Debesh Bhatta

Low-cost wideband periodic signal reconstruction using incoherent undersampling and back-end cost optimization

Acquisition of wide bandwidth signals is a significant problem in manufacturing test due to the cost of test equipment driven by the use of high-speed sample and hold circuitry and difficulty in data-clock synchronization. We propose to combine frequency interleaved down conversion (to overcome the bandwidth limitations of sample and hold circuitry) with incoherent undersampling (to overcome data-clock synchronization and ADC speed issues) to design a low cost instrumentation for high speed signal capture. A novel signal reconstruction algorithm is developed along with a method for calibrating the effects of unknown delays in data acquisition hardware due to mismatch in signal path lengths on the reconstructed signal. Simulation results and preliminary hardware validation prove the feasibility of the proposed technique.

Periodic jitter and bounded uncorrelated jitter decomposition using incoherent undersampling

Jitter measurement is an essential part for testing high speed digital I/O and clock distribution networks. Precise jitter characterization of signals at critical internal nodes provides valuable information for hardware fault diagnosis and next generation design. Recently, incoherent undersampling has been proposed as a low-cost solution for signal integrity characterization at high data rate. Incoherent undersampling drastically reduces the sampling rate compared to Nyquist rate sampling without relying on the availability of a data synchronous clock. In this paper, we propose a jitter decomposition and characterization method based on incoherent undersampling. Associated fundamental period estimation techniques along with properties of incoherent undersampling, are used to isolate the effects of periodic and periodic crosstalk jitter. Mathematical analysis and hardware experiments using commercial off-the-shelf components are performed to prove the viability of the proposed method.

Time Domain Characterization and Test of High Speed Signals Using Incoherent Sub-sampling

High speed signal acquisition and characterization contributes a significant amount to the total test cost of the finished product in modern high speed systems. Incoherent under-sampling allows robust and low cost signal acquisition without requiring a prior accurate knowledge of signal period. In this paper we propose a frequency estimation and signal reconstruction technique for incoherently sub-sampled periodic waveforms that is based on a time domain cost function. The method reduces the per iteration cost by a factor of log N compared to frequency domain cost functions. The proposed method estimates the period of a test signal with much fewer samples without degradation of accuracy.

Framework for analog test coverage

Measurement of the quality of tests run during high volume manufacturing of microprocessors is important to ensure desired outgoing product quality. For digital logic on die, such measurement is performed using techniques such as fast event-driven fault simulation using mature fault models such as stuck-at and transition faults. For analog modules on die, such test quality measurement is not performed in practice due to lack of (a) mature fault models to describe analog failures, and (b) automated, efficient and accurate fault simulation methods. This work is a first step towards our objective of establishing a practical methodology to measure analog test quality. We show promising results of a semi-automated fault simulation approach on analog modules of a high speed serial IO receiver that compares (a) two manufacturing tests in terms of their defect detection capability as measured by their fault coverages for gross and parametric faults, and, (b) the accuracy and performance of using models versus schematics for fault effect propagation.

An adaptive broadband BiCMOS active spur canceller

An active interference cancellation system is presented in BiCMOS 8HP capable of eliminating undesired signals resulting from an on-chip clock whose fundamental and/or harmonics couple into a receiver path. The cancellation system is capable of eliminating undesired clock fundamentals from 6 – 10 GHz and its harmonics up to 30 GHz. By modeling the coupling channel, the system recovers receiver sensitivity and is independent of the aggressing clock rate and amplitude fluctuations. A variable phase rotator, comb generator, and an amplitude modulator are used to achieve the cancellation. The use of a comb generator in a cancellation system is novel and enables harmonic cancellation. Over 25 dB of cancellation for an aggressing clock and its harmonics have been achieved. A series-to-parallel bus that communicates with all system sub-blocks provides a healing aspect to the system. The total power consumption of the canceller system is 71 mW.

