Thermal Noise Performance Research Articles

Submillimeter balanced SSFP BOLD-functional MRI accelerated with 3D stack-of-spirals at 9.4 T.

This work aims to improve the speed of balanced SSFP (bSSFP) acquisition with segmented 3D stack-of-spirals for functional brain studies at ultrahigh field. Functional experiments were performed with an accelerated 3D stack-of-spirals sequence with water excitation for fat suppression. The resulting data were reconstructed using an iterative algorithm with corrections for system imperfections such as trajectory deviations and B0 inhomogeneity. In the first set of experiments, we evaluated the signal change and stability with respect to echo and TR for a full-field checkerboard stimulus. To demonstrate the high spatio-temporal resolution of the developed method, the results of three optimized protocols at submillimeter resolution (0.6-mm isotropic and 0.8-mm isotropic) and at 1.2 mm isotropic resolution for whole-brain coverage were shown. Water excitation and the model-based iterative reconstruction improved image quality. The BOLD-related signal changes increased with longer TE and longer TR. We observed an increase in thermal noise performance at lower TE and higher TR. However, signal stability deteriorates at higher TE and TR. Therefore, optimized protocols used shorter TE and moderately long TR to maximize the sensitivity and speed. Reproducible activations were detected along the gray-matter gyri in the submillimeter protocols with a median signal change of approximately 4% across subjects. Three-dimensional stack-of-spirals enables passband balanced SSFP functional imaging at a much higher spatial and temporal scale, compared with conventional spoiled gradient-echo train sequences.

Multipath Mitigation for BOC Signals Based on Prompt-Assisted-Offset Correlator

Tracking ambiguity and multipath are two serious threats in processing binary offset carrier (BOC) signals that are widely used in global navigation satellite systems (GNSS). Three-loop tracking methods, such as dual binary phase-shift keying tracking (DBT), use a delay lock loop (DLL), a phase lock loop (PLL), and a subcarrier PLL (SPLL) to track the code, carrier, and subcarrier, respectively, thus solving the tracking ambiguity problem. However, the multipath remains an important factor in the ranging performance deterioration of these tracking methods. Using the offset correlator (OC) technique in the SPLL can effectively mitigate subcarrier multipath errors, but it substantially raises thermal noise errors. To solve this contradiction, this paper proposes a prompt-assisted-offset correlator (PAOC) technique that combines the prompt and offset correlators in a complementary way to mitigate multipath for BOC signals. Compared with the original OC technique, the PAOC technique has less thermal noise performance loss. Moreover, this paper discovers and quantifies the interaction between carrier and subcarrier multipath errors by analyzing the coupling effect between SPLL and PLL. Most multipath mitigation methods for BOC signals ignore the carrier multipath in the PLL, so their subcarrier multipath performance is not optimal. Thus, this paper further proposes applying the PAOC technique in both PLL and SPLL to mitigate carrier and subcarrier multipath errors simultaneously. Theoretical analysis and experimental results demonstrate that the proposed method can significantly improve the multipath performance with small noise performance loss.

Dual High Resolution Correlators for Multipath Mitigation in BOC Signals

Multipath is a dominant source of positioning errors in global navigation satellite system receivers. It is difficult to design multipath mitigation techniques for binary offset carrier (BOC) signals due to the complicated autocorrelation function of BOC signals and localized variations of multipath. In this paper, a dual high resolution correlators (DHRC) technique is proposed for BOC signals to mitigate code and subcarrier multipath errors, as well as the carrier multipath error that is neglected by common techniques. Based on the concept of dual binary phase-shift keying (BPSK) tracking, the BOC signal is decomposed into two BPSK signals located in the upper and lower sidebands, respectively, thereby transforming the multipath mitigation in the BOC signal into the multipath mitigation in the upper and lower sidebands. Two narrow correlation functions of the upper and lower sidebands are synthesized using multiple correlation values, so the multipath outside the main peak of the synthesized correlation function is eliminated. The peak value of the synthesized correlation function is reduced from the original ACF, resulting in a large noise error. Therefore, a complementary filter is designed to compensate the noise error. Theoretical analysis and simulation results demonstrate that the DHRC technique with complementary filters can significantly mitigate multipath errors for BOC signals with slight thermal noise performance loss.

