Spur-free Dynamic Range Research Articles

A low power 16-bit 50 MS/s pipeline ADC with 104 dB SFDR in 0.18 μm CMOS

This paper presents a low-power 16-bit 50-MS/s pipeline analog-to-digital converter (ADC). An improved switched-capacitor bias technique is proposed to reduce power consumption while maintaining excellent performance, and a novel bootstrapped switch is implemented to improve the linearity further. An INL-(integral nonlinearity) based capacitor mismatch calibration is proposed to calibrate the capacitor match accuracy to the 16-bit level. The pipeline ADC is fabricated in a 0.18 μm mixed-signal CMOS process and the core size is 1.8 mm2. The implemented ADC is measured at a 50 MHz sampling rate under 1.8 V supply. After foreground calibration for capacitor mismatch errors, it achieves beyond 100 dB spur-free dynamic range (SFDR) and 75.3 dB signal-to-noise ratio (SNR). At the same time, the power consumption is 70.2 mW with 0.29 pJ/conv-step FOM (Figures-of-Merit).

Compact Slow-Light Integrated Silicon Electro-Optic Modulators With Low Driving Voltage

In this work, we report two slow-light enhanced silicon optical modulators with electro-optic (EO) phase shifter lengths of 500 μm and 150 μm that were fabricated using the AIM Photonics foundry. Tunable power splitters give a threefold increase in the extinction ratio (ER). An eye diagram operating at 10 Gbps was achieved using a very low signal voltage of V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</sub> = 500mV. The energy per bit is 57.5 fJ/bit and 45 fJ/bit for the 500 μm and 150 μm modulators, respectively. The nonlinearity of the devices were measured yielding spur-free dynamic range of 113.2 dB/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2/3</sup> . The 150 μm length MZM is among the smallest optical modulators using Bragg gratings for slow light ever reported and results in an estimated modulation efficiency of 0.33 V·cm.

Ultralinear Magnetic-Flux-To-Voltage Conversion in Superconducting Quantum Interference Proximity Transistors

Superconducting interferometers are quantum devices able to transduce a magnetic flux into an electrical output with excellent sensitivity, integrability, and power consumption. Yet, their voltage response is intrinsically nonlinear, a limitation which is conventionally circumvented through the introduction of compensation inductances or by the construction of complex device arrays. Here we propose an intrinsically linear flux-to-voltage mesoscopic transducer, exploiting the superconducting quantum interference proximity transistor (SQUIPT) as a fundamental building block, called bi-SQUIPT. It provides a voltage-noise spectral density as low as approximately ${10}^{\ensuremath{-}16}\phantom{\rule{0.2em}{0ex}}{\mathrm{V}/\mathrm{Hz}}^{1/2}$ and, more interestingly, under a proper operation parameter selection, exhibits a spur-free dynamic range as large as approximately $60$ dB, a value on par with that obtained with state-of-the-art linear flux-to-voltage superconducting transducers based on superconducting quantum interference devices (SQUIDs). Furthermore, thanks to its peculiar measurement configuration, the bi-SQUIPT is tolerant to imperfections and nonidealities in general. For the above reasons, we believe that the bi-SQUIPT could provide a relevant step beyond in the field of low-dissipation and low-noise current amplification with a special emphasis on applications in cryogenic quantum electronics.

Reconfigurable microwave photonic mixer for hybrid macro-micro cellular systems.

Microwave photonic mixing can realize the frequency conversion of microwave signals in the optical domain, breaking through the bandwidth bottleneck and electromagnetic interference problems of traditional microwave mixing methods. In the background of the hybrid macro-micro cellular system, a wideband, large dynamic range and reconfigurable microwave photonic mixer is proposed, theoretically analyzed and experimentally demonstrated in this paper. By adjusting the modulator bias voltages and matching the proper digital domain operations, a microwave photonic mixer with reconfigurable functions including single-ended dispersion immune mixing, I/Q frequency down-conversion, image rejection mixing, and double-balanced mixing are realized, respectively. Meanwhile, optimizing the electrical attenuator using convex optimization can suppress the third-order intermodulation distortion (IMD3), maximize the conversion gain, and finally improve the spur-free dynamic range (SFDR). Experimental results show that the proposed scheme can be operated with a frequency from 5 to 20 GHz, and the SFDR can achieve 118.3 dB·Hz4/5. Over the whole frequency range, I/Q frequency down-conversion can be well conducted with an amplitude imbalance below 0.7 dB and a phase imbalance below ±0.7°. After an I/Q imbalance compensation algorithm, the image rejection ratio of over 60 dB is produced. The power fading caused by fiber dispersion is also compensated successfully. For a vector signal with 16 quadrature amplitude modulation, the best error vector magnitude (EVM) reaches 3.4%.

