Thermal Noise Contribution Research Articles

SNR and Standard Deviation of cGNSS-R and iGNSS-R Scatterometric Measurements

This work addresses the accuracy of the Global Navigation Satellite Systems (GNSS)-Reflectometry (GNSS-R) scatterometric measurements considering the presence of both coherent and incoherent scattered components, for both conventional GNSS-R (cGNSS-R) and interferometric GNSS-R (iGNSS-R) techniques. The coherent component is present for some type of surfaces, and it has been neglected until now because it vanishes for the sea surface scattering case. Taking into account the presence of both scattering components, the estimated Signal-to-Noise Ratio (SNR) for both techniques is computed based on the detectability criterion, as it is done in conventional GNSS applications. The non-coherent averaging operation is considered from a general point of view, taking into account that thermal noise contributions can be reduced by an extra factor of 0.88 dB when using partially overlapped or partially correlated samples. After the SNRs are derived, the received waveform’s peak variability is computed, which determines the system’s capability to measure geophysical parameters. This theoretical derivations are applied to the United Kingdom (UK) TechDemoSat-1 (UK TDS-1) and to the future GNSS REflectometry, Radio Occultation and Scatterometry on board the International Space Station (ISS) (GEROS-ISS) scenarios, in order to estimate the expected scatterometric performance of both missions.

Receptivity to Thermal Noise of the Boundary Layer over a Swept Wing

The commonly accepted description of spatial transition to turbulence in a convectively unstable boundary layer involves as one of its stages the permanent excitation of instabilities by external noise (receptivity). The idea that thermal fluctuations of microscopic origin provide a sufficient amount of permanent noise to play a role in this process has made only scant and isolated appearances in the literature. Yet, the contribution of thermal noise to receptivity has its amplitude determined from physical first principles; it provides the lower bound beyond which disturbances cannot be reduced (and the upper bound beyond which transition cannot be delayed) lest thermodynamics is violated. As it is also not significantly harder to compute than a typical -factor plot, thermal-noise receptivity can be adopted as a standard against which to compare other forms of receptivity through a noise figure analogous to the one that is of commonplace use in electronics. Elaborating on previous computations of the effect of thermal noise upon the two-dimensional boundary layer past a flat plate, it is shown by a few real-world examples that the receptivity to thermal noise of the crossflow boundary layer over a swept wing is even larger, and by a substantial amount.

A Low Phase Noise NMOS LC-VCO using New Degeneration Structure

Background/Objectives: It is necessary to reduce the contribution of a differential pair for a low phase noise LC-VCO because previous works are mainly focused on the LC tank and tail current source. Methods/Statistical Analysis: A new NMOS LC-VCO with an LC degenerated structure is proposed for linearizing a cross coupled differential pair. The NMOS LC-VCO with the proposed structure is designed using a 0.18 μm CMOS process and simulated by Spectre RF. Findings: The proposed structure is very useful for suppressing the flicker and thermal noise contribution of a cross coupled differential pair which forms a negative transconductance. Especially, the proposed structure overcomes the limitation the previous capacitive degeneration structure when it operates at higher oscillator frequency above 5 GHz. Application/Improvements: Its phase noise performance is improved about 3 dB compared with that of the conventional NMOS LC-VCO structure with no degeneration consuming the same bias current of 2mA from a 1.8 V supply.

A Compact L-Band Orthomode Transducer for Radio Astronomical Receivers at Cryogenic Temperature

We describe the design, construction, and performance of a compact orthomode transducer (OMT) for the L-band receiver (1.3–1.8 GHz) of the Sardinia Radio Telescope (SRT). The complete OMT consists of a cylindrical orthomode junction (OMJ), which is presented in this paper, and two identical 180 $^{\circ}$ hybrid power combiners in a double-ridged waveguide, which have been proposed in a previous work. The OMT operates at the cryogenic temperature of 20 K to reduce its thermal noise contribution to the noise of the receiving chain. Therefore, particular care has been taken in the design of the OMJ to minimize its dimension and insure a good thermalization. The proposed OMT has been designed and optimized by using CST Microwave Studio, and then manufactured, tested at room temperature, and installed on the L-band receiver of the SRT. The measured results fully comply with the design specifications. In particular, the isolation between the OMT output ports is more than 40 dB, and the cross polarization is less than $-{\hbox{35 dB}}$ for both polarization channels.

