Marina Topa

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.

Echo cancelling using adaptive algorithms

In the paper, an echo canceller based on a system identification scheme with adaptive algorithms is presented. The algorithms consider FIR filters with the taps chosen to minimize an error signal derived from the system according to a stochastic gradient-based method. The following adaptive filters are discussed and simulated in Matlab: classical LMS and its variants: NLMS, VSLMS, VSNLMS and RLS. The echo was simulated by adding several delayed and attenuated replica to a vocal input. The comparison between the algorithms was made taking into account the mean-square error (MSE) and average attenuation (AV), as well as the required number of multiplications.

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.

HDL implementation of the variable step size N-LMS adaptive algorithm

This paper proposes a Verilog implementation of the normalized least mean square (NLMS) adaptive algorithm, having a variable step size. The envisaged application is the identification of an unknown system. First the convergence of derived LMS algorithms was analyzed in a Simulink application. The NLMS was chosen for its good convergence and stability. Next a HDL implementation was developed for a 4th order NLMS adaptive filter. ModelSim simulations results altogether with plots obtained in Matlab prove the good behavior of the NLMS algorithm, making it an important competitor for the conventional LMS.

Analysis of Lossless Differential Microstrip Line

In this paper several aspects regarding the analysis of lossless differential microstrip lines are presented. Starting from the physical structure of a coupled microstrip transmission line, the per-unit-length parameters were computed. Then a PSPICE model for a three-conductor transmission line was developed and tested in time and frequency domain. For the exact analysis a system of partial differential equations was set up and a general solution was given.

Adaptive algorithms for double-talk echo cancelling

In the paper, an echo canceller based on a system identification scheme with adaptive algorithms is presented. The algorithms consider FIR filters with the taps chosen to minimize an error signal derived from the system according to a stochastic gradient-based method. The following adaptive filters are discussed and simulated in Matlab: classical LMS and its variants: NLMS, VSLMS, VSNLMS and RLS. The echo was simulated by adding several delayed and attenuated replica to a vocal input. The comparison between the algorithms was made taking into account the mean-square error (MSE) and average attenuation (AV), as well as the required number of multiplications.

Comparative Performance Analysis of Artificial Reverberation Algorithms

The paper considers two efficient improved artificial reverberation algorithms built with an early Schroeders and a late reverberator: modified Schroeders or an absorbent all-pass. Both of them simulate the behavior of a real concert hall by taking into account the variation with frequency of the reverberation time. Several reverberation algorithms can be found in the literature, but very few design and performance analysis hints are given. Our paper includes a detailed design of the reverberators, simulations are run for each algorithm, acoustic parameters are computed and an objective performance analysis is carried out

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.