Two-channel Time-interleaved ADC Research Articles

Direct Complex Envelope Sampling of Bandpass Signals With M-Channel Time-Interleaved ADCs

Classical receiver architectures demodulate a bandpass signal to baseband before sampling the in-phase and quadrature components. With the advent of faster analog-todigital converters (ADCs) and wide bandwidth sample and hold (S/H) circuits, it has become practicable to sample a bandpass signal directly without any demodulation operation and then process it with robust DSP technology. Direct sampling methods do present their own challenges. When a single channel is used to digitize the signal, not all frequencies above the Nyquist rate are allowed. As first shown by Kohlenberg, this restriction can be removed with a two-channel time-interleaved ADC (TIADC) where two ADCs separated by a timing offset independently sample the signal. In this paper, we propose a general and flexible technique for sampling the complex envelope of a bandpass signal using a nonuniformly interleaved <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -channel TIADC. The signal is sampled directly by the sub-ADCs which are separated by timing skews. Reconstruction of the envelope entails inverting a matrix of filters resulting in some timing skews being forbidden. The proposed sampling scheme requires the implementation of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> digital FIR filters and can be used to digitize bandpass signals with any carrier frequency in software defined radio applications. Reconstruction analysis is provided for the case of two, three, and four channels. Multi-tone and MSK signals are used in simulations to validate the proposed method and assess its performance.

All-Digital Bandwidth Mismatch Calibration of TI-ADCs Based on Optimally Induced Minimization

The problem of parameter mismatch in time-interleaved-analog-to-digital converters (TI-ADCs) has become a significant concern to guarantee output linearity. Several solutions have been presented for offset, gain, time skew, and bandwidth mismatches, but they can rely on hardware expensive methods. This article proposes an all-digital calibration algorithm for the TI-ADC bandwidth mismatch, which is capable of detecting the optimal correction coefficients for the derivative-based digital filters. The analyzed convergence logic further relaxes the hardware requirements. Moreover, numerical simulations and experimental results validate the calibration efficiency. A commercial 12-bit 3.6-GS/s two-channel TI-ADC was used to verify the proposed calibration algorithm under real conditions.

Digital Mismatch Correction for Bandpass Sampling Four-Channel Time-Interleaved ADCs in Direct-RF Sampling Receivers

Time-interleaving analog-to-digital converters (ADCs) decrease the required sampling rate for one ADC to achieve giga samples per second (GS/s) rates. The gain and timing mismatches among the ADCs generate aliasing signals, degrading the spurious-free dynamic range of the time-interleaved ADC (TI-ADC). The conventional digital correction methods for TI-ADCs have not considered an application to direct-radio-frequency (RF) sampling receivers. We present a digital correction method for bandpass sampling four-channel TI-ADCs in the receivers. The proposed method, based on a correction method for two-channel TI-ADCs, reduces the in-band aliasing signals of the four-channel TI-ADCs by using in-phase/quadrature-phase (I/Q) downconversion mixers, cascaded integrator-comb (CIC) filters, and automatic gain control (AGC) in the receiver. This allows the correction circuit including mismatch estimation to have fewer building blocks than the conventional methods: seven adders, seven multipliers, and no finite-impulse-response filters. Simulations and measurements show that the proposed method reduces the aliasing signals of 1.2 GS/s 12-bit four-channel TI-ADCs to less than −80 dBFS.

A new adaptive blind background calibration of gain and timing mismatch for a two-channel time-interleaved ADC

This study investigates a new adaptive blind calibration structure for the calibration of gain and timing mismatch error, which is applied for a two-channel Time-Interleaved Analog-to-Digital Converter (TI-ADC). This technique calibrates the mismatch error in the output of the converter without any interrupt at the typical performance of the TI-ADC. The output of the time-interleaved ADC is approximated by modeling based on Taylor's series. The effect of gain and timing mismatch in the output is represented by the first order of Taylor's series. By using a Least-Mean Square (LMS) and correlation-based algorithm, the approximate coefficients of the Taylor's series are identified. Input signal and its chopped image or input signal and its chopped and delayed image will be correlated in the proposed algorithm. This algorithm can identify the coefficients blindly. It is simpler than the previous similar techniques. Also, this technique does not have the drawbacks of a technique which uses the Taylor's series approximation and filtered-X LMS algorithm. Simulation results confirm these claims and show that by entering six sinusoidal inputs in the whole bandwidth of the proposed technique, 48.2 dB improvement in Spur Free Dynamic Range (SFDR) is achieved.