Noise and Sensitivity of Harmonic Radar Architecture for Remote Sensing and Detection of Vital Signs

This paper addresses noise and sensitivity issues in remote sensing and detection of vital signs based on a continuous wave biomedical radar operating at multiple harmonic carrier frequencies or channels. This Doppler radar makes use of a single mixer, taking advantage of the inherent nonlinearity and harmonic characteristics of the mixer. Other system building elements such as antennas, amplifiers, and circulators can also operate at and comply with multiple harmonic frequencies or channels requirements, which makes the system compact. Noise is one of the most important factors that affect the sensitivity of this type of system. The total noise is the combined contribution of thermal noise, residual phase noise, and flicker noise. Flicker noise is found to be the critical parameter for the baseband detection. Experimental results show that with the use of the harmonic radar technique, the flicker noise can be reduced by 20 dB around 1-Hz baseband frequency compared with the counterpart in a conventional radar operating at single frequency. The noise and sensitivity of a harmonic radar system operating at 12 and 24 GHz for vital signs detection are studied theoretically and experimentally. It is proven that the harmonic radar solution is able to increase detection sensitivity by increasing the signal-to-noise ratio. The performance of the harmonic radar is tested experimentally with a moving plate and also a real patient. For the heartbeat detection, an oximeter giving the oxygen saturation of blood and heart rate is used as the reference.

Update on quadruple suspension design for Advanced LIGO

We describe the design of the suspension systems for the major optics for Advanced LIGO, the upgrade to LIGO—the Laser Interferometric Gravitational-Wave Observatory. The design is based on that used in GEO600—the German/UK interferometric gravitational wave detector, with further development to meet the more stringent noise requirements for Advanced LIGO. The test mass suspensions consist of a four-stage or quadruple pendulum for enhanced seismic isolation. To minimize suspension thermal noise, the final stage consists of a silica mirror, 40 kg in mass, suspended from another silica mass by four silica fibres welded to silica ears attached to the sides of the masses using hydroxide-catalysis bonding. The design is chosen to achieve a displacement noise level for each of the seismic and thermal noise contributions of 10−19 m/√Hz at 10 Hz, for each test mass. We discuss features of the design which has been developed as a result of experience with prototypes and associated investigations.

Experimental and modelling investigation of the influence of noise transit time and self-heating on microwave noise of Si/SiGe:C and InP/InGaAs HBTs

The results of a comprehensive experimental and modelling investigation of high-frequency noise (HFN) of Si/SiGe:C and InP/InGaAs heterojunction bipolar transistors (HBTs) demonstrate that the key parameters to correctly model HFN for both HBTs are the self-heating (SH) effect at high collector injection levels, i.e. JC > 2–3 mA µm−2, and the noise transport time (τ) which must be included at all bias levels. In contrast, thermal noise contribution of the base–emitter junction of the Si/SiGe:C HBT, associated with the diffusive nature of electron transport at this junction because of the lack of conduction band discontinuity, is only important at operation frequencies lying in the frequency range f > fT/2. The noise analysis indicates that in order to correctly model HFN performances at any injection level and operation frequency, τ must be constant with bias and equal to the addition of base and collector transit times (τB + τC). On the other hand, at high JC levels if SH is neglected, then the amplitude of the equivalent noise resistance (Rn) will be the most impacted noise parameter for both HBTs. Concerning the InP/InGaAs HBT, if SH is off at JC = 5.5 mA µm−2, the minimum noise figure (NFmin) of the InP HBT will be underestimated by 10%.