Carrier loop with multi-pulse coherent integration for tracking pulsed pseudolite signal.

The pseudolite system, an alternative to global navigation satellite systems, can provide centimeter-level positioning accuracy using carrier phase measurements. The carrier phase-based positioning accuracy is determined by the tracking performance of the phase-locked loop (PLL), since cycle slip may cause carrier phase measurement bias. To overcome the near-far problem, the pulsing scheme is often adopted in pseudolite positioning systems. For the pulsed pseudolite signal, the carrier tracking loop is more prone to cycle slip compared to the conventional continuous signal. For improving the performance of the PLL tracking the pulsed pseudolite signal at low carrier-to-noise ratios, a PLL with multi-pulse coherent integration (MPCI-PLL) is proposed. The thermal noise performance of the digital PLL (DPLL) is determined by its equivalent noise bandwidth. When the product of the design noise bandwidth and the loop update interval increases, the true equivalent noise bandwidth of DPLL tends to be larger than the design value. Then, the closed-form expression of the single-sided equivalent noise bandwidth of first-, second-, and third-order DPLL is derived by the discrete frequency domain approach. The proposed MPCI-PLL is tested in both simulations and a real experiment. Compared to the conventional method of tracking the pulsed pseudolite signal, the proposed carrier tracking loop significantly decreases the probability of a cycle slip at low signal-to-noise ratios. Simulation and experimental results show that the improvement is about 3-4 dB.

Multipath Mitigation Methods of BOC-Family Signals Based on Dual BPSK Tracking Techniques

It is well known that for binary offset carrier (BOC) signals the notorious ambiguity problem due to the multipeaked autocorrelation function must be tackled first in order to obtain the potential of improving ranging performance promised by its broader Gabor bandwidth. The existence of multipath makes it more difficult to solve. Dual binary phase-shift keying (BPSK) tracking (DBT) is a kind of low complexity, unambiguous tracking method for BOC-family signals, which makes full use of the coherence between the upper and lower sidebands and employs subcarrier phase locked loop (SLL) and delay locked loop (DLL) to track subcarrier and code components, respectively. Considering that the thermal noise performance of DBT has been fully studied, this article focuses on the analysis of multipath performance and the design of antimultipath methods under DBT framework. The influence of multipath on SLL and DLL of DBT is analyzed first and the corresponding subcarrier and code multipath error envelopes are obtained. Then according to different requirements of subcarrier and code multipath errors, a multipath mitigation scheme specially designed for DBT is proposed, in which SLL and DLL adopt offset correlator and narrow correlation, respectively. Theoretical analysis and simulation are given to verify the effectiveness of the proposed methods.

Large-scale Monolithic Fused-Silica Mirror Suspension for Third-Generation Gravitational-Wave Detectors

Thermal noise from the suspension fibers used in the mirror pendulums in current gravitational-wave detectors is a critical noise source. Future detectors will require improved suspension performance with the specific ability to suspend much heavier masses to reduce radiation pressure noise, while retaining good thermal noise performance. In this paper we propose and experimentally demonstrate a design for a large-scale fused-silica suspension, demonstrating its suitability for holding an increased mass of 160 kg. We demonstrate the concepts for improving thermal noise via longer suspension fibers supporting a higher static stress. We present a full thermal noise analysis of our prototype, meeting requirements for conceptual third-generation detector designs such as the high frequency interferometer of the Einstein Telescope, and closely approaching that required for Cosmic Explorer.