Signal-Independent Background Calibration with Fast Convergence Speed in Pipeline-SAR ADC

This brief proposes a signal-independent background calibration in pipeline-SAR analog-to-digital converters (ADCs) with a convergence-accelerated technique. To achieve signal independence, an auxiliary capacitor array CA is introduced to pre-inject a pseudo-random noise (PN) in the sampling phase to cancel out the opposite PN injection of the calibrated capacitor in the conversion phase, and CA is also used to realize the D/A function of the calibrated capacitor in the conversion phase. In this way, no matter what the signal is, the residue headroom remains unchanged even with PN injection. Moreover, the first sub-ADC is designed with extended conversion bits to quantize its own residue after delivering the conversion bits required by the first stage. Afterwards, this result is provided to the calibration algorithm to reduce the signal component and accelerate the convergence. Based on the simulation, the signal-to-noise and distortion ratio (SNDR) and spur-free dynamic range (SFDR) improve from 45.3 dB and 56.4 dB to 68.2 dB and 88.4 dB, respectively, after calibration. In addition, with the acceleration technique, convergence cycles decrease from 1.7 × 108 to 5.8 × 106. Moreover, no matter whether the input signal is DC, sine wave or band-limited white noise, the calibration all works normally.

A ‘DOMZM–Filter–BPD’-Based Microwave Photonic Link for Eliminating Even Harmonics Distortion

The ‘Mach-Zehnder modulator (MZM)–wavelength division demultiplexer (WDM)–photodetector (PD)’ structure in photonic analog-to-digital converters (PADCs) may result in residual even harmonics after photodetection. In this paper, we build the model for the ‘Dual-output MZM (DOMZM)–filter–balanced PD (BPD)’-based microwave photonic link, where the amplitude and the phase responses of the filter can be arbitrary. It is proved that this link can eliminate the aforementioned even harmonics. Theoretical analysis is verified by the simulation and experimental results. By using this structure, the spur-free dynamic range (SFDR) limited by the even harmonics can be improved and the sampling frequency range can be expanded for the PADCs. As the spectrum contains only odd harmonics, the inverse cosine transform for calibrating the nonlinearity of the MZM can be applied to further improve the SFDR.

Ultraflat bandpass, high extinction, and tunable silicon photonic filters.

We report on the design and the demonstration of silicon photonic ultraflat bandpass filters with low insertion loss and high out-of-band rejection for an operation near the 1550 nm wavelength band. These filters are based on cascading low (2nd) order Ring-Assisted Mach-Zehnder Interferometer (RAMZI) filter stages. The cascade design enables high out-of-band rejection while keeping the unit cells of each stage low order to be more tolerant to fabrication imperfections. The characterization of filters shows an insertion loss of ∼1 dB, an in-band ripple of <0.1 dB, an out-of-band rejection of >50 dB for a filter 3-dB bandwidth of ∼1.1 nm, and tunable up to ∼6 nm. We also investigate the filter's spur-free dynamic range at high input optical powers, which is important for RF photonics applications, and quantify a dynamic range of >60 dB for a laser power as high as ∼11.6 mW sent to the filter. Such integrated filters are promising for applications in pump wavelength rejection in four-wave mixing photon pair generation, and in RF antenna remoting where multiple RF signals are carried on different coarse wavelengths to be separated.

Deep-learning-assisted linearization for the broadband photonic scanning channelized receiver.

An autoencoder-residual (AE-Res) network is designated to assist the linearization of the wideband photonic scanning channelized receiver. It is capable of adaptively suppressing spurious distortions over multiple octaves of signal bandwidth, obviating the need for calculating the multifactorial nonlinear transfer functions. Proof-of-concept experiments indicate that the improvement of the third-order spur-free dynamic range (SFDR2/3) is 17.44 dB. Moreover, the results for real wireless communication signals demonstrate that the improvement of the spurious suppression ratio (SSR) is 39.69 dB and the reduction of the noise floor is ∼10 dB.