System- and circuit-level optimization of PLL designs for DVB-T/H receivers

This article proposes a framework for the optimization of voltage-controlled oscillator (VCO) designs in frequency synthesizers for digital video broadcasting --- terrestrial/handheld (DVB-T/H) receivers. Linear time-invariant phase-domain model of a charge-pump phase-locked loop (PLL) is devised and includes both flicker (1/f) and thermal noise contributions from the loop oscillators. By modeling the entire receiver, it is shown that there are combinations of flicker and thermal noise contributions that result in a constant sum of inter-carrier interference (ICI) and adjacent channel interference, and constant symbol error rate as well. Consequently, optimization of the VCO phase noise spectrum is defined while maintaining the standard-specified symbol-error rate. Link-level performance evaluation is carried out to validate the stipulated trade-off. The effect of ICI mitigation schemes is discussed. Circuit-level VCO design approaches utilizing the derived trade-off are finally presented. The proposed optimization procedure is generic and is also applicable to other systems based on Orthogonal Frequency-Division Multiplexing.

Phase stability in fMRI time series: Effect of noise regression, off-resonance correction and spatial filtering techniques

Although the majority of fMRI studies exploit magnitude changes only, there is an increasing interest regarding the potential additive information conveyed by the phase signal. This integrated part of the complex number furnished by the MR scanners can also be used for exploring direct detection of neuronal activity and for thermography. Few studies have explicitly addressed the issue of the available signal stability in the context of phase time-series, and therefore we explored the spatial pattern of frequency specific phase fluctuations, and evaluated the effect of physiological noise components (heart beat and respiration) on the phase signal. Three categories of retrospective noise reduction techniques were explored and the temporal signal stability was evaluated in terms of a physiologic noise model, for seven fMRI measurement protocols in eight healthy subjects at 3T, for segmented CSF, gray and white matter voxels. We confirmed that for most processing methods, an efficient use of the phase information is hampered by the fact that noise from physiological and instrumental sources contributes significantly more to the phase than to the magnitude instability. Noise regression based on the phase evolution of the central k-space point, RETROICOR, or an orthonormalized combination of these were able to reduce their impact, but without bringing phase stability down to levels expected from the magnitude signal. Similar results were obtained after targeted removal of scan-to-scan variations in the bulk magnetic field by the dynamic off-resonance in k-space (DORK) method and by the temporal off-resonance alignment of single-echo time series technique (TOAST). We found that spatial high-pass filtering was necessary, and in vivo a Gaussian filter width of 20mm was sufficient to suppress physiological noise and bring the phase fluctuations to magnitude levels. Stronger filters brought the fluctuations down to levels dictated by thermal noise contributions, and for 62.5mm(3) voxels the phase stability was as low as 5 mrad (0.27°). In conditions of low SNR(o) and high temporal sampling rate (short TR); we achieved an upper bound for the phase instabilities at 0.0017 ppm, which is close to the dHb contribution to the GM/WM phase contrast.

Virgo gravitational wave detector: Results and perspectives

The Virgo detector reached during the past science run a sensitivity very close to the design one. During the last year the detector has been improved by suspending the main interferometer mirrors with monolithic fibers, with the goal of reducing the thermal noise contribution and testing the new technology. At the same time the design of the next detector improvements are on-going and they will be implemented during the construction of Advanced Virgo.

Analysis of Direct-Conversion IQ Transmitters With 25% Duty-Cycle Passive Mixers

The performance of direct-conversion IQ transmitters with 25% duty-cycle passive mixers is analyzed. The up-conversion transfer function is calculated and it is shown that due to lack of reverse isolation of the passive mixer, the high- and low-side conversion gains can be different. The contribution of thermal noise from mixer switches to the total output noise of the transmitter is formulated. It is also revealed that the up-conversion of the baseband signal to higher order harmonics places more stringent linearity requirements on the post-mixer driver.