Extended Double-delta Correlator Technique for GNSS Multipath Mitigation

Multipath is one of the main error sources in global navigation satellite system (GNSS). Many multipath mitigation methods have been proposed, such as the narrow correlator technique and the double-delta (DD) correlator technique. In light of the BPSK modulated signals used in many GNSS, an extended DD (EDD) correlator technique expanding from the DD correlator, which uses multiple pairs of correlators with more general spacing and weight, is proposed in this paper. Furthermore, the paper provides the discriminator structure, the closed-form expressions for multipath error envelope under infinite bandwidth and the closed-form expressions for the relationship between multipath error, thermal noise performance and design parameters of the discriminator. Based on the explicit performance expressions, the appropriate selection principles of parameters are given and verified by theoretical analysis and simulation. The results show that the proposed method can improve the performance optimization freedom of the DD technique without significantly increasing the design complexity. Moreover, the multipath performance is better than DD under infinite bandwidth and has great adaptability to limited bandwidth and the next generation GNSS signals, such as BOC and MBOC. The EDD technique can also be utilized as a noncoherent discriminator and has better anti-multipath performance. Furthermore, the EDD technique can achieve balance between multipath mitigation performance and thermal noise performance by adjusting correlator spacing and weight and can be self-adaptive to multipath relative amplitude in the environment.

General Design Method for Two-dimensional Multi-correlator Anti-multipath Tracking Loop for BOC Signals

As binary offset carrier (BOC) signals are widely used in global navigation satellite system (GNSS), tracking ambiguity threats and multipath become two main factors which affect ranging precision. Two-dimensional (2-D) tracking loops, such as double estimate tracking (DET) and double phase estimator (DPE), estimate the code delay and subcarrier delay by the delay locked loop (DLL) and the subcarrier locked loop (SLL), respectively, to obtain high precision and unambiguous propagation delay. Although 2-D tracking techniques can effectively alleviate the impact of tracking ambiguity threats, their anti-multipath performance has not been improved compared with one-dimension (1-D) tracking techniques. This paper proposes a general design method for two-dimensional multi-correlator anti-multipath tracking (TMAT) structures based on heuristic optimization. Compared with 2-D tracking loops equipped with traditional anti-multipath structures, such as narrow early minus late (NEML) and double delta (DD) structures, TMAT structures designed for different signals with different bandwidths always have better multipath mitigation performance without any degradation of tracking robustness. According to case studies, because of the great anti-multipath improvement of TMAT structures, the slight degradation in thermal noise performance can be acceptable. Also, the designed TMAT structures are insensitive to relative amplitude of multipath and bandwidth of signals, indicating the great practicality of this technique.

Gravitational wave detectors with broadband high frequency sensitivity

Gravitational waves from the neutron star coalescence GW170817 were observed from the inspiral, but not the high frequency postmerger nuclear matter motion. Optomechanical white light signal recycling has been proposed for achieving broadband sensitivity in gravitational wave detectors, but has been reliant on development of suitable ultra-low loss mechanical components. Here we show demonstrated optomechanical resonators that meet loss requirements for a white light signal recycling interferometer with strain sensitivity below 10−24 Hz−1/2 at a few kHz. Experimental data for two resonators are combined with analytic models of interferometers similar to LIGO to demonstrate enhancement across a broader band of frequencies versus dual-recycled Fabry-Perot Michelson detectors. Candidate resonators are a silicon nitride membrane acoustically isolated by a phononic crystal, and a single-crystal quartz acoustic cavity. Optical power requirements favour the membrane resonator, while thermal noise performance favours the quartz resonator. Both could be implemented as add-on components to existing detectors.

Fabrication and characterization of Al-Mn superconducting films for applications in TES bolometers* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11653001 and 11653004).

Superconducting transition edge sensor (TES) bolometers require superconducting films to have controllable transition temperatures T c in different practical applications. The value of T c strongly affects thermal conductivity and thermal noise performance of TES detectors. Al films doped with Mn (Al–Mn) of different concentrations can accomplish tunable T c. A magnetron sputtering machine is used to deposit the Al–Mn films in this study. Fabrication parameters including sputtering pressure and annealing process are studied and their influences on T c and superconducting transition width ΔT c are optimized. The Al–Mn films with ΔT c below 1.0 mK for T c in a range of 520 mK–580 mK are successfully fabricated.

Intelligent vectorised architecture for performance enhancement of GNSS receivers in signal blocking situations

Navigation in harsh environments mostly encounters many problems such as signal blocking due to a variety of obstacles around the receiver. Vectorised receiver has an architecture in which the channels can share the information. As a result, the strong channels aid the blocked one to reacquire the signal immediately after returning. However, if the output of a channel is undesirable, it may disrupt the operation of the system especially for the weak blocked channels. In this paper, an intelligent vectorised architecture is proposed to solve this problem. Three popular architectures including federated, adaptive, and vectorised are combined and then evidence theory is utilised to select the result with higher certainty. The experimental results in urban canyons show the improvements in signal availability and thermal noise performance.

