Showing papers on "Frequency drift published in 2017"

PWM and PFM Hybrid Control Method for LLC Resonant Converters in High Switching Frequency Operation

Abstract: High switching frequency is an effective method to improve power density for LLC resonant converters. However, conventional digital controllers, such as general-purpose digital signal processors and microprocessors, have limited frequency resolution, which induces high primary- and secondary-side current variation and leads to poor output voltage regulation. In this paper, a hybrid control method combining pulse frequency modulation (PFM) and pulse width modulation is proposed to overcome the limited frequency resolution issue. The proposed hybrid control method focuses on steady-state operation, and its operating principles are introduced and analyzed. In addition, the proper magnetizing inductance and dead time duration are derived to ensure that the power mosfet s achieve zero voltage switching with the proposed control method. The improved voltage regulation performance is compared with the conventional PFM control and verified through simulation and experimental results using a 240 W prototype converter operating at a switching frequency of 1 MHz.

An evaluation of frequency methods for creating a system to control semiconductor converters

Abstract: I explore the possibility of increasing the response speed of the semiconductor converters that operate in the pulse mode by increasing the carrier frequency or rejecting pulse-width modulation (PWM) in favor of frequency PWM. The relation between the PWM carrier frequency and the limit response speed of the current loop achievable in closed control systems using classical frequency methods for creation of linear systems has been established. A model of mathematical simulation of a semiconductor device is proposed. The model is approximated by ideal elements without considering the voltage drop on the semiconductor keys and the dead-time interval. Using the simulation model, the frequency characteristics of of the frequency converters are compared with one converter operating in the linear mode and the other in the pulse mode. The limiting value of the frequency starting from which the frequency characteristics of the linear and pulse systems diverge has been determined for different kinds of the transfer functions of the “unchangeable” part. It has been established that this frequency approaches the Nyquist frequency most closely at resonance-peak amplitude A m > 1.

Low-Drift Coherent Population Trapping Clock Based on Laser-Cooled Atoms and High-Coherence Excitation Fields

Abstract: Atomic clocks based on coherent population trapping (CPT) are of great interest for portable applications, due to their high performance, low power requirements, and potential for miniaturization. Slow frequency drift, however, limits their holdover ability on long time scales. This study reports a CPT clock that minimizes long-term drift, with a fractional frequency stability of $3\ifmmode\times\else\texttimes\fi{}{10}^{-13}$ over the span of an hour---ten times as good as previous CPT clocks. After successful shrinking of the laser-cooling apparatus, this clock could see commercial use, as the atoms fall just a few millimeters during a typical probe period.

Simulation of VLF chorus emissions in the magnetosphere and comparison with THEMIS spacecraft data

Abstract: We present results of numerical simulations of VLF chorus emissions based on the backward wave oscillator (BWO) model and compare them with THEMIS spacecraft data from the equatorial chorus source region on the early morning side at a radial distance of 6 Earth radii. Specific attention is paid to the choice of simulation parameters based on experimental data. We show that with known parameters of the geomagnetic field, plasma density, and the initial wave frequency, one can successfully reproduce individual chorus elements in the simulation. In particular, the measured growth rate, wave amplitude, and frequency drift rate are in agreement with observed values. The characteristic interval between the elements has a mismatch of factor 2. The agreement becomes perfect if we assume that the inhomogeneity scale of the magnetic field along the field line is half of that obtained from the T96 model. Such an assumption can be justified since the T96 model does not fit well for the time of chorus observations, and there is a shear in the observed field which indicates the presence of local currents.

