Accurate spectral testing without accurate instrumentation

Analog-to-digital converters (ADCs) are becoming increasingly common in many systems in integrated circuits. Spectral testing is widely used to test the dynamic linearity performance of ADCs and waveform generators. With improvements in the performance of ADCs, it is becoming an expensive and challenging task to perform spectral testing using standard methods because of the requirement that the test instrumentation environment must satisfy several stringent conditions. In order to address these challenges and to decrease the test cost, in this dissertation, four new algorithms are proposed to perform accurate spectral testing of ADCs by relaxing three conditions required for standard spectral testing methods. The first method developed is relaxing the requirements on precise control of coherent sampling and input signal amplitude. The efficiency and accuracy of this method is similar to the straightforward FFT, but it can simultaneously handle amplitude clipping and noncoherent sampling. By replacing a noncoherent and clipped fundamental with a coherent and unclipped one, correct spectral specifications can be obtained. Both simulation and measurement results validated the proposed method. The second algorithm can simultaneously perform the linearity test and the spectral test with only one-time data acquisition. Targeted for realizing the cotest of linearity and spectral performance under noncoherent sampling and amplitude clipping, a new accurate method for identifying the noncoherent and clipped fundamental is introduced. The residue after removing the identified fundamental from raw data is used to obtain the linearity and spectral characterizations. Simulation and measurement results against the standard test methods collaborate to validate the accuracy and robustness of the new solution. The third method proposes an efficient and accurate jitter estimation method based on one frequency measurement. Applying a simple mathematical processing to the ADC output in time domain, the RMS of jitter and noise power are obtained. Furthermore, prior information of harmonics need not be known before the processing. The algorithm is robust enough that nonharmonic spurs do not affect the estimation result. Using the proposed algorithm, specifications of the ADC under test can be obtained without the jitter effect. Simulation results of ADCs with different resolutions show the functionality and accuracy of the method. The last method is developed to accurately estimate the SNR with sampling clock jitter. This method does not require a precise sampling clock and thus reduces the test cost. The ADC output sequence is separated into two segments. By analyzing the difference of the two segments, the RMS of jitter and the noise power are estimated, and then the SNR is obtained. Simulation and measurement results against the standard test methods collaborate to validate the accuracy and robustness of the new solution

Non-sliced Optical Arbitrary Waveform Measurement (OAWM) Using a Silicon Photonic Receiver Chip

Comb-based optical arbitrary waveform measurement (OAWM) techniques can overcome the bandwidth limitations of conventional coherent detection schemes and may have disruptive impact on a wide range of scientific and industrial applications. Over the previous years, different OAWM schemes have been demonstrated, showing the performance and the application potential of the concept in laboratory experiments. However, these demonstrations still relied on discrete fiber-optic components or on combinations of discrete coherent receivers with integrated optical slicing filters that require complex tuning procedures to achieve the desired performance. In this paper, we demonstrate the first wavelength-agnostic OAWM front-end that is integrated on a compact silicon photonic chip and that neither requires slicing filters nor active controls. Our OAWM system comprises four IQ receivers, which are accurately calibrated using a femtosecond mode-locked laser and which offer a total acquisition bandwidth of 170 GHz. Using sinusoidal test signals, we measure a signal-to-noise-and-distortion ratio (SINAD) of 30 dB for the reconstructed signal, which corresponds to an effective number of bits (ENOB) of 4.7 bit, where the underlying electronic analog-to-digital converters (ADC) turn out to be the main limitation. The performance of the OAWM system is further demonstrated by receiving 64QAM data signals at symbol rates of up to 100 GBd, achieving constellation signal-to-noise ratios (CSNR) that are on par with those obtained for conventional coherent receivers. In a theoretical scalability analysis, we show that increasing the channel count of non-sliced OAWM systems can improve both the acquisition bandwidth and the signal quality. We believe that our work represents a key step towards out-of-lab use of highly compact OAWM systems that rely on chip-scale integrated optical front-ends