Low Cost Sparse Multiband Signal Characterization Using Asynchronous Multi-Rate Sampling: Algorithms and Hardware

Characterizing the spectrum of sparse wideband signals of high-speed devices efficiently and precisely is critical in high-speed test instrumentation design. Recently proposed sub-Nyquist rate sampling systems have the potential to significantly reduce the cost and complexity of sparse spectrum characterization; however, due to imperfections and variations in hardware design, numerous implementation and calibration issues have risen and need to be solved for robust and stable signal acquisition. In this paper, we propose a low-cost and low-complexity hardware architecture and associated asynchronous multi-rate sub-Nyquist rate sampling based algorithms for sparse spectrum characterization. The proposed scheme can be implemented with a single ADC or with multiple ADCs as in multi-channel or band-interleaved sensing architectures. Compared to other sub-Nyquist rate sampling methods, the proposed hardware scheme can achieve wideband sparse spectrum characterization with minimum cost and calibration effort. A hardware prototype built using off-the-shelf components is used to demonstrate the feasibility of the proposed approach.

Performance Analysis of Balanced and Unbalanced Feed-Forward Equalizer Structures for Multi-Gigabit Applications in 0.18μm CMOS Process

In this paper, a comparative study of two different structures for the Feed Forward Equalizer (FFE) is presented to emphasize the effect of structural differences on the performance of the passive delay line based FFEs with large number of taps. Both FFEs are designed for compensation of Inter Symbol Interference (ISI) in multi-Gb/s data link. The two test structures use the same building blocks but differ in the implementation. Both of them have nine taps with passive delay cells and are designed in 0.18 mum CMOS technology.

Incoherent Undersampling-Based Waveform Reconstruction Using a Time-Domain Zero-Crossing Metric

Incoherent undersampling-based waveform acquisition provides a low-cost test setup for characterizing high-speed systems. A periodic waveform reconstruction using incoherent undersampling remaps time indices of samples using the modulus of the suspected period of the signal, effectively folding the signal into a time window equal to one period. The major cost and accuracy limitations of the reconstruction technique arise from estimation of the waveform period. Multiple cost functions have been proposed to estimate the period, including frequency domain metrics, which are computationally intensive. In this paper, we propose a new time domain zero-crossing (ZC)-based metric, where the metric gives the number of ZC in the reconstructed waveform for an assumed period of the waveform. The reconstruction technique is also extended to beyond the track-and-hold amplifier bandwidth using a novel test setup combining the incoherent undersampling with multichannel bandwidth interleaving.

Time Domain Reconstruction of Incoherently Undersampled Periodic Waveforms Using Bandwidth Interleaving

Incoherent undersampling provides a low cost solution for wideband periodic waveform acquisition without the requirement for synchronization with the source clock. The bandwidth of a traditional incoherent undersampling based test setup is limited by the the bandwidth of the track and hold amplifier. In this work, a test setup is proposed combining incoherent undersampling and bandwidth interleaving to break the bandwidth barrier of the track and hold amplifier. The high frequency components of the signal waveform beyond the track and hold bandwidth are down converted using mixers and undersampled. While bandwidth interleaved frequency domain signal reconstruction techniques have been proposed before, this is the first time that fast time domain reconstruction techniques are used for periodic waveform reconstruction of wideband signals in the absence of any synchronization between the test signal and the tester oscillator/sampling clock over multiple frequency bands by choosing the local oscillator frequency to be a multiple of the sampling frequency. The periodic test signal waveform is acquired over multiple channels each covering only part of the total bandwidth of the signal. Feasibility of the proposed technique is shown through simulation and hardware results.

Spectral Estimation Based Acquisition of Incoherently Under-sampled Periodic Signals: Application to Bandwidth Interleaving

Acquisition of periodic waveforms is an integral part of characterizing high speed system performance. Various techniques are used to reduce the equipment cost, dominated by the cost of the digitizer. Incoherent under-sampling provides an attractive solution for signal acquisition. However, due to the aliasing present in under-sampled signal and the windowing effects, high resolution spectral estimation is difficult. This is especially true when under-sampling is combined with techniques such as bandwidth interleaving to extend the bandwidth of the test setup beyond the track and hold bandwidth. In this paper we propose a high resolution spectral estimation technique and propose a new setup that combines incoherent under-sampling with the bandwidth interleaving without requiring a synchronization with the signal clock. This enables waveform acquisition with bandwidth greater than the sampling track-and-hold bandwidth.