Wavelet-based Characterization of Small-scale Solar Emission Features at Low Radio Frequencies

Abstract: Low radio frequency solar observations using the Murchison Widefield Array have recently revealed the presence of numerous weak short-lived narrowband emission features, even during moderately quiet solar conditions. These nonthermal features occur at rates of many thousands per hour in the 30.72 MHz observing bandwidth, and hence necessarily require an automated approach for their detection and characterization. Here, we employ continuous wavelet transform using a mother Ricker wavelet for feature detection from the dynamic spectrum. We establish the efficacy of this approach and present the first statistically robust characterization of the properties of these features. In particular, we examine distributions of their peak flux densities, spectral spans, temporal spans, and peak frequencies. We can reliably detect features weaker than 1 SFU, making them, to the best of our knowledge, the weakest bursts reported in literature. The distribution of their peak flux densities follows a power law with an index of −2.23 in the 12–155 SFU range, implying that they can provide an energetically significant contribution to coronal and chromospheric heating. These features typically last for 1–2 s and possess bandwidths of about 4–5 MHz. Their occurrence rate remains fairly flat in the 140–210 MHz frequency range. At the time resolution of the data, they appear as stationary bursts, exhibiting no perceptible frequency drift. These features also appear to ride on a broadband background continuum, hinting at the likelihood of them being weak type-I bursts.

Parametric frequency mixing in a magnetoelastically driven linear ferromagnetic-resonance oscillator

Abstract: We demonstrate the linear frequency conversion of ferromagnetic resonance (FMR) frequency by optically excited elastic waves in a thin metallic film on dielectric substrates. Time-resolved probing of the magnetization directly witnesses magnetoelastically driven parametric second-harmonic generation, sum- and difference-frequency mixing from two distinct frequencies, as well as excitation of parametric resonances. Starting from the Landau-Lifshitz-Gilbert equations, we derive an analytical equation of an elastically driven (nonlinear) parametric oscillator and show that frequency mixing is dominated by the parametric modulation of the linear FMR oscillator.

Parametric Noise Reduction in a High-Order Nonlinear MEMS Resonator Utilizing Its Bifurcation Points

Abstract: An electrostatically actuated non-linear microelectromechanical systems (MEMS) resonator can describe double hysteresis behavior in the measured frequency response due to the interplay between electrical and mechanical non-linearities in the system. This paper provides the first experimental mapping of the stable and unstable branches of the frequency response of a MEMS resonator describing a double hysteretic frequency response using a closed-loop phase feedback oscillator. Furthermore, the frequency stability of the oscillator is compared for varying amplitude and phase feedback conditions, and it is experimentally demonstrated that parametric noise up-conversion can be suppressed in such a system by suitably biasing the resonator at one of the four bifurcation points in such a system. This result is qualitatively consistent with theoretical prediction and demonstrates that improved frequency stability in a non-linear MEMS oscillator is possible by suitably biasing the resonator using simultaneous amplitude and phase feedback.

A 51.3-MHz 21.8-ppm/°C CMOS Relaxation Oscillator With Temperature Compensation

Abstract: A 51.3-MHz 18- $\mu\text{W}$ 21.8-ppm/°C relaxation oscillator is presented in 90-nm CMOS. The proposed oscillator employs an integrated error feedback and composite resistors to minimize its sensitivity to temperature variations. For a temperature range from −20 °C to 100 °C, the fabricated circuit demonstrates a frequency variation less than ±0.13%, leading to an average frequency drift of 21.8 ppm/°C. As the supply voltage changes from 0.8 to 1.2 V, the frequency variation is ±0.53%. The measured rms jitter and phase noise at 1-MHz offset are 89.27 ps and −83.29 dBc/Hz, respectively.

Low-Drift Optoelectronic Oscillator Based on a Phase Modulator in a Sagnac Loop

Abstract: A low-drift optoelectronic oscillator (OEO) is developed and experimentally shown. In the proposed OEO, a fiber Sagnac interferometer, including an optical phase modulator (PM) and a nonreciprocal bias unit, functions as an intrinsically drift-free intensity modulator. The phase noise of the proposed OEO is modeled in phase space, which is verified by experiments. Phase noise and frequency stability of the photonically generated microwave signals are measured. The single-sideband phase noise of the generated microwave signal is −106.6 dBc/Hz at 10-kHz offset from the 10.833-GHz carrier, with 120-fs rms timing jitter integrated from 1 kHz to 10 MHz. Frequency drift measurements show ±0.85-ppm maximum fractional frequency deviation over 35 h, mainly caused by drift of the fiber delay line.