Adaptive Linearization for the Sub-Nyquist Photonic Receiver Based on Deep Learning

Due to the nonlinear and aliasing effects, the sub-Nyquist photonic receiver for radio frequency (RF) signals with large instantaneous bandwidth suffers limited dynamic range and noise performance. We designated a deep residual network (Resnet) to realize adaptive linearization across 40 GHz bandwidth. In contrast to conventional linearization methods, the deep learning method achieves the suppression of multifactorial spurious distortions and the noise floor simultaneously. It does not require an accurate calculation of the nonlinear transfer function or prior signal information. The experiments demonstrated that the proposed Resnet could improve the spur-free dynamic range (SFDR) and the signal-to-noise ratio (SNR) significantly by testing with single-tone signals, dual-tone signals, wireless communication signals, and modulated radar signals.

Integrated Slow-Light Enhanced Silicon Photonic Modulators for RF Photonic Links

An integrated RF-photonic link that consists of a slow-light enhanced silicon (Si) electro-optic modulator, a single mode Si optical waveguide and a silicon-germanium (SiGe) photodetector was fabricated using a Si photonic standard foundry service. The third-order intermodulation distortion (IMD3) of the RF signals and the link spur-free dynamic range (SFDR) were characterized through a two-tone test method. The Si Mach-Zehnder modulator architecture is designed to have unequal arms to operate on a simplified driving scheme. As a result, on-chip thermal heaters can be omitted, and the modulator can configure near its quadrature point by adjusting the wavelength to the modulator. Owning to enhanced light-matter interaction in the slow-light region, an increased slope efficiency is observed in the modulator. By selecting the optimal DC bias to the grating and the reference arms of the modulator, a SFDR of 96 dB/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2/3</sup> can be obtained in the RF link at the slow light region. The slow-light modulator also displays increased modulation efficiency, allowing for a reduced footprint for large array integration.

A 12‐bit SC3 partially segmented current steering DAC with improved SFDR and bandwidth

AbstractA 12‐bit self cascode capacitor compensated (SC3) partial segmented current steering digital to analog converter (DAC) is proposed and implemented in standard 180‐nm CMOS technology. The effect of the output capacitance at a higher frequency of operation is analyzed. Based on the analysis, a self cascode transistor is connected on the top of the differential switch with an “always‐on” biasing without compromising the voltage headroom. Besides, a compensation capacitor is also used at the output to improve spur free dynamic range (SFDR), which is obtained as 108 dB when sampled at 800 MS/s. The proposed architecture also offers high output impedance by more than 10 times compared with the cascode device. The proposed design also enhances the bandwidth of the circuit and is obtained to be 4 GHz with a power supply rejection ratio (PSRR) of 43 dB. It also has a total harmonic distortion (THD) of &lt;3% up to input power of −4 dB and is also insensitive to process variations with a sensitivity of &lt;0.5%.

High-performance dual-band radio-over-fiber link for future 5G radio access applications

For the fronthaul between the baseband unit and the remote radio unit (RRU), radio over fiber (RoF) is promising to replace the Common Public Radio Interface in beyond fifth-generation and the sixth-generation wireless communication systems. The weight, size, and power consumption of the RRU can be greatly reduced, along with easy support for the millimeter-wave band. In this paper, a dual-band RoF link is proposed, where flexible switching between high-frequency (28 GHz) and low-frequency (3.5 GHz) RF channels is realized through polarization control. Single-sideband modulation is performed on 28 GHz signals to avoid periodic power fading during long-distance fiber transmission. By adjusting the DC bias, the third-order intermodulation distortion component in the 3.5 GHz signal is suppressed, and the spur-free dynamic range (SFDR) is improved. Experimental results show a channel isolation of above 43 dB. The link gain of the 28 GHz signal channel is flat, and the SFDR of the 3.5 GHz signal channel can reach 118.6 d B ⋅ H z 4 / 5 . In the case of RF vector signal transmission, the adjacent channel power ratio of the 3.5 GHz signal channel is optimally up to 48 dB, and the error vector magnitude is 2.9%.