Behavioral modeling of pipeline ADC building blocks

SUMMARYThis paper presents accurate behavioral models for the basic building blocks of pipeline data converters with emphasis on the MDAC circuit. These models take into account major circuit‐level non‐idealities, including small‐ and large‐signal effects, as well as the impact of switch‐on resistance effects and thermal noise contributions. The behavioral models have been validated against transistor‐level simulations under different scenarios, showing in all cases a worst‐case deviation of 0.3 bit effective resolution. Copyright © 2011 John Wiley & Sons, Ltd.

The dynamics of monolithic suspensions for advanced detectors: A 3-segment model

In order to reduce the suspension thermal noise, the second generation GW interferometric detectors will employ monolithic suspensions in fused silica to hold the mirrors. The fibres are produced by melting and pulling apart a fused silica rod, obtaining a long thin wire with two thicker heads. The dynamics of such a fibre is in principle different from that of a cylindrical, regular fibre, because most of the deformation energy is stored in the neck region where the diameter is variable. This is an advantage, since adjusting the neck tapering, a thermoelastic noise cancellation effect can be obtained. Therefore, a careful study of the suspensions behavior is necessary to estimate the overall noise and to optimize the control strategy. To simplify the control design, a simple three segment model for the silica fibres has been developed, fully equivalent to the beam equation at low frequencies. The model, analytically proved for a regular cylindrical fibre, can be extended to a fibre with tapered necks, provided that the equivalent bending length is suitably measured. We developed a tool to measure the position of the bending point for each fibre, thus allowing to experimentally check the validity of the model. A numerical code has been written to solve the beam equation for wires with varying diameter. This code confirms the validity of the three segment model. Moreover, it is possible to extend the solution to higher frequencies thus computing the transfer function and the energy distribution of the suspension system and estimating the thermal noise contribution.

Silicate bonding properties: Investigation through thermal conductivity measurements

A direct approach to reduce the thermal noise contribution to the sensitivity limit of a GW interferometric detector is the cryogenic cooling of the mirrors and mirrors suspensions. Future generations of detectors are foreseen to implement this solution. Silicon has been proposed as a candidate material, thanks to its very low intrinsic loss angle at low temperatures and due to its very high thermal conductivity, allowing the heat deposited in the mirrors by high power lasers to be efficiently extracted. To accomplish such a scheme, both mirror masses and suspension elements must be made of silicon, then bonded together forming a quasi-monolithic stage. Elements can be assembled using hydroxide-catalysis silicate bonding, as for silica monolithic joints. The effect of Si to Si bonding on suspension thermal conductance has therefore to be experimentally studied. A measurement of the effect of silicate bonding on thermal conductance carried out on 1 inch thick silicon bonded samples, from room temperature down to 77 K, is reported. In the explored temperature range, the silicate bonding does not seem to affect in a relevant way the sample conductance.

Investigation of mechanical dissipation in CO2 laser-drawn fused silica fibres and welds

The planned upgrades to the LIGO gravitational wave detectors include monolithic mirror suspensions to reduce thermal noise. The mirrors will be suspended using CO2 laser-drawn fused silica fibres. We present here measurements of mechanical dissipation in synthetic fused silica fibres drawn using a CO2 laser. The level of dissipation in the surface layer is investigated and is found to be at a similar level to fibres produced using a gas flame. Also presented is a method for examining dissipation at welded interfaces, showing clear evidence of the existence of this loss mechanism which forms an additional component of the total detector thermal noise. Modelling of a typical detector suspension configuration shows that the thermal noise contribution from this loss source will be negligible.

