Radio Frequency Oscillator Research Articles

High spectral purity Kerr frequency comb radio frequency photonic oscillator.

Femtosecond laser-based generation of radio frequency signals has produced astonishing improvements in achievable spectral purity, one of the basic features characterizing the performance of an radio frequency oscillator. Kerr frequency combs hold promise for transforming these lab-scale oscillators to chip-scale level. In this work we demonstrate a miniature 10 GHz radio frequency photonic oscillator characterized with phase noise better than −60 dBc Hz−1 at 10 Hz, −90 dBc Hz−1 at 100 Hz and −170 dBc Hz−1 at 10 MHz. The frequency stability of this device, as represented by Allan deviation measurements, is at the level of 10−10 at 1–100 s integration time—orders of magnitude better than existing radio frequency photonic devices of similar size, weight and power consumption.

Synthetic-wavelength interferometry improved with frequency calibration and unambiguity range extension.

We improve the accuracy of distance measurements with synthetic-wavelength interferometry by referencing the spectral spacing of the free-running light sources to a high-precision radio-frequency oscillator. In addition, we increase the unambiguity range with a time-of-flight technique. Distances to scattering technical surfaces can be measured with micrometer accuracy and an unambiguity range of 1.17m. The measurement rate amounts to 300Hz.

Validation of radio frequency treatments as alternative non-chemical methods for disinfesting chestnuts

Chemical fumigation has been widely used to control insects in postharvest chestnuts but is inherent dangers when using fumigants. The purpose of this study was to validate application of radio frequency (RF) treatments for disinfesting chestnuts as an alternative to chemical fumigation. A practical process protocol was developed to control insect pests in chestnuts using a 27.12 MHz free-running oscillator RF system. Fifth-instar yellow peach moth, Conogethes punctiferalis, more heat tolerant than chestnut weevil, Curculio elephas, under three temperature and time combinations using a heating block system, was selected as the targeted insect to validate the RF treatment protocol. Mortality of fifth-instar C. punctiferalis increased with increasing holding time at 55 °C using RF heating and reached 100% while holding in hot air for at least 5 min. Furthermore, there was no significant quality difference in color, fat, firmness, moisture content, protein, and soluble sugar content of chestnuts observed between RF treatments and controls. RF treatment methods hold potential to scale up for industrial applications of disinfesting chestnuts.

Finite Element Modeling and Analysis of Radio Frequency Heating Rate in Mung Beans

Abstract. Radio frequency (RF) heating has been extensively studied as a novel disinfestation method for dry agricultural products. A major difficulty in using this method is that different heating rates at the corners and edges of materials may cause negative effects on product quality. A systematic analysis of factors that influence the RF heating rate is desirable to help in designing effective treatment protocols. A finite element model using COMSOL Multiphysics software was developed and experimentally validated with 3 kg of mung beans in a 6 kW, 27.12 MHz free-running oscillator RF system to study the influence of sample moisture content, density, specific heat capacity, thermal conductivity, dielectric properties, top electrode voltage, and electrode gap on RF heating rate. Simulation results demonstrated that the variation in sample density and specific heat capacity, especially thermal conductivity, had a relatively slight effect on RF heating rate. The RF heating rate was significantly influenced by electrode gap, top electrode voltage, and the dielectric properties and moisture content of the sample. These heating rate distributions might be valuable in guiding and optimizing RF treatment conditions, which are helpful to improve RF heating uniformity for disinfecting dry products.

Validation of top electrode voltage in free-running oscillator radio frequency systems with different moisture content soybeans

Precisely estimating the top electrode voltage of a radio frequency (RF) system is important for computer simulation of sample temperatures. The goal of this study was to validate whether the developed equations can be applied to determine the top electrode for RF heated soybeans with different moisture contents. Soybeans with moisture contents of 6.7% and 8.7% (w.b.) were treated in a 27.12 MHz, 6 kW RF system under 5 electrode gaps. The results showed that the top electrode voltage increased almost linearly both with increasing moisture content and decreasing electrode gap. The maximum difference of the estimated top electrode voltages was less than 10% relative error between the anode current and the measured current in the same conditions. The temperature distributions in three representative layers from simulation were in good agreement with those from experiment, as was the average root mean square error (RMSE) of 1.6% for temperature profiles in three selected positions. The validation results demonstrated that the established equation using anode current was suitable for estimating the top electrode voltage when soybeans of different moisture contents were heated in RF systems.