Coherent Amplitude-Modulated RF Photonic Link

The advantages of optical fiber as a transmission medium for microwave signals have led to an increasing interest in implementing RF photonic links in radar frontends. Many critical radar applications, such as those found in modern military systems, often require a large spur-free dynamic range (SFDR) that is beyond the reach of conventional intensity modulated-direct detection (IM-DD) fiber-optic links. For a solution, we propose a coherent amplitude-modulated link with optical down-conversion that can achieve high dynamic range by employing a receiver with balanced detection and digital linearization. The link is presented along with experimental validation. The measurements show notable dynamic range enhancements over prior arts in down-conversion RF photonic links.

Visualizing and simplifying convolutional recurrent autoencoder for mismatch compensation of channel-interleaved photonic analog-to-digital converter

Deep learning (DL) has been used to successfully solve numerous problems and challenges in a wide range of fields. The architecture of DL is complex and treated as a black box, making it difficult to understand the principles behind it. Here, we visualize the process of compensating for time mismatches for a two-channel photonic analog-to-digital converter (PADC) by a convolutional recurrent autoencoder (CRAE) with excellent generalizability and robustness. Besides, we explore the effects of different modules of the CRAE on the generalizability. Based on the analysis of the above two operations, we simplify the CRAE and then apply it to a four-channel PADC, which is a more complex channel-interleaved system. Consequently, for both PADC systems, the performance of the simplified CRAE is as good as that of the original CRAE. Moreover, for the two-channel PADC, after simplification, the frame rate of the CRAE is increased from 460 frames/second to 975 frames/second, 20,000 points in each frame. For the four-channel PADC, the spur-free dynamic range is enhanced to 24.6 dBc from 5.2 dBc.

Effects of the Nonlinearity Caused by 'MZM-WDM' Structure in Time-Wavelength Interleaved Photonic Analog-to-Digital Converters.

The effects of the nonlinearity caused by the structure of ‘Mach-Zehnder modulator (MZM) - wavelength division demultiplexer (WDM)’ in Time-Wavelength Interleaved Photonic Analog-to-Digital Converters is studied for the first time. Even if the MZM is biased at quadrature, the narrow passband width of the WDM after modulation may result in the presentation of even harmonics in the spectrum of the sampling results. As the frequency of the signal being sampled increases and the passband width of the WDM decreases, the magnitudes of the even harmonics increase. As a result, the spur-free dynamic range (SFDR) would be determined by the magnitudes of the signal and the 2nd harmonic instead of the 3rd harmonic. When the passband width of the WDM is less than 100 GHz, the variation of the SFDR can be as much as 6 dB over 0~25 GHz-frequency range of the signal being sampled. Theoretical analysis is verified by the simulation and experimental results. When the passband width of the WDM is greater than 150 GHz, the SFDR over 0~25 GHz-RF range is always dominated by the 3rd harmonic. When the passband width is 50 GHz, the SFDR is dominated by the 2nd harmonic for RF frequencies above 10 GHz, and can be as much as 20 dB higher than the 3rd harmonic term at 25 GHz-RF frequency. The requirement for the passband width of the WDM can be specified according to the frequency range of the signal being sampled to avoid the presentation of even harmonics and optimize the design of TWIPADCs.

Photonic analog-to-digital converter powered by a generalized and robust convolutional recurrent autoencoder.

We propose a convolutional recurrent autoencoder (CRAE) to compensate for time mismatches in a photonic analog-to-digital converter (PADC). In contrast of other neural networks, the proposed CRAE is generalized to untrained mismatches and untrained category of signals while remaining robust to system states. We train the CRAE using mismatched linear frequency modulated (LFM) signals with mismatches of 35 ps and 57 ps under one system state. It can effectively compensate for mismatches of both LFM and Costas frequency modulated signals with mismatches ranging from 35 ps to 137 ps under another system state. When the spur-free dynamic range (SFDR) of the unpowered PADC decreases from 10.2 dBc to -3.0 dBc, the SFDR of the CRAE-powered PADC is over 31.6 dBc.