Laser frequency stabilization by combining modulation transfer and frequency modulation spectroscopy.

Abstract: We present a hybrid laser frequency stabilization method combining modulation transfer spectroscopy (MTS) and frequency modulation spectroscopy (FMS) for the cesium D2 transition. In a typical pump-probe setup, the error signal is a combination of the DC-coupled MTS error signal and the AC-coupled FMS error signal. This combines the long-term stability of the former with the high signal-to-noise ratio of the latter. In addition, we enhance the long-term frequency stability with laser intensity stabilization. By measuring the frequency difference between two independent hybrid spectroscopies, we investigate the short-and long-term stability. We find a long-term stability of 7.8 kHz characterized by a standard deviation of the beating frequency drift over the course of 10 h and a short-term stability of 1.9 kHz characterized by an Allan deviation of that at 2 s of integration time.

A Unified Approach for Automatic Resonant Frequency Tracking in LLC DC–DC Converter

Abstract: This paper presents a unified control scheme for automatic resonant frequency tracking (ARFT) in unregulated LLC series resonant dc–dc converters The proposed strategy detects frequency drift by monitoring either phase or gain relationship of an exclusive variable pair in an LLC tank Considering both phase angle and amplitude ratio of the state variables, the plant is modeled for a realistic ramp variation in resonant frequency A two-step approach is proposed to choose the most suitable variable pair for ARFT First, the best candidates for phase and gain comparison are selected individually based on the sensitivity criteria Then, the final choice of variable pair is made by analytically deriving the frequency tracking error due to realistic variation in tank parameters and nonidealities The proposed controller is implemented using analog electronics and the practical limit for circuit delays during low cost realization is clarified Performance evaluation of the proposed control scheme is carried out on a lab prototype of the LLC converter

Stability-Oriented Design of Frequency Drift Anti-Islanding and Phase-Locked Loop Under Weak Grid

Abstract: The stability of a grid-tied, power conditioning system (PCS)—equipped with frequency drift anti-islanding—is analyzed in this paper. To reduce the nondetection zone (NDZ), the positive feedback gain (PFG) of the frequency drift method should be increased. However, it was reported that PFG is restricted due to stability issues of the system under a weak grid. Careful analysis revealed that such a restriction is also related to the parameters of the phase-locked loop (PLL). As the parameters of PLL are represented by the damping coefficient and natural frequency, which demonstrate the second-order filter characteristics, the investigation shows that the damping coefficient has a close relation to the maximum PFG for a stable operation of the system in the weak grid. To enhance the stability margin and increase PFG, an alternative method is proposed to estimate grid frequency. Since the proposed method reduces the negative resistance region in PCS, it can be stabilized more than the conventional estimation method. Furthermore, the PFG can be increased to reduce NDZ.

A 1-MHz on-chip relaxation oscillator with comparator delay cancelation

Abstract: In this work, an improved topology of a relaxation oscillator is proposed, dealing with the non-idealities of the comparator stage. The oscillator test chips, manufactured in 0.35-μm CMOS process, have the nominal frequency of 1 MHz and typical power consumption of 210 μW. The area of the oscillator core is 0.04 mm2. The measured temperature variation of the output frequency is ±0.4% in the temperature range from −40 to 125 °C. The line sensitivity is 0.28 %/V with the supply ranging from 3.0 to 4.5 V. The output frequency stabilizes after one cycle.

Distributed Injection-Locked Frequency Dividers

Abstract: Distributed injection-locked frequency division is introduced as a method to increase the locking range beyond that of conventional injection-locked frequency dividers It is analytically shown that continuous frequency division can be achieved over a frequency range that spans over multiples of the self-oscillation frequency of the core divider Design techniques in millimeter-waves are discussed in detail A proof-of-concept prototype, realized in a foundry 130-nm BiCMOS SiGe HBT technology, achieves a measured locking range of 35–44 and 41–595 GHz while consuming 38 mW from a 115-V supply

Crystal-free narrow-band radios for low-cost IoT

Abstract: A transceiver was designed and fabricated in 65 nm CMOS to verify the feasibility of using a free running, on-chip LC tank as the local oscillator in an IEEE 802.15.4 transceiver. The elimination of the off-chip frequency reference is possible while still using a standards based narrow-band architecture. A free running LC tank is shown to have frequency stability better than ± 40 ppm in the absence of temperature changes. Demodulator-based feedback is implemented to allow a receiver to track transmitter drift due to varying environmental factors and phase noise. The modulation accuracy of a free-running open loop Minimum Shift Key (MSK) transmitter is shown to be within the limits set by IEEE 802.15.4.