A Software-based Low-Jitter Servo Clock for Inexpensive Phasor Measurement Units

This paper presents the design and the implementation of a servo-clock (SC) for low-cost Phasor Measurement Units (PMUs). The SC relies on a classic Proportional Integral (PI) controller, which has been properly tuned to minimize the synchronization error due to the local oscillator triggering the on-board timer. The SC has been implemented into a PMU prototype developed within the OpenPMU project using a BeagleBone Black (BBB) board. The distinctive feature of the proposed solution is its ability to track an input Pulse-Per-Second (PPS) reference with good long-term stability and with no need for specific on-board synchronization circuitry. Indeed, the SC implementation relies only on one co-processor for real-time application and requires just an input PPS signal that could be distributed from a single substation clock

Software breadboard study

The overall goal of this study was to develop new concepts and technology for the Comet Rendezvous Asteroid Flyby (CRAF), Cassini, and other future deep space missions which maximally conform to the Functional Specification for the NASA X-Band Transponder (NXT), FM513778 (preliminary, revised July 26, 1988). The study is composed of two tasks. The first task was to investigate a new digital signal processing technique which involves the processing of 1-bit samples and has the potential for significant size, mass, power, and electrical performance improvements over conventional analog approaches. The entire X-band receiver tracking loop was simulated on a digital computer using a high-level programming language. Simulations on this 'software breadboard' showed the technique to be well-behaved and a good approximation to its analog predecessor from threshold to strong signal levels in terms of tracking-loop performance, command signal-to-noise ratio and ranging signal-to-noise ratio. The successful completion of this task paves the way for building a hardware breadboard, the recommended next step in confirming this approach is ready for incorporation into flight hardware. The second task in this study was to investigate another technique which provides considerable simplification in the synthesis of the receiver first LO over conventional phase-locked multiplier schemes and in this approach, provides down-conversion for an S-band emergency receive mode without the need of an additional LO. The objective of this study was to develop methodology and models to predict the conversion loss, input RF bandwidth, and output RF bandwidth of a series GaAs FET sampling mixer and to breadboard and test a circuit design suitable for the X and S-band down-conversion applications

Accurate spectral test algorithms with relaxed instrumentation requirements

Spectral testing is widely used to test the dynamic linearity performance of Analog-to-Digital Converters (ADC) and waveform generators. Dynamic specifications for ADCs are very important in high speed applications such as digital communications, ultrasound imaging and instrumentation. With improvements in the performance of ADCs, it is becoming an expensive and challenging task to perform spectral testing using standard methods due to the requirement that the test instrumentation environment must satisfy several stringent conditions. In order to address these challenges and to decrease the test cost, in this dissertation, three new algorithms are proposed to perform accurate spectral testing of ADCs by relaxing three necessary conditions required for standard spectral testing methods. The testing is done using uniformly sampled points. The first method introduces a new fundamental identification and replacement (FIRE) method, which eliminates the requirement of coherent sampling when using the DFT for testing the spectral response of an ADC. The robustness and accuracy of the proposed FIRE method is verified using simulation and measurement results obtained with non-coherently sampled data. The second method, namely, the Fundamental Estimation, Removal and Residue Interpolation (FERARI) method, is proposed to eliminate the requirement of precise control over amplitude and frequency of the input signal to the ADC. This method can be used when the ADC output is both non-coherently sampled and clipped. Simulation and measurement results using the FERARI method with non-coherently sampled and clipped outputs of the ADC are used to validate this approach. A third spectral test method is proposed that simultaneously relaxes the conditions of using a spectrally pure input source and coherent sampling. Using this method, the spectral characteristics of a high resolution ADC can be accurately tested using a non-coherently sampled output obtained with a sinusoidal input signal that has significant and unknown levels of nonlinear distortion. Simulation results are presented that show the accuracy and robustness of the proposed method. Finally, the issue of metastability in comparators and Successive Approximation Register (SAR) ADCs is analyzed. The analysis of probability of metastability in SAR ADCs with and without using metastable detection circuits is provided. Using this analysis, it is shown that as the frequency of sampling clock increases, using a metastable detection circuit decreases the probability of metastability in SAR ADC