A 91.0-dB SFDR Single-Coarse Dual-Fine Pipelined-SAR ADC With Split-Based Background Calibration in 28-nm CMOS

This paper presents a single-coarse dual-fine architecture that improves energy-efficiency of pipelined-SAR analog-to-digital converters (ADCs). A coarse and fast sub-ADC is used to quantize the most significant bits (MSBs), which are encoded with a proposed residue transformation method to control the residue generation of the first stages in two fine channels. The residue voltages generate on the capacitive digital-to-analog converters (C-DACs) of split fine channels directly without successive approximation processes. Therefore, the conversion rate is increased and the power is reduced. A shuffle mechanism is introduced into split-ADC based digital background calibration to avoid the divergence of the conventional algorithm in the proposed architecture. A high-energy-efficiency dynamic amplifier is also introduced as the residue amplifier. A 14-bit 60-MS/s ADC is prototyped in a 28-nm CMOS process. The digital calibration engine operates under 0.9-V supply, other parts of the ADC core operate under 1.05-V supply. The ADC core consumes 4.26 mW. Measurement results show that the calibration improved the signal-to-noise and distortion ratio (SNDR) and spur-free dynamic range (SFDR) dramatically, the calibrated ADC achieves SNDR and SFDR of 66.9 dB and of 91.0 dB respectively, translating to a Schreier FoM of 165.4 dB and a Walden FoM of 39.3 fJ/conversion-step.

Switchable and filter‐free photonic microwave single‐sideband frequency converter

AbstractA switchable and filter‐free photonic microwave single‐sideband frequency converter based on a dual‐parallel dual‐drive Mach‐Zehnder modulator is proposed and experimentally demonstrated. Two first‐order carrier‐suppression single sidebands for input signal and local oscillator signal are achieved by adjusting the DC bias voltages of the modulator. The frequency upconversion and downconversion can be switched by adjusting one DC bias voltage of the modulator. Thanks to the carrier‐suppression single‐sideband modulation without using any filter, the frequency converter has a large operating bandwidth but no power fading caused by chromatic dispersion, and the mixing spurs can be effectively suppressed. Experimental results show that the frequency converted signal is pure over a wide frequency range, and the sideband suppression ratio of desired signal is over 20 dB. The spur free dynamic range achieves 108.3 dB Hz2/3.

Modified deep-learning-powered photonic analog-to-digital converter for wideband complicated signal receiving.

We propose and demonstrate a modified deep-learning-powered photonic analog-to-digital converter (DL-PADC) in which a neural network is used to eliminate the signal distortions of the photonic system. This work broadens the receiving capability from simple waveforms to complicated waveforms via implementing a modified deep learning algorithm. Thus, the modified DL-PADC can be applied in real scenarios with wideband complicated signals. Testing results show that the trained neural network eliminates the signal distortions with high quality, improving the spur-free dynamic range by ∼20dB. An experiment for echo detection is conducted as an example, which shows that the neural network enhances the quality of detailed target profile detection. Furthermore, the modified DL-PADC only comprises a low-complexity photonic system, which obviates the requirement for redundant hardware setup while maintaining the processing quality. It is expected that the modified DL-PADC can perform as a promising photonic wideband signal receiver with low hardware complexity.

A broadband digital receiving system with large dynamic range for solar radio observation

Solar radio spectra and their temporal evolution provide important clues to understand the energy release and electron acceleration process in the corona, and are commonly used to diagnose critical parameters such as the magnetic field strength. However, previous solar radio telescopes cannot provide high-quality data with complete frequency coverage. Aiming to develop a generalized solar radio observing system, in this study, we designed a digital receiving system that could capture solar radio bursts with a broad bandwidth and a large dynamic range. A dual-channel analog-to-digital converter (ADC) printed circuit board assembly (PCBA) with a sampling rate of 14-bit, 1.25 Giga samples per second (GSPS) cooperates with the field-programmable-gate-array (FPGA) chip XC7K410T in the design. This receiver could realize the real-time acquisition and preprocessing of high-speed data of up to 5 GB s−1, which ensures high time and spectral resolutions in observations. This receiver has been used in the solar radio spectrometer working in the frequency range of 35 to 40 GHz in Chashan Solar Observatory (CSO) established by Shandong University, and will be further developed and used in the solar radio interferometers. The full-power bandwidth of the PCBA in this receiving system could reach up to 1.5 GHz, and the performance parameters (DC–1.5 GHz) are obtained as follows: spur free dynamic range (SFDR) of 64.7–78.4 dB, signal-to-noise and distortion (SINAD) of 49.1–57.2 dB, and effective number of bits (ENOB) of > 7.86 bit. Based on the receiver that we designed, real-time solar microwave dynamic spectra have been acquired and more solar microwave bursts with fine spectral structures are hopeful to be detected in the coming solar maximum.