Oscillation Maps in the Broadband Radio Spectrum of the 1 August 2010 Event

Abstract: We search for indications of waves in the 25 – 2000 MHz radio spectrum of the 1 August 2010 event (SOL2010-08-01T08:57:00L075C013), where fast propagating waves in the solar corona with periods of 181, 69, and 40 seconds were detected in UV observations. Using the wavelet technique, we construct a new type of oscillation map for selected periods in the whole domain of the radio spectrum. While an oscillation with a period of 181 seconds was recognized in the whole 25 – 2000 MHz radio spectrum, oscillations with periods of 69 and 40 seconds were only confirmed in the 250 – 870 MHz frequency range. In the 800 – 2000 MHz range we found periods of 50 and 80 seconds. Moreover, in the 250 – 870 MHz frequency range, an oscillation with a period of about 420 seconds was detected. We also made maps of phases of the 181-second oscillations in order to analyze their frequency drift. At the beginning of the radio event, the phase of the 181-second oscillation in the 2000 – 500 MHz frequency range drifts toward lower frequencies. On the other hand, we found that the phase is nearly synchronous at frequencies 25 – 500 MHz. While the phase drift at higher frequencies can be interpreted as being caused by the UV wave, the synchronization of the phase on lower frequencies is explained by the fast electron beams, whose acceleration is modulated by the UV wave. Owing to this modulation, the electron beams are accelerated with the period of the UV wave (181 seconds). These beams propagate upward through the solar corona and generate the 25 – 500 MHz radio emission with the 181-second period. The 25 – 500 MHz radio emission, which corresponds to a large interval of heights in the solar corona, is nearly synchronous because of the high beam velocity ( ${\approx}\,\mathrm{c}/3$ , where c is the light speed).

Frequency stability improvement for piezoresistive micromechanical oscillators via synchronization

Abstract: Synchronization phenomenon first discovered in Huygens’ clock shows that the rhythms of oscillating objects can be adjusted via an interaction. Here we show that the frequency stability of a piezoresistive micromechanical oscillator can be enhanced via synchronization. The micromechanical clamped-clamped beam oscillator is built up using the electrostatic driving and piezoresistive sensing technique and the synchronization phenomenon is observed after coupling it to an external oscillator. An enhancement of frequency stability is obtained in the synchronization state. The influences of the synchronizing perturbation intensity and frequency detuning applied on the oscillator are studied experimentally. A theoretical analysis of phase noise leads to an analytical formula for predicting Allan deviation of the frequency output of the piezoresistive oscillator, which successfully explains the experimental observations and the mechanism of frequency stability enhancement via synchronization.

Tunable Low-Jitter Low-Drift Spurious-Free Transposed-Frequency Optoelectronic Oscillator

Abstract: We propose and theoretically and experimentally demonstrate a novel tunable spurious-free single-loop optoelectronic oscillator (OEO) with low drift and low-phase noise. In the proposed transposed-frequency OEO (TF-OEO), a nonreciprocal bias unit and an optical phase modulator in a fiber Sagnac interferometer function jointly as an intrinsically drift-free intensity modulator, which improves the long-term drift. Besides, a transposed-frequency low-noise filtered amplifier is used which replaces the conventional radio frequency (RF) bandpass filter (BPF) and RF amplifier with an intermediate frequency (IF) BPF, an ultralow phase noise IF amplifier, and a tunable local oscillator, to attain frequency tuning and single-frequency selection with ultralow phase noise at the same time. The quality of the generated microwave signals is theoretically investigated and verified by experiments. Preliminary phase noise, frequency stability, spurious noise levels, and frequency tunability of the photonically generated microwave signal are also investigated. A microwave signal with a frequency tunable range of 15 MHz around 10.833 GHz is generated with no spurs. The generated microwave oscillation has a single-sideband phase noise of −120 dBc/Hz at 10 kHz offset from 10.833 GHz carrier, with 36 fs RMS timing jitter integrated from 1 kHz to 10 MHz. Long-term frequency stability measurements show ±0.05 ppm maximum fractional frequency deviation over 60 h, which is mainly limited by drift of the fiber delay line. The measured results show the long-term frequency stability (in terms of overlapping Allan deviation) within $8.7\times 10^{-9}$ at 1000 s averaging time.