Near-Optimal Decoding of Incremental Delta-Sigma ADC Output

This paper presents a nonlinear digital decoder (reconstruction filter) for incremental delta-sigma modulators. This decoder utilizes both the magnitude and pattern information of the modulator output to achieve accurate input estimation. Compared to the conventional linear filters with the same oversampling ratio (OSR), it can improve the converter’s signal-to-quantization noise ratio by a few dB to a few 10’s of dB with slight thermal noise performance degradation. Using the proposed decoder, the modulator’s OSR can be a few times less while achieving the same resolution and data rate, thus minimizing the modulator as well as its peripheral circuits’ energy consumption. In this paper, the proposed decoder is optimized for digital implementation, with its function being verified using a modulator prototype. This decoder is mainly designed for dc or near-dc signal conversions and it does not provide frequency notches.

Thermal noise performance analysis for dual binary phase‐shift keying tracking of standard BOC signals

Among the existing binary offset carrier (BOC) signal receiving technologies, the dual binary phase-shift keying tracking (DBT), originally designed for alternate BOC (AltBOC) modulated Galileo E5 signal and then extended to standard BOC signals, has its unique characteristics. It incorporates both single-band and dual-band processing schemes into a unified and simple structure to meet both requirements of improving ranging precision and anti-jamming. However, in this study, the authors propose that it is problematic to directly extend the DBT technique from the AltBOC signal to the standard BOC signal. The root cause of this problem lies in the fact that the cross-correlation functions between the received standard BOC signal and the local replicas in the upper and lower sidebands are not an ideal correlation triangle as the AltBOC case, but with some ripples. This leads to more complex behaviours when the DBT technique is applied to standard BOC signals. The rigorous theoretical analysis of this technique is given in respect of thermal noise performance in this study, which can be used not only to optimise the tracking loop parameters of DBT but also to compare the tracking performance of DBT and other tracking technologies.

Enhanced Sensing and Nondegraded Thermal Noise Performance Based on PT-Symmetric Electronic Circuits with a Sixth-Order Exceptional Point.

An exceptional point (EP) is a non-Hermitian degeneracy where both eigenvalues and their corresponding eigenvectors coalesce. It was recently proposed and demonstrated that such spectral singularity can be utilized for enhanced sensing. Potential drawbacks of EP sensing include both fundamental resolution limit and noise effects that might mask the hypersensitive resonant splitting. Here, we address these issues by proposing a parity-time (PT)-symmetric sensing circuit bearing a sixth-order EP. By employing capacitive coupling channel as a sensing platform, we achieve an enhanced resonance shift proportional to the fourth-order root of the perturbation strength and maintain a high resolution for weak perturbation. Due to the low-pass feature of our circuit, thermal noise is mitigated down to a level comparable to its Hermitian counterpart, despite the presence of highly noisy gain and loss elements. Our EP sensing scheme offers combined enhanced sensitivity, improved resolution and nondegraded thermal noise performance, showing an exciting prospect for next-generation sensing technologies.

Investigation on noise performance of InAsxSb1−x MOSFETs with compositional variations

Using numerical analysis we investigate the thermal and low frequency noise (LFN) performance of InAsxSb1−x-channel nMOSFETs having various As mole fractions x ranging 0.15–0.85 at channel length of 65 nm. The models used in the simulation are calibrated by comparing simulated transfer characteristic curve with experimental characteristic reported elsewhere. The numerically obtained values of drain current ID and transconductance gM are used to compute power spectral density of drain current SID/ID2, noise resistance Rn and minimum noise figure NFmin as a function of As composition fraction x. The minimum noise figure NFmin of 1.35 dB is obtained for thermal noise with x = 0.15 at frequency of 5 GHz and gate bias of 1 V. Our investigation reveals that LFN as well as thermal noise can be suppressed by proper selection of As mole fraction in InAsxSb1−x n-channel MOSFETs.

