Arbitrary Radio Frequency Research Articles

New unifying metric for NMR/MRI probe evaluation based on optimized solenoid coil geometry

A three-dimensional numerical simulation of the magnetic field distribution and Bloch equations for arbitrary radio frequency (RF) coils is developed and compared against nuclear magnetic resonance (NMR) experimental results to evaluate the NMR signal intensity. Because NMR is inherently insensitive and its signal intensity is dependent on RF coil geometry, the investigation of RF coil geometry to maximize signal intensity for a given sample volume is important for improving the signal-to-noise ratio (SNR) and shortening the accumulation time. The developed simulation can optimize the RF coil geometry, specifically a single-layer solenoid coil with a constant winding pitch, and the result of the solenoid coil simulation serves as a new unifying metric for evaluating NMR/MRI probes. It is found that the most efficient sample aspect ratio (ratio of sample length to sample diameter) and pitch to wire diameter ratio for the highest signal intensity are around 2.2 and 1.65, respectively. Some discrepancies from the solenoid coil geometry ratios for higher signal intensity in previous studies can be explained by the difference in the gap between the inner diameter of the solenoid coil and the sample diameter. These results are confirmed through NMR signal intensity expressed in voltages with three approaches: 3D simulation, experiment, and estimation based on probe parameters. The simulated signal intensity shows a maximum error of approximately 5 % and an average error of 1 % when compared to the experimental results. This result suggests that the developed methods hold the potential for application in quantitative NMR (qNMR) without relying on standard reference materials. Finally, this study introduces a standardized geometry for the optimized solenoid coil for higher signal intensity and uses it to establish an evaluation metric called the signal-to-optimized-solenoid-signal ratio (3SR). The 3SR addresses the volume-dependence problem in conventional metrics like SNR and SNR per sample volume. It provides a standardized approach for the unified evaluation of all RF coils and probe designs, regardless of sample volume and measurement frequency. Therefore, 3SR can be utilized as a useful metric in the search for optimal coil geometry, while metrics such as SNR or SNR per sample volume are currently used for such purpose. This metric is expected to be useful for NMR/magnetic resonance imaging (MRI) users and developers.

Advanced control of an acousto-optic filter performance through arbitrary RF signals.

The proposed study consists of using the capabilities of an arbitrary waveform generator to create a specific radio frequency (RF) signal to be applied to an acousto-optic tunable filter based on quasicollinear interaction. By controlling the duration and shape of the acoustic pulse, the filter transmission function can be changed in terms of selectivity and side-lobe levels. The measurement results show good agreement with the simulation model based on the fast Fourier transform. In particular, the Hamming window has been found to be a good option for suppressing side lobes in the transmission function, even when using high acoustic powers.

CBRAM technology: transition from a memory cell to a programmable and non-volatile impedance for new radiofrequency applications

Electrical resistance control programming of conductive bridging random access memory (CBRAM) radio frequency (RF) switches could benefit the development of electronically controlled non-volatile RF attenuators and other reconfigurable devices. The object of this study is to adapt a conventional CBRAM based memory cell to be used as an RF switch, and to demonstrate the feasibility of programming non-volatile RF CBRAM switches to achieve specific target resistances within a range of continuous values. The memory-RF technologic transition implies a drastic increase of the geometry in order to handle a much higher power, a decrease of the transition capacitance in order to operate at much higher frequencies, and a decrease of the LRS to a few ohms, which is critical for RF applications. These studies are initially performed on an in-house made RF CBRAM cell array at DC frequency, and then extended successfully to a co-planar waveguide (CPW) based shunt mode RF switch with an integrated CBRAM cell. Reliability of the proposed technique is validated through detailed analysis of factors like repeatability of the process, time stability of programmed states, and statistics of time taken to converge to a desired resistance value for an arbitrary RF CBRAM switch.

A high frequency builder software for arbitrary radio frequency signals.

