4,781 research outputs found
Robustness of circadian clocks to daylight fluctuations: hints from the picoeucaryote Ostreococcus tauri
The development of systemic approaches in biology has put emphasis on
identifying genetic modules whose behavior can be modeled accurately so as to
gain insight into their structure and function. However most gene circuits in a
cell are under control of external signals and thus quantitative agreement
between experimental data and a mathematical model is difficult. Circadian
biology has been one notable exception: quantitative models of the internal
clock that orchestrates biological processes over the 24-hour diurnal cycle
have been constructed for a few organisms, from cyanobacteria to plants and
mammals. In most cases, a complex architecture with interlocked feedback loops
has been evidenced. Here we present first modeling results for the circadian
clock of the green unicellular alga Ostreococcus tauri. Two plant-like clock
genes have been shown to play a central role in Ostreococcus clock. We find
that their expression time profiles can be accurately reproduced by a minimal
model of a two-gene transcriptional feedback loop. Remarkably, best adjustment
of data recorded under light/dark alternation is obtained when assuming that
the oscillator is not coupled to the diurnal cycle. This suggests that coupling
to light is confined to specific time intervals and has no dynamical effect
when the oscillator is entrained by the diurnal cycle. This intringuing
property may reflect a strategy to minimize the impact of fluctuations in
daylight intensity on the core circadian oscillator, a type of perturbation
that has been rarely considered when assessing the robustness of circadian
clocks
Digital phase-lock loop
An improved digital phase lock loop incorporates several distinctive features that attain better performance at high loop gain and better phase accuracy. These features include: phase feedback to a number-controlled oscillator in addition to phase rate; analytical tracking of phase (both integer and fractional cycles); an amplitude-insensitive phase extractor; a more accurate method for extracting measured phase; a method for changing loop gain during a track without loss of lock; and a method for avoiding loss of sampled data during computation delay, while maintaining excellent tracking performance. The advantages of using phase and phase-rate feedback are demonstrated by comparing performance with that of rate-only feedback. Extraction of phase by the method of modeling provides accurate phase measurements even when the number-controlled oscillator phase is discontinuously updated
Hardware simulation of KU-band spacecraft receiver and bit synchronizer, phase 2, volume 1
The acquisition behavior of the PN subsystem of an automatically acquiring spacecraft receiver was studied. A symbol synchronizer subsystem was constructed and integrated into the composite simulation of the receiver. The overall performance of the receiver when subjected to anomalies such as signal fades was evaluated. Potential problems associated with PN/carrier sweep interactions were investigated
Ultra-stable long distance optical frequency distribution using the Internet fiber network
We report an optical link of 540 km for ultrastable frequency distribution
over the Internet fiber network. The stable frequency optical signal is
processed enabling uninterrupted propagation on both directions. The robustness
and the performance of the link are enhanced by a cost effective fully
automated optoelectronic station. This device is able to coherently regenerate
the return optical signal with a heterodyne optical phase locking of a low
noise laser diode. Moreover the incoming signal polarization variation are
tracked and processed in order to maintain beat note amplitudes within the
operation range. Stable fibered optical interferometer enables optical
detection of the link round trip phase signal. The phase-noise compensated link
shows a fractional frequency instability in 10 Hz bandwidth of 5x10-15 at one
second measurement time and 2x10-19 at 30 000 s. This work is a significant
step towards a sustainable wide area ultrastable optical frequency distribution
and comparison network
Time Domain Simulations of Arm Locking in LISA
Arm locking is a technique that has been proposed for reducing laser
frequency fluctuations in the Laser Interferometer Space Antenna (LISA), a
gravitational-wave observatory sensitive in the milliHertz frequency band. Arm
locking takes advantage of the geometric stability of the triangular
constellation of three spacecraft that comprise LISA to provide a frequency
reference with a stability in the LISA measurement band that exceeds that
available from a standard reference such as an optical cavity or molecular
absorption line. We have implemented a time-domain simulation of arm locking
including the expected limiting noise sources (shot noise, clock noise,
spacecraft jitter noise, and residual laser frequency noise). The effect of
imperfect a priori knowledge of the LISA heterodyne frequencies and the
associated 'pulling' of an arm locked laser is included. We find that our
implementation meets requirements both on the noise and dynamic range of the
laser frequency.Comment: Revised to address reviewer comments. Accepted by Phys. Rev.