Designing RF Ring Oscillator Using Current-Mode Technology

Abstract: In this paper, a new method of designing RF ring oscillator using current-mode technology is presented. The current-mode low frequency oscillator theory is transferred to facilitate the design of RF ring oscillator. As an example, a current-mode RF quadrature ring oscillator (RFQRO) for $L$ -band wireless communication applications is presented. The current-mode RFQRO consists of two first-order all-pass filters, and it provides four quadrature RF sinusoidal outputs. The proposed current-mode RFQRO is verified with GlobalFoundries’ $0.18~\mu \text{m}$ 1P6M CMOS RF process. The simulated and measured results demonstrate that the proposed RFQRO works at 1.03 GHz, and its frequency tuning range is from 648.9 MHz to 1.17 GHz by adjusting the bias voltage from 0.62 to 1 V. The measured phase noise of the RFQRO is −105.5 dBc/Hz at 1 MHz offset, and it consumes 2.5 mW from 1 V supply voltage.

Laser frequency stabilization by combining modulation transfer and frequency modulation spectroscopy

Abstract: We present a hybrid laser frequency stabilization method combining modulation transfer spectroscopy (MTS) and frequency modulation spectroscopy (FMS) for the cesium D2 transition. In a typical pump-probe setup, the error signal is a combination of the DC-coupled MTS error signal and the AC-coupled FMS error signal. This combines the long-term stability of the former with the high signal-to-noise ratio of the latter. In addition, we enhance the long-term frequency stability with laser intensity stabilization. By measuring the frequency difference between two independent hybrid spectroscopies, we investigate the short-term and long-term stability. We find a long-term stability of 7.8 kHz characterized by a standard deviation of the beating frequency drift over the course of 10 hours, and a short-term stability of 1.9 kHz characterized by an Allan deviation of that at 2 seconds of integration time.

Retrospective correction of frequency drift in spectral editing: The GABA editing example.

Abstract: GABA levels can be measured using proton MRS with a two-step editing sequence. However due to the low concentration of GABA, long acquisition time is usually needed to achieve sufficient SNR to detect small differences in many psychiatric disorders. During this long scan time the frequency offset of the measured voxel can change because of magnetic field drift and patient movement. This drift will change the frequency of the editing pulse relative to that of metabolites, leading to errors in quantification. In this article we describe a retrospective method to correct for frequency drift in spectral editing. A series of reference signals for each metabolite was generated for a range of frequency offsets and then averaged together based on the history of frequency changes over the scan. These customized basis sets were used to fit the in vivo data. Our results demonstrate the effectiveness of the correction method and the remarkable robustness of a GABA editing technique with a top hat editing profile in the presence of frequency drift.

A Temperature Compensated Triple-Path PLL With $K_{\mathrm {VCO}}$ Non-Linearity Desensitization Capable of Operating at 77 K