While the frequencies accessible by signal generators steadily rise, the synthesization of complex and arbitrary waveforms with high frequency components remains challenging, especially when restricted by an external reference clock. In this article, we present a comprehensive software package combined with state-of-the-art hardware as a solution for the generation of highly sampled, arbitrary radio frequency waveforms. The software can be used to conduct both synchronous and heterodyne pump-probe experiments due to a variety of different synchronization modules. While both kinds of modules allow for standard waveforms, such as sines, pulses, and bursts, as well as any arbitrary signal, the heterodyne modules additionally are not restricted by the reference clock frequency. Both the output and the synchronization module can be adapted to support additional measurement devices. Due to the modular software structure, individual classes can be exchanged while maintaining all functionalities. The software provides a user friendly graphical interface that allows us to compose, save, and load complex arbitrary waveforms within only a few steps. The frequency selectivity provided by the software-hardware combination allows us to directly target specific excitation states of physical systems. Conducting a heterodyne scanning transmission x-ray microscopy experiment, we are able to demonstrate the capabilities of the software when paired with a high sample rate arbitrary waveform generator. The heterodyne synchronization modules allow for unlimited flexibility leveraging arbitrary waveform generation to their full power. By solving the challenges of synthesizing highly complex electromagnetic waves, the software enables a large variety of experiments to be performed more conveniently.

Effect of cross polarization radiofrequency phases on signal phase

Utilizing phases of radio frequency (RF) pulses to manipulate spin dynamics is routine in NMR and MRI, leading to spectacular techniques like phase cycling. In a very different area, cross polarization (CP) also has a long history as part of a vast number of solid-state NMR pulse sequences. However, a detailed study devoted to the effect of CP RF phases on NMR signal, seems not to be readily available. From first principles, we arrive at a simple dependence of NMR signal on arbitrary CP RF phases, for static and MAS conditions, accompanied by experimental verification. In the process, the CP propagator emerges as a product of RF “pulses” and a period of “free precession”, conforming to coherence transfer pathway theory. The theoretical expressions may lend confidence for dealing with CP blocks with tunable phases in pulse sequences.

Millimeter-Wave Massive MIMO Systems Relying on Generalized Sub-Array-Connected Hybrid Precoding

In this paper, we consider a generalized sub-array-connected (GSAC) architecture for arbitrary radio frequency (RF) chain and antenna configurations, where the number of RF chains connected to a sub-array and the number of antennas in each sub-array can be arbitrary. Our design objective is to improve the energy-efficiency of the hybrid precoder of millimeter-wave massive multiple input multiple output (MIMO) systems. We firstly propose a successive interference cancellation based hybrid precoding algorithm to maximize the achievable rate for any given RF chain and antenna configuration. This algorithm firstly decomposes the total achievable rate optimization problem into multiple sub-rate optimization problems, then it successively maximizes these sub-rates. Since the number of RF chains is limited, we can afford using an exhaustive search scheme to evaluate all configurations and identify the one having the best energy efficiency. Moreover, to rely on an attractive limited feedback, we also propose a beamsteering codebook for our hybrid precoding aided GSAC architecture. Our simulation results demonstrate that the proposed scheme achieves a similar rate as the corresponding optimal unconstrained precoder. Furthermore, we show that the energy-efficiency of the proposed scheme is better than that of the existing schemes in the fully-connected and sub-array-connected architectures.

Chip-based arbitrary radio-frequency photonic filter with algorithm-driven reconfigurable resolution.

All programmable microwave photonic filters to date have been limited by the trade-off between the resolution and the operation bandwidth to construct arbitrary filter shapes. Here, we propose a new approach to synthesize a radio-frequency photonic filter with an arbitrary response and reconfigurable resolution through phase-only tailoring, which is implemented in a low-loss photonic chip. Based on an algorithm, incorporating the target transfer function of the filter optimum resolution can be intelligently chosen by tuning the coupling coefficients and the round-trip phase of cascaded ring resonators. We achieved filtering with tunable resolution from 300MHz to 25GHz along with various filtering shapes and multiple passbands.

Electrically Tunable Bandpass Filter With Patterned Permalloy Thin-Film-Enabled Engineered Substrate

An electrically tunable bandpass filter based on an engineered substrate embedded with patterned permalloy (Py) thin film has been designed, implemented, and characterized. The bandpass filter is designed with metamaterial resonators to achieve a compact size; the designed filter is fabricated on a multilayered engineered substrate, which is composed of a layer of liquid crystal polymer, a layer of patterned Py thin film, and silicon substrate. The tunability of center frequency for the designed bandpass filter is dependent on the radio frequency (RF) characteristics of the implemented engineered substrate. The equivalent permeability of the Py thin film embedded in the engineering substrate is electrically tunable with the dc current, which thus provides high and tunable inductance densities for the split-ring resonators in the designed filter; the center frequency of the implemented bandpass filter is thus electrically tunable. The measured results show that the center frequency of the designed bandpass filter shifts from 2.37 to 2.45 GHz continuously when the applied tuning dc current is changed from 0 to 500 mA. The introduced unique concept of the engineered substrate provides design feasibility of filters with continuous frequency tunability, and the implementation of the engineered substrate enabled with the patterned Py thin film is suitable for the cost-effective fabrication of the arbitrary tunable RF devices.

