, which have to match given human audiograms. Two metrics for the differences between the synthesized and wanted magnitude characteristics are minimized by the optimization algorithm: the peak magnitude error and the total squared error. A design example is shown, with the IIR filter yielded by the proposed method for a particular audiogram compared against the FIR filter synthesized by using a previously reported method, for the same audiogram.

Comparative Analysis of Simulation-Based Methods for Deriving the Phase- and Gain-Margins of Feedback Circuits With Op-Amps

Ten methods for finding through simulations the small-signal phase and gain margins of feedback circuits based on op-amps are described and analyzed in this mostly tutorial paper. The testbenches employed by these methods are presented and the corresponding analytical expressions of the return ratio are derived and compared against their “ideal” counterpart, obtained with standard circuit analysis; the requirement that the return ratio should not depend on the point it was measured at is also verified. These analyses are performed on a fairly general case: a generic reciprocal two-port network that closes a feedback loop around an op-amp acting as the forward amplifier. The four main types of op-amps were considered. The limitations of some of the tested methods are then highlighted by simulations. Besides the detailed analysis of previously reported methods, the paper proposes a novel method for deriving the return ratio of feedback circuits, that employs only current stimuli; it is demonstrated analytically that this method can be used for bilateral circuits, not only for op-amp-based (unilateral) ones. Also, a recent method for deriving directly the phase margin of a circuit is extended to estimating the gain margin, too. Conclusions on the accuracy and suitability of the analyzed methods for practical circuit cases are drawn. These results are then extended to other circuit topologies.