Multi-antenna GNSS Receiver Tracking Algorithm for Vehicles With Unconstrained Three-dimensional Motion
An algorithm for GNSS GPS/GLONASS receivers is presented that allows the receiver to seamlessly combine the inputs from multiple antennas during signal tracking in order to keep full sky visibility at all times. This algorithm has applications for both aeronautical and space applications in all kinds of vehicles with unconstrained three-dimensional motion capabilities: high maneuverability jet aircraft, rockets, satellites, unmanned aerial vehicles (UAVs), etc. The algorithm presented here keeps track of the received signal-to-noise ratios (SNR) and carrier phase on each of the antennas. For each set of correlation values the receiver selects the input antenna with the best SNR in order to do navigation message decoding and ranging. This fast-switching antenna selection process allows the receiver to stay synchronized with the incoming satellite signal for as long as at least one of the antennas of the set is within line-of-sight of the transmitting satellite. Distributing enough antennas throughout the fuselage of a vehicle, this algorithm will ensure that the receiver does not lose synchronization with the GNSS signals even during threedimensional maneuvers like spins and attitude changes. This algorithm was implemented on a four-antenna GNSS receiver prototype hardware, and tested using a GNSS signal simulator. During these tests the multi-antenna tracking algorithm performed successfully. The results of some of these tests are presented in this paper.Instituto de Investigaciones en Electrónica, Control y Procesamiento de Señale
Spur Reduction Techniques for Phase-Locked Loops Exploiting A Sub-Sampling Phase Detector
This paper presents phase-locked loop (PLL) reference-spur reduction design techniques exploiting a sub-sampling phase detector (SSPD) (which is also referred to as a sampling phase detector). The VCO is sampled by the reference clock without using a frequency divider and an amplitude controlled charge pump is used which is inherently insensitive to mismatch. The main remaining source of the VCO reference spur is the periodic disturbance of the VCO by the sampling at the reference frequency. The underlying VCO sampling spur mechanisms are analyzed and their effect is minimized by using dummy samplers and isolation buffers. A duty-cycle-controlled reference buffer and delay-locked loop (DLL) tuning are proposed to further reduce the worst case spur level. To demonstrate the effectiveness of the\ud
proposed spur reduction techniques, a 2.21 GHz PLL is designed and fabricated in 0.18 m CMOS technology. While using a high loop-bandwidth-to-reference-frequency ratio of 1/20, the reference spur measured from 20 chips is 80 dBc. The PLL consumes 3.8 mW while the in-band phase noise is 121 dBc/Hz at 200 kHz and the output jitter integrated from 10 kHz to 100 MHz is 0.3 ps rms
A study of Radiation-Tolerant Voltage-Controlled Oscillators designs in 65 nm bulk and 28 nm FDSOI CMOS technologies
Phase-locked loop (PLL) systems are widely employed in integrated circuits for space analog devices and communications systems that operate in radiation environments, where significant perturbations, especially in terms of phase noise, can be generated due to radiation particles. Among all the blocks that form a PLL system, previous research suggests the voltage-controlled oscillator (VCO) is one of the most critical components in terms of radiation tolerance and electric performance. Ring oscillators (ROs) and LC-tank VCOs have been commonly employed in high-performance PLLs. Nevertheless, both structures have drawbacks including a limited tuning range, high sensitivity to phase noise, limited radiation tolerance, and large design areas. In order to fulfill these high-performance requirements, a current-model logic (CML) based RO-VCO is presented as a possible solution capable of reducing the limitations of the commonly used structures and exploiting their advantages. The proposed hybrid VCO model includes passive components in its design which are the key parameters that define oscillation frequency of this structure. This tunable oscillator has been designed and tested in 65nm Bulk and 28 nm Fully depleted silicon-on-insulator (FDSOI) CMOS technologies
The 65nm testchip was designed to compare the behavior of the proposed CML VCO with a current-starved RO and a radiation hardened by design (RHBD) LC-tank VCO in terms of tuning range, phase noise, Single event effect (SEE) sensitivity and design area. Simulations were carried out by applying a double exponential current pulse into different sensitive nodes of the three VCOs. In addition, SEE tests were conducted using pulsed laser experiments. Simulation and test results show that a CML VCO can effectively overcome the limitations presented by a RO-VCO and LC-tank VCO, achieving a wide range of tuning, and low sensitivity to noise and SEEs without the need for a large cross-section.