Stable closed-loop fiber-optic delay of arbitrary radio-frequency waveforms.

Thermal drifts in long fiber-optic delay lines are compensated based on chromatic dispersion. An arbitrary input radio-frequency (RF) waveform and a control RF sine wave modulate two different tunable laser sources and are coupled into the fiber delay line. The RF phase of the control tone at the output of the delay line is monitored and used to adjust the wavelengths of both sources, so that the effects of thermal drifts and dispersion cancel out. The input and control waveforms are separated in the optical domain, and no restrictions are imposed on their RF spectra. A figure of merit is proposed, in terms of the fiber delay, range of temperature changes that may be compensated for, and residual delay variations. An upper bound on performance is established in terms of the specifications of the tunable lasers. The principle is used in the stable distribution of sine waves and of broadband linear frequency-modulated (LFM) waveforms, which are commonly employed in radar systems. Lastly, the method is incorporated in stable interrogation of a localized hot-spot within a high-resolution, distributed Brillouin fiber sensing setup. The results demonstrate the applicability of the proposed protocol in the processing of arbitrary waveforms, as part of larger, more complex systems.

On the RF heating of coronary stents at 7.0 Tesla MRI

PurposeExamine radiofrequency (RF) induced heating of coronary stents at 7.0 Tesla (T) to derive an analytical approach which supports RF heating assessment of arbitrary stent geometries and RF coils.MethodsSimulations are performed to detail electromagnetic fields (EMF), local specific absorption rates (SAR) and temperature changes. For validation E‐field measurements and RF heating experiments are conducted. To progress to clinical setups RF coils tailored for cardiac MRI at 7.0T and coronary stents are incorporated into EMF simulations using a human voxel model.ResultsOur simulations of coronary stents at 297 MHz were confirmed by E‐field and temperature measurements. An analytical solution which describes SAR(1g tissue voxel) induced by an arbitrary coronary stent interfering with E‐fields generated by an arbitrary RF coil was derived. The analytical approach yielded a conservative estimation of induced SAR(1g tissue voxel) maxima without the need for integrating the stent into EMF simulations of the human voxel model.ConclusionThe proposed analytical approach can be applied for any patient, coronary stent type, RF coil configuration and RF transmission regime. The generalized approach is of value for RF heating assessment of other passive electrically conductive implants and provides a novel design criterion for RF coils. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc. Magn Reson Med 74:999–1010, 2015. © 2014 Wiley Periodicals, Inc.

On the RF heating of coronary stents at 7.0 Tesla MRI.

Examine radiofrequency (RF) induced heating of coronary stents at 7.0 Tesla (T) to derive an analytical approach which supports RF heating assessment of arbitrary stent geometries and RF coils. Simulations are performed to detail electromagnetic fields (EMF), local specific absorption rates (SAR) and temperature changes. For validation E-field measurements and RF heating experiments are conducted. To progress to clinical setups RF coils tailored for cardiac MRI at 7.0T and coronary stents are incorporated into EMF simulations using a human voxel model. Our simulations of coronary stents at 297 MHz were confirmed by E-field and temperature measurements. An analytical solution which describes SAR(1g tissue voxel) induced by an arbitrary coronary stent interfering with E-fields generated by an arbitrary RF coil was derived. The analytical approach yielded a conservative estimation of induced SAR(1g tissue voxel) maxima without the need for integrating the stent into EMF simulations of the human voxel model. The proposed analytical approach can be applied for any patient, coronary stent type, RF coil configuration and RF transmission regime. The generalized approach is of value for RF heating assessment of other passive electrically conductive implants and provides a novel design criterion for RF coils.