Abstract: A novel triple-path PLL (TPPLL) is presented to compensate the VCO frequency drift caused by the large temperature variations meanwhile maintaining a stable bandwidth and good jitter performance The proposed PLL architecture splits the VCO tuning loop into three paths as the proportional, the integral, and the temperature compensation (TC) path, respectively The feed-forward TC path with a large VCO gain but a small bandwidth is adopted to realize the compensation for the VCO frequency temperature drift in a closed-loop manner without affecting the high-frequency performance of the VCO The fixed control voltage on the proportional path and limited control-voltage variation on the integral path desensitize the VCO gain ( $K_{\mathrm {VCO}}$ ) non-linearity and stabilizes the loop bandwidth over large temperature range The small VCO gain on the proportional and integral paths contributes to low phase noise and spurs In addition, the different gain settings for the separate proportional and integral paths work as a capacitor multiplier, leading to saving on the silicon area of the loop filter A prototype TPPLL at 256 GHz using a 65-nm CMOS process has been implemented and measured The core circuits occupy an area of 008 mm2, and consume 85 mW The silicon measurement results show that this PLL can continuously work (without calibration) when the temperature changes from 300 K down to 77 K, and has a frequency drift reduction by 99%, while keeping good jitter performance across the entire temperature range

Frequency-stabilized laser system at 1572 nm for space-borne CO 2 detection LIDAR

Abstract: A frequency-stabilized laser system at 1572 nm for space-borne carbon dioxide (CO2) detection LIDAR to realize the precise measurement of the global atmospheric CO2 concentration is presented in this Letter. A distributed-feedback laser diode serves as the master laser (ML) and is wavelength locked to the CO2 line center at 1572.0179 nm using the external frequency modulation technique. The root mean square frequency drift is suppressed to about 50 kHz at an average time of 0.1 s over 8 h. Based on optical phase-locked loops, an online seeder and an offline seeder are offset locked to the reference laser at 1572.024 and 1572.081 nm, respectively, retaining virtually the same frequency stability as the ML.

40 GHz narrow linewidth frequency-switched microwave signal generation based on a single-longitudinal-mode double-Brillouin-frequency spaced Brillouin fiber laser.

Abstract: A novel approach to generating 40 GHz narrow linewidth frequency-switched microwave signals is proposed and demonstrated. In this scheme, a single-longitudinal-mode (SLM) double-Brillouin-frequency spaced Brillouin fiber laser with dual-ring configuration and unpumped erbium-doped fiber (EDF) is used to generate dual-wavelength lasers, and a fiber Bragg grating is used to select the laser for different Brillouin frequency spacings. Dual-ring configuration and unpumped EDF are designed to select the mode for the SLM laser. Dual-wavelength lasers are inserted into a photodetector, and microwave signals at 10.66, 21.39, 32.12, or 42.85 GHz can be obtained. The linewidth of the generated microwave signals is less than 69 kHz. The frequency drift at each frequency is less than 0.83 MHz. The frequency noise and linewidth of Stokes signals are measured, and the linewidth broadening effect of microwave signals is analyzed.

Field programmable gate array implementation of fuzzy variable step size adaptive linear element for adaptive frequency estimation

Abstract: Accurate estimation of power signal frequency is an important requirement for many application areas that include system protection, energy quality monitoring and instrumentation. Though significant efforts have been made since long to develop potent algorithms for accurate estimation of power signal frequency, still their accuracy and convergence speed are a challenge under sudden frequency drift and variations. Therefore, this study focuses on a low complexity adaptive linear element filter using quadratic signal model, whose parameters are adjusted using a fast variable step size fuzzy logic-based learning algorithm to provide better convergence and noise rejection properties for the estimation of frequency from noisy and distorted signals. In addition, the new filter has also been implemented on a field programmable gate array hardware and Xilinx 14.2 with Sysgen software for the tracking of dynamic signal parameters in single and three phase power networks. Various numerical and experimental results are addressed for estimation of frequency of time varying sinusoids.

Reduction of far off-resonance laser frequency drifts based on the second harmonic of electro-optic modulator detection in the optically pumped magnetometer.

Abstract: The frequency drifts of the probe laser could be coupled into the calibrated scale factor of the optically pumped magnetometer (OPM) and induce an error of the measurement accuracy. We propose a method to reduce the far off-resonance laser frequency drifts based on the second harmonic of the electro-optic modulator (EOM) detection system in the all-optical K-Rb hybrid pumping magnetometer. Adopting the closed-loop feedback by monitoring the second-harmonic component in real time, the frequency drift of the probe laser has been effectively reduced by about five times to ∼30 MHz/0.5 h at the detuning of 130 GHz and the cell temperature of 443 K. Besides, this technique has been demonstrated to be helpful for reducing the frequency drifts at different detuning points and temperatures. This method is not only suitable for the development of more compact, high-sensitivity OPMs due to the long-term stability improvement with no extra optical path, but also can be applied to other atomic devices and EOM detection systems for reducing the influence of the laser.

