Anu Kalidas Muralidharan Pillai

Prefilter-Based Reconfigurable Reconstructor for Time-Interleaved ADCs With Missing Samples

This brief proposes a reconstruction scheme for the compensation of frequency-response mismatch errors at the output of a time-interleaved analog-to-digital converter (TI-ADC) with missing samples. The missing samples are due to sampling instants reserved for estimating the channel mismatch errors in the TI-ADC. Compared with previous solutions, the proposed scheme offers substantially lower computational complexity.

Efficient signal reconstruction scheme for time-interleaved ADCs

Time-interleaved analog-to-digital converters (ADCs) exhibit offset, gain, and time-skew errors due to channel mismatches. The time skews give rise to a nonuniformly sampled signal instead of the desired uniformly sampled signal. This introduces the need for a digital signal reconstructor that takes the “nonuniform samples” and generates the “uniform samples”. In the general case, the time skews are frequency dependent, in which case a generalization of nonuniform sampling applies. When the bandwidth of a digital reconstructor approaches the whole Nyquist band, the computational complexity may become prohibitive. This paper introduces a new scheme with reduced complexity. The idea stems from recent multirate-based efficient realizations of linear and time-invariant system . However, a time-interleaved ADC (without correction) is a time-varying system which means that these multirate-based techniques cannot be used straightforwardly but need to be appropriately analyzed and extended for this context.

A sub-band based reconstructor for M-channel time-interleaved ADCS with missing samples

This paper proposes a scheme for the recovery of a uniformly sampled sequence from the output of a time-interleaved analog-to-digital converter (TI-ADC) with static time-skew errors and missing samples. Nonuniform sampling occurs due to timing mismatches between the individual channel ADCs and also due to missing input samples as some of the sampling instants are reserved for estimating the mismatches in the TI-ADC. In addition to using a non-recursive structure, the proposed reconstruction scheme supports online reconfigurability and reduces the computational complexity of the reconstructor as compared to a previous solution.

Efficient reconfigurable scheme for the recovery of sub-Nyquist sampled sparse multi-band signals

Sub-Nyquist sampling makes use of sparsities in analog signals to sample them at a rate lower than the Nyquist rate. The reduction in sampling rate, however, comes at the cost of additional digital signal processing (DSP) which is required to reconstruct the uniformly sampled sequence at the output of the sub-Nyquist sampling analog-to-digital converter. At present, this additional processing is computationally intensive and time consuming and offsets the gains obtained from the reduced sampling rate. This paper focuses on sparse multi-band signals where the user band locations can change from time to time and the reconstructor requires real-time redesign. We propose a technique that can reduce the computational complexity of the reconstructor. At the same time, the proposed scheme simplifies the online reconfigurability of the reconstructor.

Time-skew error correction in two-channel time-interleaved ADCs based on a two-rate approach and polynomial impulse responses

Even though time-interleaved analog-to-digital converters (ADCs) help to achieve higher bandwidth with simpler individual ADCs, gain, offset, and time-skew mismatch between the channels degrade the achievable resolution. Of particular interest is the time-skew error between channels which results in nonuniform samples and thereby introducing distortion tones at the output of the time-interleaved ADC. Time-varying digital reconstructors can be used to correct the time-skew errors between the channels in a time-interleaved ADC. However, the complexity of such reconstructors increases as their bandwidth approaches the Nyquist band. In addition to this, the reconstructor needs to be redesigned online every time the time-skew error varies. Design methods that result in minimum reconstructor order require expensive online redesign while those methods that simplify online redesign result in higher reconstructor complexity. This paper proposes a technique that can be used to simplify the online redesign and achieve a low complexity reconstructor at the same time.