Theory for the self-bias formation in capacitively coupled plasmas excited by arbitrary waveforms

We develop a semi-analytical theory for the self-bias formation in capacitively coupled plasmas excited by arbitrary radio-frequency (rf) waveforms. The requirement of rf current continuity and voltage balance across the discharge results in the need for a self-bias voltage to develop with non-sinusoidal excitations, even in geometrically symmetric systems. The theory is compared extensively with a wide range of experimental and particle-in-cell (PIC) simulation data within the literature, and is found to be in excellent agreement. Furthermore, it is shown that the present theory is formally equivalent to the original model proposed by Heil et al (2008 J. Phys. D: Appl. Phys. 41 165202), but goes further by explicitly allowing the time-varying sheath voltages and symmetry parameter to be evaluated without input from PIC simulations.

Principle and key technology of generalized high precision simulation of TT&C channel

A generalized simulation method of the tracking, telemetry and control (TT&C) channel, which is applicable to wideband and arbitrary radio frequency (RF) signal, is proposed. It can accurately simulate the dynamic transmission delay of the arbitrary RF signal in channels, especially regardless of any prior knowledge including signal form, signal parameters, and so on. The proposed method orthogonally demodulates the wideband and arbitrary RF signal to complex baseband by a known local oscillator (LO) signal. Whereafter, it takes measures to obtain the delay reconstruction signal of baseband signals based on the dynamic transmission delay between a ground station and a responder. Meanwhile, it manages to obtain the delay reconstruction signal of LO signals. The simulation output signal (the delayed RF signal) can be achieved through the synthesis of the two delay reconstruction signals mentioned above. The principle and its related key technology are described in detail, and the realizable system architecture is given.

B1‐based specific energy absorption rate determination for nonquadrature radiofrequency excitation

The current gold standard to estimate local and global specific energy absorption rate for MRI involves numerically modeling the patient and the transmit radiofrequency coil. Recently, a patient-individual method was presented, which estimated specific energy absorption rate from individually measured B(1) maps. This method, however, was restricted to quadrature volume coils due to difficulties distinguishing phase contributions from radiofrequency transmission and reception. In this study, a method separating these two phase contributions by comparing the electric conductivity reconstructed from different transmit channels of a parallel radiofrequency transmission system is presented. This enables specific energy absorption rate estimation not only for quadrature excitation but also for the nonquadrature excitation of the single elements of the transmit array. Though the contributions of the different phases are known, unknown magnetic field components and tissue boundary artifacts limit the technique. Nevertheless, the high agreement between simulated and experimental results found in this study is promising. B(1)-based specific energy absorption rate determination might become possible for arbitrary radiofrequency excitation on a patient-individual basis.

Ultrabroad-bandwidth arbitrary radiofrequency waveform generation with a silicon photonic chip-based spectral shaper

Ultrabroad-bandwidth radiofrequency pulses offer significant applications potential, such as increased data transmission rate and multipath tolerance in wireless communications. Such ultrabroad-bandwidth pulses are inherently difficult to generate with chip-based electronics due to limits in digital-to-analog converter technology and high timing jitter. Photonic means of radiofrequency waveform generation, for example, by spectral shaping and frequency–time mapping, can overcome the bandwidth limit in electronic generation. However, previous bulk optic systems for radiofrequency arbitrary waveform generation do not offer the integration advantage of electronics. Here, we report a chip-scale, fully programmable spectral shaper consisting of cascaded multiple-channel microring resonators, on a silicon photonics platform that is compatible with electronic integrated circuit technology. Using such a spectral shaper, we demonstrate the generation of burst radiofrequency waveforms with programmable time-dependent amplitude, frequency and phase profiles, for frequencies up to 60 GHz. Our demonstration suggests potential for chip-scale photonic generation of ultrabroad-bandwidth arbitrary radiofrequency waveforms.

20 GHz arbitrary radio-frequency waveform generator based on incoherent pulse shaping

We demonstrate a new approach of photonically assisted radio-frequency (RF) waveform generation using a spectrally incoherent light source. The system is based on the so-called generalized frequency-to-time mapping operation. In this work, external modulation of the source is done by concatenating two electro-optic Mach-Zehnder modulators properly biased to achieve short pulse gates which allow for broad bandwidth electrical signals. Also, the spectral shaping stage is performed prior to O/E conversion. A detailed theoretical analysis demonstrates that even in this case the frequency-to-time mapping is preserved. The key is that the noise level is greatly reduced because the amplitude filter serves as noise rejecter. Experimental results show up to approximately 20 GHz electrical bandwidth signals using 10 Gb/s standard telecommunication equipment with the nice feature of repetition rate control on the generated electrical waveform.