Further studies of the proposed CML VCO were done on 28nm FDSOI in order to reduce the leakage current and increase the switching speed. the same current-starved VCO and CML VCO were implemented on this testchip, and simulations were performed by injecting a double exponential current pulse energy into the previously defined sensitive nodes. The results show SEE sensitivity improvement without narrowing the tuning range or affecting the phase noise response
Influence of Resonances on the Noise Performance of SQUID Susceptometers
Scanning Superconducting Quantum Interference Device (SQUID) Susceptometry simultaneously images the local magnetic fields and susceptibilities above a sample with sub-micron spatial resolution. Further development of this technique requires a thorough understanding of the current, voltage, and flux ( IVΦ ) characteristics of scanning SQUID susceptometers. These sensors often have striking anomalies in their current–voltage characteristics, which we believe to be due to electromagnetic resonances. The effect of these resonances on the performance of these SQUIDs is unknown. To explore the origin and impact of the resonances, we develop a model that qualitatively reproduces the experimentally-determined IVΦ characteristics of our scanning SQUID susceptometers. We use this model to calculate the noise characteristics of SQUIDs of different designs. We find that the calculated ultimate flux noise is better in susceptometers with damping resistors that diminish the resonances than in susceptometers without damping resistors. Such calculations will enable the optimization of the signal-to-noise characteristics of scanning SQUID susceptometers
PT-symmetric cross injection dual optoelectronic oscillator
An optoelectronic oscillator (OEO) is a time delay oscillator (TDO) that uses
photonics technology to provide the long delay required to generate pristine
microwave carriers. Parity-time (PT) symmetry concepts applied to an OEO offer
the potential to achieve combined low phase noise and high sidemode
suppression. A TDO composed of a pair of identical ring resonators coupled by a
2x2 coupler is modelled, and the coupler transmission matrix required for the
oscillator to be PT- symmetric is derived. In a first configuration, the
coupler is interpreted as the composition of a gain/loss block and a
Mach-Zehnder interferometer (MZI) block. In practice, there are excess losses
that must be compensated by a special dual amplifier with saturation
characteristics compatible with PT- symmetry. The PT- symmetry phase transition
determined by the gain/loss and the MZI differential phase parameters is found
to be global and not local in its effect on modes. This is resolved by placing
a short delay line within one arm of the MZI resulting in a frequency dependent
and hence local mode-selective PT- symmetry phase transition. In addition, it
is demonstrated that the first configuration may be transformed into a second
but equivalent configuration as a cross-injection dual TDO with imbalanced
delays. The local PT- symmetry phase transition may then be understood in terms
of the Vernier effect. Advantageously, the second configuration enables the
special dual amplifier to be replaced by a pair of matched but otherwise
independent amplifiers. Thereby, the second configuration is amenable to
practical implementation as a dual OEO using standard RF-photonic and
RF-electronic components. The theory is validated by complex envelope model
simulations using Simulink and phase model analytic results evaluated using
MATLAB. There is excellent agreement between the theoretical and simulation
results.Comment: 40 pages, 13 figure
- …