A 3-Phase AC–AC Matrix Converter GaN Chipset With Drive-by-Microwave Technology

This paper describes an ultracompact GaN 3 × 3 matrix power converter with drive-by-microwave (DBM) technology, which comprises a radio frequency (RF)-triggered GaN-gate injection transistor (GIT) bidirectional power switches integration chip with co-integrated RF rectifiers, novel isolated dividing couplers in a printed circuit board to reduce complicated gate lines, and low-consumption GaN/Si DBM gate driver on a chip that controls nine bi-directional power switches. The proposed 4.0-kW GaN 3 × 3 matrix power converter is extremely compact, measuring only 18 × 25 mm due to the use of GaN-GIT power device integration technology and DBM technology that provides isolated gate signals by microwave wireless power transmission and eliminates the need for photo-couplers and isolated power supplies. The GaN/Si DBM driver realizes low power consumption of only 2.0 W as a result of gate power sharing with three RF oscillators. The sequential switching operation by the fabricated GaN 3 × 3 matrix converter was successfully achieved.

Wavelength conversion in modulated coupled-resonator systems and their design via an equivalent linear filter representation.

We propose wavelength converters based on modulated coupled resonators that achieve conversion by matching the modulation frequency to the frequency splitting of the supermodes of the unmodulated system. Using temporal coupled-mode theory, we show that these time-variant systems have an equivalent linear, time-invariant filter representation that simplifies the optimal engineering of design parameters for realistic systems. Applying our model to carrier plasma-dispersion modulators as an example implementation, we calculate conversion efficiencies between -5.4 and -1.7 dB for intrinsic quality factors of 10(4)-10(6). We show that the ratio of the resonance shift to the total linewidth is the most important parameter when determining conversion efficiency. Finally, we discuss how this model can be used to design devices such as frequency shifters, widely tunable radio frequency oscillators, and frequency combs.

Thermally driven magnetic precession in spin valves

We investigate the angular dependence of the spin torque generated when applying a temperature difference across a spin-valve. Our study shows the presence of a non-trivial fixed point in this angular dependence, i.e. the possibility for a temperature gradient to stabilize radio frequency oscillations without the need for an external magnetic field. This so called "wavy" behavior can already be found upon applying a voltage difference across a spin-valve but we find that this effect is much more pronounced with a temperature difference. Our semi-classical theory is parametrized with experimentally measured parameters and allows one to predict the amplitude of the torque with good precision. Although thermal spin torque is by nature less effective than its voltage counterpart, we find that in certain geometries, temperature differences as low as a few degrees should be sufficient to trigger the switching of the magnetization.

Radio-frequency thawing of food products – A computational study

Main goal of optimal thawing is to minimize thawing time with least damage to the quality of frozen food products. Microwave (MW) and radio frequency (RF) applications have potential for their use in industrial thawing. Higher penetration depths of RF contribute to a better distribution of energy generated by the interaction between food and electromagnetic field, and thus help to improve the heating uniformity and to minimize runaway heating. Modeling is one way to design and to optimize such process where complexities due to coupling the heat transfer with phase change and the solution of electric field are faced. Therefore, the objectives of this study were to develop a computational model to determine temperature distribution in frozen lean beef during thawing and experimentally validate the model. For this purpose, a commercial software, based on finite element method, was used to solve coupled heat conduction and electric field in a 3D domain with temperature dependent thermo-physical and dielectric properties. Experimental data used to validate the model referred to a 50Ω and a free-running oscillator RF systems with various sized samples. Comparison of simulation results agreed well with experimental data, and the mathematical model was reported to be used for designing RF systems to mitigate the effect of overheating at the surfaces of the sample.

Spin-torque diode radio-frequency detector with voltage tuned resonance

We report on a voltage-tunable radio-frequency (RF) detector based on a magnetic tunnel junction (MTJ). The spin-torque diode effect is used to excite and/or detect RF oscillations in the magnetic free layer of the MTJ. In order to reduce the overall in-plane magnetic anisotropy of the free layer, we take advantage of the perpendicular magnetic anisotropy at the interface between ferromagnetic and insulating layers. The applied bias voltage is shown to have a significant influence on the magnetic anisotropy, and thus on the resonance frequency of the device. This influence also depends on the voltage polarity. The obtained results are accounted for in terms of the interplay of spin-transfer-torque and voltage-controlled magnetic anisotropy effects.