COGNITIVE RADIOS FOR DYNAMIC SPECTRUM ACCESS - Polyphase Multipath Radio Circuits for Dynamic Spectrum Access

Dynamic access of unused spectrum via a cognitive radio asks for flexible radio circuits that can work at an arbitrary radio frequency. This article reviews techniques to realize radios without resorting to frequency selective dedicated filters. In particular, a recently proposed polyphase multipath technique canceling harmonics and sidebands is discussed. Using this technique, a wideband and flexible power upconverter with a clean output spectrum has been realized on a CMOS chip, aiming at flexible radio transmitter application. Prototype chips can transmit at an arbitrary frequency between DC and 2.4 GHz. Unwanted harmonics and sidebands are more than 40 dB lower than the desired signal up to the 17th harmonic of the transmit frequency

Nonlinear plasma dynamics in capacitive radio frequency discharges

The excitation of harmonics in the current of capacitive radio frequency (RF) discharges is a frequently observed phenomenon. The effect is of interest for several reasons. It forms, for instance, the basis of a successful diagnostic concept for technical plasmas, and it is intimately connected to the process of electron heating in capacitive discharges. Recently, mathematical models were proposed which interpret the phenomenon as the self-excitation of the plasma series resonance by the nonlinearity of the boundary sheath. These models are surprisingly successful but suffer from the limitation that they analyse the plasma dynamics in terms of global equations with concentrated parameters. They are unable to account for the complex multi-mode current waveforms seen in experiments and also cannot resolve the electromagnetic effects which dominate contemporary/large area, high frequency processing discharges. This paper aims to correct the deficiency by presenting a spatially resolved, fully electromagnetic model of the nonlinear RF dynamics of a bounded plasma. The model holds for arbitrary plasma reactor geometries and external RF excitations and makes no assumptions on the homogeneity of the plasma or the characteristics of the boundary sheath. A functional analytic (Hilbert space) formulation of the model is given which allows for an exact solution in terms of an infinite power/Fourier series. For the case of an idealized cylindrical reactor, the model is also explicitly evaluated. The calculated RF current wave forms exhibit the complex multi-mode structure of the currents observed in experiments and follow the same scaling laws. It is concluded that the presented model is capable of describing the nonlinear dynamics of capacitive RF discharges of all sizes and that it provides a significant improvement over both the established nonlinear global models and linear models with spatial resolution.

A New Generation Method of Slowly Rotating-EM Fields for Radiated Immunity/Susceptibility Test

A new generation method of rotating electromagnetic fields (rotating-EM fields) for radio frequency (RF) radiated immunity/susceptibility test and its basic characteristics are described. Two different double-side-band suppressed-carrier (DSB-SC) signals are required for generating the slowly rotating-EM field for the immuntty/susceptibility test. These DSB-SC signals are generated by a DSB-SC-signal generator based on the new concept which consists of voltage-variable attenuators, bi-phase switches, a direct-digital synthesizer and a micro processor. Using the DSB-SC-signal generator, the DSB-SC signal of arbitrary RF frequency can be generated more easily than the conventional system. In this paper, the principle of the DSB-SC signal generator and the basic characteristics of the DSB-SC signals generated by the generator are clarified. The measured basic characteristics of the rotating-EM field generated using the new concept are shown and it is confirmed that the field can be applied for the RF immunity/susceptibility test. In addition, the susceptibility test of an equipment under test is made as an example, the validity of our proposed system is established.

4<tex>$,times,$</tex>2.5 GHz Repetitive Photonic Sampler for High-Speed Analog-to-Digital Signal Conversion

This letter presents a 10-gigasample/s (GS/s) photonic analog-to-digital converter (ADC) system constructed using a four-wavelength picosecond pulsed source. The lasing-to-nonlasing modes suppression ratio of the optical source is over 24 dB. By using the 10-GHz optical source, a 10-GS/s photonic ADC has been demonstrated and was used to sample an arbitrary radio-frequency signal. The system was further investigated by sampling a 2.4-GHz sinusoidal signal. Important parameters including the signal-to-noise and distortion ratio and the spurious-free dynamic range have been determined.