All-Digital Calibration of Timing Mismatch Error in Time-Interleaved Analog-to-Digital Converters

This paper presents an all-digital background calibration for timing mismatch in time-interleaved analog-to-digital converters (TI-ADCs). It combines digital adaptive timing mismatch estimation and digital derivative-based correction, achieving lower hardware cost and better suppression of timing mismatch tones than previous work. In addition, for the first time closed-form exact expressions for the signal-to-noise and distortion ratio (SNDR) of a four-channel TI-ADC with timing mismatch after derivative-based digital correction are obtained, which can be used to guide the design. Simulation results of a four-channel TI-ADC behavioral model and measurement results from a commercial 12-bit 3.6-GS/s two-channel TI-ADC show that the proposed all-digital calibration can accurately estimate the timing skew and effectively correct the timing mismatch errors, while also confirming the analytic SNDR expressions.

Adaptive Background Estimation for Static Nonlinearity Mismatches in Two-Channel TIADCs

Due to channel mismatches in time-interleaved analog-to-digital converters (TIADCs), estimation and compensation methods are required to restore the resolution of the individual converters. Whereas several methods exist for linear mismatches, nonlinearity mismatches have not been widely investigated. This brief presents an adaptive background estimation method for nonlinearity mismatches in two-channel TIADCs. It utilizes a normalized least-mean-square algorithm and assumes slight oversampling as well as a polynomial nonlinearity model that is appropriate when smooth errors dominate. Furthermore, two implementation strategies are proposed to enhance its ability for different applications. The estimation performance of the proposed method is evaluated through behavioral-level simulations.

Joint Blind Calibration for Mixed Mismatches in Two-Channel Time-Interleaved ADCs

To further enhance the dynamic performance of time-interleaved analog-to-digital converters (TI-ADCs), both linear and nonlinear mismatches should be estimated and compensated for. This paper introduces a method for joint calibration of several types of linear and nonlinear mismatch errors in two-channel TI-ADCs. To demonstrate the generality of this method, we take different scenarios into account, including static and dynamic mixed mismatch models. The proposed method utilizes a normalized least-mean square (N-LMS) algorithm as well as a certain low degree of oversampling for the overall converter to estimate and compensate for the mixed mismatch errors. The calibration performance and computational complexity are investigated and evaluated through simulations.

Relations Between Zero-IF Receiver I/Q and TI-ADC Channel Mismatches

This paper shows that the in-phase and quadrature (I/Q) channel mismatch problem for complex signals in zero-IF receivers (ZIFRs) is related to the real-signal channel-mismatch problem in two-channel time-interleaved analog-to-digital converters (TI-ADCs). The problems are related in that, given one of the problems, it can be converted to the other problem via relatively simple signal processing operations. This offers more options for the estimation of and compensation for I/Q and TI-ADC channel mismatches. In particular, if there is a need to perform both I/Q and TI-ADC channel mismatch correction, one can make use of the same basic estimation and/or compensation principles for both applications which may save computational resources. The use of TI-ADC mismatch estimation and/or compensation also offers real-signal processing techniques to the complex-signal mismatch estimation and compensation problem in ZIFRs.

Adaptive Blind Background Calibration of Polynomial-Represented Frequency Response Mismatches in a Two-Channel Time-Interleaved ADC

This paper introduces an adaptive calibration structure for the blind calibration of frequency response mismatches in a two-channel time-interleaved analog-to-digital converter (TI-ADC). By representing frequency response mismatches as polynomials, we can exploit slight oversampling to estimate the coefficients of the polynomials by using the filtered-X least-mean square (FxLMS) algorithm. Utilizing the coefficients in an adaptive structure, we can compensate frequency response mismatches including time offset and bandwidth mismatches. We develop an analytical framework for the calibration structure and analyze its performance. We show the efficiency of the calibration structure by simulations, where we include examples from the literature.

Adaptive blind calibration of timing offset and gain mismatch for two-channel time-interleaved ADCs

In this paper, we describe a blind calibration method for gain and timing mismatches in a two-channel time-interleaved low-pass analog-to-digital converters (ADC). The method requires that the input signal should be slightly oversampled. This ensures that there exists a frequency band around the zero frequency where the Fourier transforms of the ADC subchannels are alias free. Low-pass filtering the ADC subchannels to this alias-free band reduces the blind calibration problem to a conventional gain and time delay estimation problem for an unknown signal in noise. An adaptive filtering structure with three fixed FIR filters and two adaptive gain and delay parameters is employed to achieve the calibration. A convergence analysis is presented for the blind calibration technique. Numerical simulations for a bandlimited white noise input and for inputs containing several sinusoidal components demonstrate the effectiveness of the proposed method