Development of thermal treatment protocol for disinfesting chestnuts using radio frequency energy

Methyl bromide fumigation, widely used for disinfesting chestnuts, will be banned in developing countries by 2015 under Montreal Protocol due to its adverse effects on human health and environment. The purpose of this research was to study possible applications of radio frequency (RF) heating for disinfestations of chestnuts to replace chemical fumigation. A 6kW, 27MHz free-running oscillator RF system was used to determine the effect of a developed RF treatment protocol on quality of chestnuts. The results showed that the heating time needed only 5.4min to heat the 2.5kg chestnuts from 20°C to 55°C using RF energy, and 170min for chestnuts to reach 52.5°C using hot air at 55°C and 1.6m/s. Based on the heating uniformity studies, a RF treatment protocol was finally developed to combine 0.6kW RF powers with a forced hot air at 55°C, movement of the conveyor, mixing twice, and holding at 55°C hot air for 5min, followed by forced room air cooling through single-layer samples. Quality of chestnuts was not affected by the RF treatments because no significant differences in moisture, protein, fat, soluble sugar, firmness, and color were observed between RF treatments and untreated controls after 8 days at 35°C, simulating one year of storage at 4°C. The RF treatments may provide a rapid and environmentally friendly method to replace chemical fumigation for disinfesting chestnuts.

Physical Layer Authentication for Mobile Systems with Time-Varying Carrier Frequency Offsets

A novel physical layer authentication scheme is proposed in this paper by exploiting the time-varying carrier frequency offset (CFO) associated with each pair of wireless communications devices. In realistic scenarios, radio frequency oscillators in each transmitter-and-receiver pair always present device-dependent biases to the nominal oscillating frequency. The combination of these biases and mobility-induced Doppler shift, characterized as a time-varying CFO, can be used as a radiometric signature for wireless device authentication. In the proposed authentication scheme, the variable CFO values at different communication times are first estimated. Kalman filtering is then employed to predict the current value by tracking the past CFO variation, which is modeled as an autoregressive random process. To achieve the proposed authentication, the current CFO estimate is compared with the Kalman predicted CFO using hypothesis testing to determine whether the signal has followed a consistent CFO pattern. An adaptive CFO variation threshold is derived for device discrimination according to the signal-to-noise ratio and the Kalman prediction error. In addition, a software-defined radio (SDR) based prototype platform has been developed to validate the feasibility of using CFO for authentication. Simulation results further confirm the effectiveness of the proposed scheme in multipath fading channels.

Optical length change measurement via RF frequency shift analysis of incoherent light source based optoelectronic oscillator

Radio-frequency (RF) frequency shift of incoherent light source based optoelectronic oscillator (OEO) is employed to measure the optical length change. In the proposed OEO using an incoherent light source, the optical length under test is inserted in the optoelectronic hybrid loop. The frequency shift of RF oscillation modes at the output of the OEO reflects the optical length change, with the change being measured via frequency shift analysis. Two OEO configurations are theoretically designed and experimentally performed, while an amplified spontaneous emission (ASE) source serves as the incoherent light source. A linear relationship between the frequency shift and the optical length change has been confirmed for measurement, and a reconfigurable measurement sensitivity is available by selecting different oscillation modes. Moreover, the use of ASE greatly reduces the complexity and the cost for stabilization control on light source, while the derived results are consistent with that obtained in a laser source based OEO both in the measured optical length changes and the phase noise performance. A sensitivity of -28 KHz/cm, -480 KHz/cm or higher, and a resolution of nano-meter scale are obtained, which can be used to monitor the displacement, the changes in refractive index, temperature.

Distribution of high-stability 10 GHz local oscillator over 100 km optical fiber with accurate phase-correction system

We have developed a radio-frequency local oscillator remote distribution system, which transfers a phase-stabilized 10.03 GHz signal over 100 km optical fiber. The phase noise of the remote signal caused by temperature and mechanical stress variations on the fiber is compensated by a high-precision phase-correction system, which is achieved using a single sideband modulator to transfer the phase correction from intermediate frequency to radio frequency, thus enabling accurate phase control of the 10 GHz signal. The residual phase noise of the remote 10.03 GHz signal is measured to be -70 dBc/Hz at 1 Hz offset, and long-term stability of less than 1×10⁻¹⁶ at 10,000 s averaging time is achieved. Phase error is less than ±0.03π.

Experimental and simulated top electrode voltage in free-running oscillator radio frequency systems

Top electrode voltage of a radio frequency (RF) heating system is an important parameter to accurately determine final sample temperatures using computer simulation. The purpose of this study was to establish the correlation between the top electrode voltage obtained by analytical method or simulation and the measured electrical currents. A measuring circuit was designed and developed to directly measure the top electrode voltage in the 27 MHz 6 kW RF unit together with the anode current read from the RF generator. The top electrode voltage was experimentally determined with 3 kg soybeans under five electrode gaps using computer simulation based on the matched temperature profiles in three layers, and analytical methods based on the heating rates measured by infrared camera, fiber optic sensors and thermocouples. Results showed that the electrode voltage decreased with increasing electrode gap or decreasing heating rate. The correlation between the electrode voltage estimated by temperature distributions from three-layer thermal imaging and the electrical currents both from the anode reading and direct measurement was the best one among the estimation methods and could be used for future computer simulations.

