A review on low-complexity structures and algorithms for the correction of mismatch errors in time-interleaved ADCs

Abstract—In this paper we review the progress in the design of low-complexity digital correction structures and algorithms for time-interleaved ADCs over the last five years. We devise a discrete-time model, state the design problem, and finally derive the algorithms and structures. In particular, we discuss efficient algorithms to design time-varying correction filters as well as iterative structures utilizing polynomial based filters. Finally, we give an outlook to future research questions. I

Time-Interleaved Analog-to-Digital-Converters: Modeling, Blind Identification and Digital Correction of Frequency Response Mismatches

Analog-to-digital-conversion enables utilization of digital signal processing (DSP) in many applications today such as wireless communication, radar and electronic warfare. DSP is the favored choice for processing information over analog signal processing (ASP) because it can typically offer more flexibility, computational power, reproducibility, speed and accuracy when processing and extracting information. Software defined radio (SDR) receiver is one clear example of this, where radio frequency waveforms are converted into digital form as close to the antenna as possible and all the processing of the information contained in the received signal is extracted in a configurable manner using DSP. In order to achieve such goals, the information collected from the real world signals, which are commonly analog in their nature, must be converted into digital form before it can be processed using DSP in the respective systems. The common trend in these systems is to not only process ever larger bandwidths of data but also to process data in digital format at ever higher processing speeds with sufficient conversion accuracy. So the analog-to-digital-converter (ADC), which converts real world analog waveforms into digital form, is one of the most important cornerstones in these systems.The ADC must perform data conversion at higher and higher rates and digitize ever-increasing bandwidths of data. In accordance with the Nyquist-Shannon theorem, the conversion rate of the ADC must be suffcient to accomodate the BW of the signal to be digitized, in order to avoid aliasing. The conversion rate of the ADC can in general be increased by using parallel ADCs with each ADC performing the sampling at mutually different points in time. Interleaving the outputs of each of the individual ADCs provides then a higher digitization output rate. Such ADCs are referred to as TI-ADC. However, the mismatches between the ADCs cause unwanted spurious artifacts in the TI-ADC’s spectrum, ultimately leading to a loss in accuracy in the TI-ADC compared to the individual ADCs. Therefore, the removal or correction of these unwanted spurious artifacts is essential in having a high performance TI-ADC system.In order to remove the unwanted interleaving artifacts, a model that describes the behavior of the spurious distortion products is of the utmost importance as it can then facilitate the development of efficient digital post-processing schemes. One major contribution of this thesis consists of the novel and comprehensive modeling of the spurious interleaving mismatches in different TI-ADC scenarios. This novel and comprehensive modeling is then utilized in developing digital estimation and correction methods to remove the mismatch induced spurious artifacts in the TI-ADC’s spectrum and recovering its lost accuracy. Novel and first of its kind digital estimation and correction methods are developed and tested to suppress the frequency dependent mismatch spurs found in the TI-ADCs. The developed methods, in terms of the estimation of the unknown mismatches, build on statistical I/Q signal processing principles, applicable without specifically tailored calibration signals or waveforms. Techniques to increase the analog BW of the ADC are also analyzed and novel solutions are presented. The interesting combination of utilizing I/Q downconversion in conjunction with TI-ADC is examined, which not only extends the TI-ADC’s analog BW but also provides flexibility in accessing the radio spectrum. Unwanted spurious components created during the ADC’s bandwidth extension process are also analyzed and digital correction methods are developed to remove these spurs from the spectrum. The developed correction techniques for the removal of the undesired interleaving mismatch artifacts are validated and tested using various HW platforms, with up to 1 GHz instantaneous bandwidth. Comprehensive test scenarios are created using measurement data obtained from HW platforms, which are used to test and evaluate the performance of the developed interleaving mismatch estimation and correction schemes, evidencing excellent performance in all studied scenarios. The findings and results presented in this thesis contribute towards increasing the analog BW and conversion rate of ADC systems without losing conversion accuracy. Overall, these developments pave the way towards fulfilling the ever growing demands on the ADCs in terms of higher conversion BW, accuracy and speed

Research of algorithms for the reconstruction of non-uniform sampled discrete-time signals with unknown sampling locations

В монографии изложены результаты исследований алгоритмов восстановления дискретных сигналов (ДС), заданных в узлах временной сетки (ВС) с точно неизвестными значениями координат ее узлов. Проведен анализ состояния предметной области, включая: существующие виды неравномерной дискретизации сигналов по времени и причины ее возникновения. Предложена классификация видов неравномерной дискретизации сигналов во времени. Приведены примеры реальных измерительных систем, на выходе которых получают ДС, заданные в узлах неравномерной временной сетки (НВС), а также методы восстановления ДС данного типа. Приведена постановка задачи восстановления ДС, заданного в узлах НВС, с неизвестными точно значениями координат ее узлов. Изложены результаты исследования особенностей восстановления ДС, заданных в узлах НВС с неизвестными точно значениями координат ее узлов, с помощью интерполяционных методов, а также методов, основанных на уточнении значений координат узлов НВС, и оценки их точности. Предложены новые алгоритмы восстановления ДС, заданных на НВС с точно неизвестными значениями координат ее узлов, продемонстрирована их работоспособность и получены оценки точности восстановления ДС с помощью данных алгоритмов. Проведен анализ точности восстановления периодических ДС, получаемых на выходе реальных цифровых систем (высокоскоростного 8-ми битного АЦП на основе КМОП-технологии 0.18 мкм, системы, состоящей из 8 параллельных 5-ти битных АЦП на основе КМОП-тепхнологии 65 нм), с помощью разработанных алгоритмов восстановления.In this treatise are considered the results of research of algorithms for the reconstruction of non-uniform sampled band-limited discrete-time signal with unknown sampling location. The condition of problem domain is analyzed, including: applying signal sampling schemes and causes of origin of the non-uniform sampling signals. Also is proposed classification of signal sampling schemes. Examples of real measurement systems, on the output of which registers non-uniform sampled signals with unknown sampling location as well as methods to reconstruct this type signals are given. The problem of signal reconstruction from non-uniform samples is considered. Interpolation methods to reconstruct a band-limited discrete-time signal from non-uniform samples with unknown sampling location, and optimization methods by estimating unknown sampling locations values are analyzed. Estimates of accuracy of interpolation methods for recovery of irregularly sampled signals are given. Also are given estimates of accuracy of methods to reconstruct a signal from non-uniform samples with unknown sampling locations by estimating unknown sampling locations values and presenting a solution of certain multiparameter problem of global optimization. Are proposed the new algorithms to reconstruct band-limited discrete-time signal from non-uniform samples with unknown sampling location and is demonstrated their performance. Estimate of accuracy of algorithms is compared and reconstruction errors are given in the numerical examples. The results of the research on the accuracy of the periodic signals reconstruction registered at the high speed 8 parallel 5-bit ADC system output based on the 65 nm CMOS technology and 8-bit high-rate ADC output based on the 0.18µm CMOS technology by means of special algorithms to reconstruct a signal from non-uniform samples with unknown sampling locations are analyzed