Efficient Recovery of Sub-Nyquist Sampled Sparse Multi-Band Signals Using Reconfigurable Multi-Channel Analysis and Modulated Synthesis Filter Banks

Sub-Nyquist cyclic nonuniform sampling (CNUS) of a sparse multi-band signal generates a nonuniformly sampled signal. Assuming that the corresponding uniformly sampled signal satisfies the Nyquist sampling criterion, the sequence obtained via CNUS can be passed through a reconstructor to recover the missing uniform-grid samples. At present, these reconstructors have very high design and implementation complexity that offsets the gains obtained due to sub-Nyquist sampling. In this paper, we propose a scheme that reduces the design and implementation complexity of the reconstructor. In contrast to the existing reconstructors which use only a multi-channel synthesis filter bank (FB), the proposed reconstructor utilizes both analysis and synthesis FBs which makes it feasitble to achieve an order-of-magnitude reduction of the complexity. The analysis filters are implemented using polyphase networks whose branches are allpass filters with distinct fractional delays and phase shifts. In order to reduce both the design and the implementation complexity of the synthesis FB, the synthesis filters are implemented using a cosine-modulated FB. In addition to the reduced complexity of the reconstructor, the proposed multi-channel recovery scheme also supports online reconfigurability which is required in flexible (multi-mode) systems where the user subband locations vary with time.

Two reconstructors for m-channel time-interleaved ADCs with missing samples

In this paper, we explore two nonrecursive reconstructors which recover the uniform-grid samples from the output of a time-interleaved analog-to-digital converter (TI-ADC) that uses some of the sampling instants for estimating the mismatches in the TI-ADC. Nonuniform sampling occurs due to timing mismatches between the individual channel ADCs and also due to missing input samples. Compared to a previous solution, the reconstructors presented here offer substantially lower computational complexity.

Low-complexity two-rate based multivariate impulse response reconstructor for time-skew error correction in m-channel time-interleaved ADCs

Nonuniform sampling occurs in time-interleaved analog-to-digital converters (TI-ADC) due to timing mismatches between the individual channel analog-to-digital converters (ADCs). Such nonuniformly sampled output will degrade the achievable resolution in a TI-ADC. To restore the degraded performance, digital time-varying reconstructors can be used at the output of the TI-ADC, which in principle, converts the nonuniformly sampled output sequence to a uniformly sampled output. As the bandwidth of these reconstructors increases, their complexity also increases rapidly. Also, since the timing errors change occasionally, it is important to have a reconstructor architecture that requires fewer coefficient updates when the value of the timing error changes. Multivariate polynomial impulse response reconstructor is an attractive option for an M-channel reconstructor. If the channel timing error varies within a certain limit, these reconstructors do not need any online redesign of their impulse response coefficients. This paper proposes a technique that can be applied to multivariate polynomial impulse response reconstructors in order to further reduce the number of fixed-coefficient multipliers, and thereby reduce the implementation complexity.

Lower bounds on the L2-norms of digital resampling filters with zero-valued input samples

This paper derives lower bounds on the L2-norms of digital resampling filters with zero-valued input samples. This emanates from uniform-grid sampling but where some of the samples are missing. One application is found in time-interleaved analog-to-digital converters with missing samples due to calibration at certain time instances. The square of the L2-norms correspond to scaling of the round-off noise that in practice is always present at the input of the resampling filter. As will be shown through the derived bounds, the L2-norm of the corresponding filter that recovers the missing samples is generally much larger than unity. Consequently, the noise variance is generally much larger for the recovered samples than for the other samples obtained in the sampling process. Based on this observation, the paper also proposes an alternative resampling scheme for which the maximum of all L2-norms in the resampling is reduced.

A study on switched-capacitor blocks for reconfigurable ADCs

Pipelined analog-to-digital converters (ADCs) achieve low to moderate resolutions at high bandwidths while sigma-delta (ΣΔ) ADCs provide high resolution at moderate bandwidths. A switched-capacitor (SC) block which can function as an integrator or an MDAC can be used to implement a reconfigurable ADC (R-ADC) which supports both these types of architectures. Through the use of high level models this work attempts to derive the capacitance and critical opamp parameters such as DC gain and bandwidth of the SC blocks in a reconfigurable ADC. Scaling of capacitance afforded by the noise shaping property of ΣΔ loops as well as the inter-stage gain of pipelined ADCs is used to minimize the total capacitance. This work can be used as reference material to understand some of the design trade-offs in R-ADCs.