Two-Stage System Based on a Software-Defined Radio for Stabilizing of Optical Frequency Combs in Long-Term Experiments

A passive optical resonator is a special sensor used for measurement of lengths on the nanometer and sub-nanometer scale. Astabilized optical frequency comb can provide an ultimate reference for measuring the wavelength of a tunable laser locked to the optical resonator. If we lock the repetition and offset frequencies of the comb to a high-grade radiofrequency (RF) oscillator its relative frequency stability is transferred from the RF to the optical frequency domain. Experiments in the field of precise length metrology of low-expansion materials are usually of long-term nature so it is required that the optical frequency comb stay in operation for an extended period of time. The optoelectronic closed-loop systems used for stabilization of combs are usually based on traditional analog electronic circuits processing signals from photodetectors. From an experimental point of view, these setups are very complicated and sensitive to ambient conditions, especially in the optical part, therefore maintaining long-time operation is not easy. The research presented in this paper deals with a novel approach based on digital signal processing and a software-defined radio. We describe digital signal processing algorithms intended for keeping the femtosecond optical comb in a long-time stable operation. This need arose during specialized experiments involving measurements of optical frequencies of tunable continuous-wave lasers. The resulting system is capable of keeping the comb in lock for an extensive period of time (8 days or more) with the relative stability better than 1.6 × 10−11.

CdS based novel photo-impedance light sensor

A novel photo-impedance (hybrid photocapacitive-photoresistive) sensor for visible light employs gated cadmium sulfide photoresistance connecting two fixed capacitances. The changes of the photoresistance and of the metal–insulator (dielectric)-semiconductor (MIS) under illumination affect the overall device capacitance. The MIS and photoresistor frequency dispersion allows for increasing the dynamic range by measuring the capacitance response at lower frequencies. By making this device a part of the tank circuit of a radio frequency oscillator, the information on illumination intensity can be relayed as the signal frequency and detected remotely.

Tunable frequency upconversion based on a directly modulated DFB-LD and FP-LD injection

We show a simple, convenient, and cost-effective scheme for tunable frequency upconversion at millimeter-wave band without a local oscillator. By launching a 2.5-Gb/s directly modulated baseband signal into a Fabry-Perot laser diode (FP-LD), the mode of the FP-LD is locked by the high-order sideband of the injected signal. The beating frequency of the injection-locked mode and the injected signal can generate upconversion subcarriers. In our experiment, tunable frequency subcarriers of 28.4, 29.3, and 30.5 GHz are obtained without any radio-frequency local oscillator. The single sideband phase noises of ?83.88, ?76.36, and ?78.54 dBc/Hz @ 10 kHz (at 28.4-, 29.3-, and 30.5-GHz subcarriers, respectively) are shown. The proposed scheme has potential to generate much higher frequency carriers.

OFDM Inter-carrier Interference Due to Radio Frequency Synthesizer Phase Noise

Abstract This work presents the effects of radio-frequency Local Oscillator non-idealities on OFDM inter-carrier interference. The synthesizer has the main role on modulation and demodulation process of base band IQ signals to radio frequency channels which need to fulfill several requirements imposed by standards specifications. Phase and frequency stability are crucial, since the synchronization of the entire system depends on the accuracy of this circuit. The discussed OFDM system is based on a simulation scenario including radio-frequency channel conversion considering Local Oscillator with configurable phase noise power and bandwidth. This feature allows the study of global system based on Local Oscillator output spectrum. Sub-channel spacing, cyclic redundancy and pulse shaping are configurable. The obtained results indicate that free run oscillator phase noise characterization is not appropriate when the Local Oscillator in the radio-frequency front end is based on synthesizer architecture. System evaluation is based on error vector magnitude. The relation of this metric with the Local Oscillator phase noise allows the performance estimation of the global system.

Characterization of Optomechanical RF frequency Mixing/Down-Conversion and its Application in Photonic RF Receivers

We have characterized the process of radio-frequency (RF) frequency mixing and signal down-conversion in radiation pressure driven optomechanical RF oscillators. We have studied the dependence of the optomechanical mixing process on RF frequency, optical pump power and wavelength detuning and verified the existence of a linear regime where down-converted power is proportional to the RF input power. These outcomes show that optomechanical oscillator (OMO) has the potential to be used as frequency down-converter in RF over fiber links and photonic RF receivers. We have verified the fidelity of the optomechanical down-conversion process and demonstrated the first optomechanical voice down-conversion from an RF carrier.