Effects of sidewall spacer layers on thermal and low frequency noise performance of SOI UTB MOSFETs

In this paper we investigate the impact of sidewall spacer dielectric constant and length on the noise behavior of SOI UTB MOSFETs using numerical simulation techniques. The important electrical parameters of devices such as drain current, transconductance are computed and utilized to obtain normalized power spectral density of drain current, noise resistance and minimum noise figure of SOI UTB MOSFETs. Our investigations manifest that thermal noise as well as low frequency noise parameters can be reduced considerably by employing spacer layers with high dielectric constant and low value of spacer length (~ 5 nm). The minimum noise figure NFmin for thermal noise is obtained as low as 1.2 dB at frequency of 1 GHz by setting 1.2 V at the gate and drain terminals.

Coatings and surface treatments for enhanced performance suspensions for future gravitational wave detectors

Further improvements in the low frequency sensitivity of gravitational wave detectors are important for increasing the observable population of astrophysical sources, such as intermediate mass compact black hole binary systems. Improvements in the lower stage mirror and suspension systems will set challenging targets for the required thermal noise performance of the cantilever blade springs, which provide vertical softness and, thus, isolation to the mirror suspension stack. This is required due to the coupling between the vertical and horizontal axes due to the curvature of the Earth. This can be achieved through use of high mechanical Q materials, which are compatible with cryogenic cooling, such as crystalline silicon. However, such materials are brittle, posing further challenges for assembly/jointing and, more generally, for long-term robustness. Here, we report on experimental studies of the breaking strength of silicon at room temperature, via both tensile and 4-point flexural testing; and on the effects of various surface treatments and coatings on durability and strength. Single- and multi-layer DLC (diamond-like carbon) coatings, together with magnetron-sputtered silica and thermally-grown silica, are investigated, as are the effects of substrate preparation and argon plasma pre-treatment. Application of single- or multi-layer DLC coatings can significantly improve the failure stress of silicon flexures, in addition to improved robustness for handling (assessed through abrasion tests). Improvements of up to 80% in tensile strength, a twofold increase in flexural strength, in addition to a 6.4 times reduction in the vertical thermal noise contribution of the suspension stack at 10 Hz are reported (compared to current Advanced LIGO design). The use of silicon blade springs would also significantly reduce potential ‘crackling noise’ associated with the underlying discrete events associated with plastic deformation in loaded flexures.

Design of a tuned mass damper for high quality factor suspension modes in Advanced LIGO.

We discuss the requirements, design, and performance of a tuned mass damper which we have developed to damp the highest frequency pendulum modes of the quadruple suspensions which support the test masses in the two advanced detectors of the Laser Interferometric Gravitational-Wave Observatory. The design has to meet the requirements on mass, size, and level of damping to avoid unduly compromising the suspension thermal noise performance and to allow retrofitting of the dampers to the suspensions with minimal changes to the existing suspensions. We have produced a design satisfying our requirements which can reduce the quality factor of these modes from ∼500 000 to less than 10 000, reducing the time taken for the modes to damp down from several hours to a few minutes or less.

Performance of Backshort-Under-Grid Kilopixel TES Arrays for HAWC+

We present results from laboratory detector characterizations of the first kilopixel BUG arrays for the High- resolution Wideband Camera Plus (HAWC+) which is the imaging far-infrared polarimeter camera for the Stratospheric Observatory for Infrared Astronomy (SOFIA). Our tests demonstrate that the array performance is consistent with the predicted properties. Here, we highlight results obtained for the thermal conductivity, noise performance, detector speed, and first optical results demonstrating the pixel yield of the arrays.

Indium joints for cryogenic gravitational wave detectors

A viable technique for the preparation of highly thermal conductive joints between sapphire components in gravitational wave detectors is presented. The mechanical loss of such a joint was determined to be as low as 2 × 10−3 at 20 K and 2 × 10−2 at 300 K. The thermal noise performance of a typical joint is compared to the requirements of the Japanese gravitational wave detector, KAGRA. It is shown that using such an indium joint in the suspension system allows it to operate with low thermal noise. Additionally, results on the maximum amount of heat which can be extracted via indium joints are presented. It is found that sapphire parts, joined by means of indium, are able to remove the residual heat load in the mirrors of KAGRA.