A systematic design approach for low-power 10-bit 100 MS/s pipelined ADC

A systematic design approach for low-power 10-bit, 100MS/s pipelined analog-to-digital converter (ADC) is presented. At architectural level various per-stage-resolution are analyzed and most suitable architecture is selected for designing 10-bit, 100MS/s pipeline ADC. At Circuit level a modified wide-bandwidth and high-gain two-stage operational transconductance amplifier (OTA) proposed in this work is used in track-and-hold amplifier (THA) and multiplying digital-to-analog converter (MDAC) sections, to reduce power consumption and thermal noise contribution by the ADC. The signal swing of the analog functional blocks (THA and MDAC sections) is allowed to exceed the supply voltage (1.8V), which further increases the dynamic range of the circuit. Charge-sharing comparator is proposed in this work, which reduces the dynamic power dissipation and kickback noise of the comparator circuit. The bootstrap technique and bottom plate sampling technique is employed in THA and MDAC sections to reduce the nonlinearity error associated with the input signal resulting in a signal-to-noise-distortion ratio of 58.72/57.57dB at 2MHz/Nyquist frequency, respectively. The maximum differential nonlinearity (DNL) is +0.6167/−0.3151LSB and the maximum integral nonlinearity (INL) is +0.4271/−0.4712LSB. The dynamic range of the ADC is 58.72dB for full-scale input signal at 2MHz input frequency. The ADC consumes 52.6mW at 100MS/s sampling rate. The circuit is implemented using UMC-180nm digital CMOS technology.

Sources of functional magnetic resonance imaging signal fluctuations in the human brain at rest: a 7 T study

Signal fluctuations in functional magnetic resonance imaging (fMRI) can result from a number of sources that may have a neuronal, physiologic or instrumental origin. To determine the relative contribution of these sources, we recorded physiological (respiration and cardiac) signals simultaneously with fMRI in human volunteers at rest with their eyes closed. State-of-the-art technology was used including high magnetic field (7 T), a multichannel detector array and high-resolution (3 mm(3)) echo-planar imaging. We investigated the relative contribution of thermal noise and other sources of variance to the observed fMRI signal fluctuations both in the visual cortex and in the whole brain gray matter. The following sources of variance were evaluated separately: low-frequency drifts due to scanner instability, effects correlated with respiratory and cardiac cycles, effects due to variability in the respiratory flow rate and cardiac rate, and other sources, tentatively attributed to spontaneous neuronal activity. We found that low-frequency drifts are the most significant source of fMRI signal fluctuations (3.0% signal change in the visual cortex, TE=32 ms), followed by spontaneous neuronal activity (2.9%), thermal noise (2.1%), effects due to variability in physiological rates (respiration 0.9%, heartbeat 0.9%), and correlated with physiological cycles (0.6%). We suggest the selection and use of four lagged physiological noise regressors as an effective model to explain the variance related to fluctuations in the rates of respiration volume change and cardiac pulsation. Our results also indicate that, compared to the whole brain gray matter, the visual cortex has higher sensitivity to changes in both the rate of respiration and the spontaneous resting-state activity. Under the conditions of this study, spontaneous neuronal activity is one of the major contributors to the measured fMRI signal fluctuations, increasing almost twofold relative to earlier experiments under similar conditions at 3 T.

A 52.6 mW 10-bit, 100 MS/s Pipelined CMOS Analog-To-Digital Converter

The design of 10-bit, 100 MS/s, pipelined analog-to-digital converter (ADC) is presented. A wide-bandwidth and high gain two-stage operational trans-conductance amplifier (OTA) is used in Track-and-Hold Amplifier (THA) and Multiplying Digital-to-Analog Converter (MDAC) sections, to reduce power consumption and thermal noise contribution by the ADC. The signal swing of the analog functional blocks (THA and MDAC sections) is allowed to exceed the supply voltage (1.8 V), which further reduces the thermal noise contributed by the circuit and increases the dynamic range of the circuit. Charge sharing comparator is proposed in this work, which reduces the dynamic power dissipation and kick-back noise of the comparator circuit. The bootstrap technique and bottom plate sampling technique is employed in THA and MDAC sections to reduce the non-linearity error associated with the input signal resulting in a signal to noise distortion ratio of 58.72 dB/57.57 dB @ 2 MHz/ Nyquist frequency respectively. The maximum differential non-linearity (DNL) is +0.6167/ -0.3151 LSB and the maximum integral nonlinearity (INL) is +0.4271/-0.4712 LSB. The dynamic range of the ADC is 58.72 dB for full scale input signal @ 2 MHz input frequency. The ADC consumes 52.6 mW at 100 MS/s sampling rate. The circuit is implemented using UMC-180 nm digital CMOS technology.