Time and Frequency Transfer in a Coherent Multistatic Radar using a White Rabbit Network

Networks of coherent multistatic radars require accurate and stable time and frequency transfer (TFT) for range and Doppler estimation. TFT techniques based on global navigation satellite systems (GNSS), have been favoured for several reasons, such as enabling node mobility through wireless operation, geospatial referencing, and atomic clock level time and frequency stability. However, such systems are liable to GNSS-denial, where the GNSS carrier is temporarily or permanently removed. A denial-resilient system should consider alternative TFT techniques, such as the White Rabbit (WR) project. WR is an Ethernet based protocol, that is able to synchronise thousands of nodes on a fibre-optic based network with sub-nanosecond accuracy and picoseconds of jitter. This thesis evaluates WR as the TFT network for a coherent multistatic pulse-Doppler radar – NeXtRAD. To test the hypothesis that WR is suitable for TFT in a coherent multistatic radar, the time and frequency performance of a WR network was evaluated under laboratory conditions, comparing the results against a network of multi-channel GPS-disciplined oscillators (GPSDO). A WR-disciplined oscillator (WRDO) is introduced, which has the short-term stability of an ovenised crystal (OCXO), and long-term stability of the WR network. The radar references were measured using a dual mixer time difference technique (DMTD), which allows the phase to be measured with femtosecond level resolution. All references achieved the stringent time and frequency requirements for short-term coherent bistatic operation, however the GPSDOs and WRDOs had the best short-term frequency stability. The GPSDOs had the highest amount of long-term phase drift, with a peak-peak time error of 9.6 ns, whilst the WRDOs were typically stable to within 0.4 ns, but encountered transient phase excursions to 1.5 ns. The TFT networks were then used on the NeXtRAD radar, where a lighthouse, Roman Rock, was used as a static target to evaluate the time and frequency performance of the references on a real system. The results conform well to the laboratory measurements, and therefore, WR can be used for TFT in coherent radar

Accurate and robust spectral testing with relaxed instrumentation requirements

Spectral testing has been widely used to characterize the dynamic performances of the electrical signals and devices, such as Analog-to-Digital Converters (ADCs) for many decades. One of the difficulties faced is to accurately and cost-effectively test the continually higher performance devices. Standard test methods can be difficult to implement accurately and cost effectively, due to stringent requirements. To relax these necessary conditions and to reduce test costs, while achieving accurate spectral test results, several new algorithms are developed to perform accurate spectral and linearity test without requiring precise, expensive instruments. In this dissertation, three classes of methods for overcoming the above difficulties are presented. The first class of methods targeted the accurate, single-tone spectral testing. The first method targets the non-coherent sampling issue on spectral testing, especially when the non-coherently sampled signal has large distortions. The second method resolves simultaneous amplitude and frequency drift with non-coherent sampling. The third method achieves accurate linearity results for DAC-ADC co-testing, and generates high-purity sine wave using the nonlinear DAC in the system via pre-distortion. The fourth method targets ultra-pure sine wave generation with two nonlinear DACs, two simple filters, and a nonlinear ADC. These proposed methods are validated by both simulation and measurement results, and have demonstrated their high accuracy and robustness against various test conditions. The second class of methods deals with the accurate multi-tone spectral testing. The first method in this class resolves the non-coherent sampling issue in multi-tone spectral testing. The second method in this class introduces another proposed method to deal with multi-tone impure sources in spectral testing. The third method generates the multi-tone sine wave with minimum peak-to-average power ratio, which can be implemented in many applications, such as spectral testing and signal analysis. Similarly, simulation and measurement results validate the functionality and robustness of these proposed methods. Finally, the third class introduces two proposed methods to accurately test linearity characteristics of high-performance ADCs using low purity sinusoidal or ramp stimulus in the presence of flicker noise. Extensive simulation results have verified their effectiveness to reduce flicker noise influence and achieve accurate linearity results