Phase-sensitive optical time-domain reflectometric system based on a single-source dual heterodyne detection scheme.

Abstract: A phase-sensitive optical time-domain reflectometric (ϕ-OTDR) system based on a novel single-source dual heterodyne detection scheme is proposed and demonstrated. It uses the optical beat-frequency signals as the local oscillator signal containing the modulated frequency, frequency drift and phase fluctuation, while the signal to be detected contains all the forgoing spectral components, in addition to the vibration signal under measurement. Frequency mixing serves to isolate the pure vibration signal from the omnipresent residual frequency and phase fluctuations caused by a less strictly synchronous clock, inherent characteristics of the laser and the acousto-optical modulator, and environment temperature changes. With a reduced burden on data processing, better real-time performance is achieved as well. Using probe light pulses of 4 kHz repetition rate and 80 ns pulse width, a 9 m spatial resolution has been achieved on a 24.6 km sensing fiber, with a detectable frequency range from 5 Hz to 1.715 kHz, with a signal-to-noise ratio greater than 23.5 dB. All the above parameters are close to the maximum theoretical values. The drastically improved system demodulation characteristics foreshadow better performance and improved reliability in engineering applications.

A Low Power Low Phase Noise Oscillator for MICS Transceivers.

Abstract: A low-power, low-phase-noise quadrature oscillator for Medical Implantable Communications Service (MICS) transceivers is presented. The proposed quadrature oscillator generates 349~689 MHz I/Q (In-phase and Quadrature) signals covering the MICS band. The oscillator is based on a differential pair with positive feedback. Each delay cell consists of a few transistors enabling lower voltage operation. Since the oscillator is very sensitive to disturbances in the supply voltage and ground, a self-bias circuit for isolating the voltage disturbance is proposed to achieve bias voltages which can track the disturbances from the supply and ground. The oscillation frequency, which is controlled by the bias voltages, is less sensitive to the supply and ground noise, and a low phase noise is achieved. The chip is fabricated in the UMC (United Microelectronics Corporation) 0.18 μm CMOS (Complementary Metal Oxide Semiconductor) process; the core just occupies a 28.5 × 22 μm2 area. The measured phase noise is −108.45 dBc/Hz at a 1 MHz offset with a center frequency of 540 MHz. The gain of the oscillator is 0.309 MHz/mV with a control voltage from 0 V to 1.1 V. The circuit can work with a supply voltage as low as 1.2 V and the power consumption is only 0.46 mW at a 1.8 V supply voltage.

Modelling the environment around five ultracool dwarfs via the radio domain

Abstract: We present the results of a series of short radio observations of six ultracool dwarfs made using the upgraded Very Large Array in S (2–4GHz) and C (4–7GHz) bands. LSR J1835+3259 exhibits a 100 per cent right-hand circularly polarized burst that shows intense narrow-band features with a fast negative frequency drift of about −30 MHz s−1. They are superimposed on a fainter broad-band emission feature with a total duration of about 20 min, bandwidth of about 1 GHz, centred at about 3.5 GHz, and a slow positive frequency drift of about 1 MHz s−1. This makes it the first such event detected below 4 GHz and the first one exhibiting both positive and negative frequency drifts. Polarized radio emission is also seen in 2MASS J00361617+1821104 and NLTT 33370, while LP 349-25 and TVLM 513-46546 have unpolarized emission and BRI B0021-0214 was not detected. We can reproduce the main characteristics of the burst from LSR J1835+3259 using a model describing the magnetic field of the dwarf as a tilted dipole. We also analyse the origins of the quiescent radio emission and estimate the required parameters of the magnetic field and energetic electrons. Although our results are non-unique, we find a set of models that agree well with the observations.