High mechanical Q-factor measurements on silicon bulk samples

Future gravitational wave detectors will be limited by different kinds of noise. Thermal noise from the coatings and the substrate material will be a serious noise contribution within the detection band of these detectors. Cooling and the use of a high mechanical Q-factor material as a substrate material will reduce the thermal noise contribution from the substrates. Silicon is one of the most interesting materials for a third generation cryogenic detector. Due to the fact that the coefficient of thermal expansion vanishes at 18 and 125 K the thermoelastic contribution to the thermal noise will disappear. We present a systematic analysis of the mechanical Q-factor at low temperatures between 5 and 300 K on bulk silicon (100) samples which are boron doped. The thickness of the cylindrical samples is varied between 6, 12, 24, and 75mm with a constant diameter of 3 inches. For the 75mm substrate a comparison between the (100) and the (111) orientation is presented. In order to obtain the mechanical Q-factor a ring-down measurement is performed. Thus, the substrate is excited to resonant vibrations by means of an electrostatic driving plate and the subsequent ring-down is recorded using a Michelson-like interferometer. The substrate itself is suspended as a pendulum by means of a tungsten wire loop. All measurements are carried out in a special cryostat which provides a temperature stability of better than 0.1K between 5 and 300K during the experiment. The influence of the suspension on the measurements is experimentally investigated and discussed. At 5.8K a highest Q-factor of 4.5 × 108 was achieved for the 14.9 kHz mode of a silicon (100) substrate with a diameter of 3 inches and a thickness of 12 mm.

Analytical expressions for the conductance noise measured with four circular contacts placed in a square array

In the ideal case, noise measurements with four contacts minimize the contribution of the contact interface. There is a need to characterize conductance noise and noise correction factors for the different geometries provided with four contacts, as already is the case for resistivity measurements with van der Pauw structures. Here, we calculate the noise correction factors for two geometries with a pair of sensors and a pair of current driver electrodes placed in a square array. The first geometry investigated is a very large film compared to the distance L between four circular electrodes, which are placed in a square array far away from the borders of the film. The second is a square-shaped conductive film with side length L and provided with four quarter-circle corner contacts with radius l. The effect of the conductance noise in the film can be observed between current free sensors in a four-point measurement or between current carrying drivers in a two-point measurement. Our analytical expressions are based on approximations to solve the integrals ∬(J∙J̃)2dA and ∬∣J∣4dA for the voltage noise measured across a pair of sensors, SVQ, and across the drivers, SVD, respectively. The first and second integrands represent the squared dot product of the current density and adjoint current density and the modulus of the current density to the fourth power, respectively. The current density J in the samples is due to the current I passing through the driver contacts. The calculated expressions are applicable to samples with thickness t⪡l⩽0.1L. Hence, the disturbances in the neighborhood of the sensors on J and of the drivers on J̃ are ignored. Noise correction factors for two- and four-point measurements are calculated for sensors on an equipotential (transversal noise) with the driver contacts on the diagonal of a square and for sensors next to each other on one side of the square with the drivers next to each other on the other side of the square (longitudinal noise). In all cases the noise between the sensors is smaller and less sensitive to the contact size 2l∕L than the noise between the drivers. The ratio SVQ∕SVD becomes smaller with smaller contact radius l. Smaller sensors give a better suppression of interface noise at the contacts. But overly low 2l∕L values result in overly high resistance between the sensors and too strong a contribution of thermal noise at the sensors. Therefore, equations are derived to calculate the current level needed to observe 1∕f conductance fluctuations on top of the thermal noise. The results from the calculated analytical expressions show good agreement with experimental results obtained from the noise in carbon sheet resistance and numerical results. Transversal noise measurements on a square sample with corner contacts are recommended to characterize the 1∕f noise of the layer. This is due to the increased current densities in the sample compared to the open structure, which result in easier detection of the 1∕f on top of the thermal noise.