Radio Frequency Detection Research Articles

Wideband micromachined capacitive microphones with radio frequency detection

Silicon microphones based on capacitive micromachined ultrasonic transducer membranes and radio frequency detection overcome many of the limitations in bandwidth, uniformity of response, and durability associated with micromachined condenser microphones. These membranes are vacuum-sealed to withstand submersion in water and have a flat mechanical response from dc up to ultrasonic frequencies. However, a sensitive radio frequency detection scheme is necessary to detect the small changes in membrane displacement that result from utilizing small membranes. In this paper we develop a mathematical model for calculating the expected output signal and noise level and verifies the model with measurements on a fabricated microphone. Measurements on a sensor with 1.3 mm2 area demonstrate less than 0.5 dB variation in the output response between 0.1 Hz to 100 kHz under electrostatic actuation and an A-weighted equivalent noise level of 63.6 dB(A) SPL in the audio band. Because the vacuum-sealed membrane structure has a low mechanical noise floor, there is the potential for improved sensitivity using higher carrier frequencies and more sophisticated detection circuitry.

Micro-spectrometer for NMR: analysis of small quantities in vitro

This paper reports the design, fabrication and preliminary tests of planar microcoils associated with a micromachined channel in silicon. These microcoils are used as nuclear magnetic resonance (NMR) radio frequency detection coils. They allow in vitro NMR analysis of small quantities introduced into the microchannel. It is a real challenging task to develop microsystems for NMR spectrum extraction for smaller and smaller sample volumes. Moreover, it is advantageous that these microsystems could be integrated in a micro total analysing system (µ-TAS) as an analysing stage. In this paper the description, fabrication process and electrical characterization of planar microcoil receivers are described. Results obtained on NMR microspectroscopy experiments have been performed in water and ethanol, using a 500 µm × 500 µm planar microcoil tuned and matched at 85.13 MHz (proton frequency at 2 T).

A longitudinally detected high-field ESR spectrometer for the measurement of spin–lattice relaxation times

We describe a high-field longitudinally detected electron spin resonance (LOD-ESR) spectrometer operating at 35 and 75 GHz. The lack of resonant microwave circuits facilitates operation at different microwave frequencies without changing the probehead. A very low noise radio frequency detection compensates partially the resulting low sensitivity. The major elements of the LOD-ESR spectrometer are commercially available and may be adapted to usual high frequency spectrometers. The instrument allows field and frequency dependent spin lattice relaxation time ( T 1) studies. T 1 in the range of 2–80 ns can be determined from the phase sensitively detected LOD-ESR spectra. We demonstrate the performance of the apparatus by the measurement of T 1 in the normal state of RbC 60, an electrically conducting alkaline fulleride polymer.

Implantable planar rf microcoils for NMR microspectroscopy

This communication reports the design, the fabrication and preliminary tests of planar microcoils used as nuclear magnetic resonance (NMR) radio frequency (rf) detection coils. The magnetic coupling between a rf coil and a sample is maximized by physically designing the coil as small as possible as to just accommodate the sample. Maximum coupling corresponds directly to an optimal sensitivity for the signal reception. Designed for a biological application, these coils have to be as less invasive as possible and they must allow studying small quantities of metabolites surrounding the microcoil. NMR microspectroscopy experiments have been performed in water and butter samples using a 500mm � 1000 mm planar microcoil tuned and matched at 85.13 MHz (proton’s frequency at 2 T). # 2002 Elsevier Science B.V. All rights reserved.

Magnetic resonance microscopy approaches to molecular imaging: sensitivity vs. specificity.

Magnetic resonance imaging (MRI) has become a staple of diagnostic radiology. Despite its diagnostic utility the resolving power of typical clinical MRI instruments is only on the order of 1 mm. This has led to the development of magnetic resonance microscopy (MRM), which employs the same physical imaging principals used in MRI, but with instrumentation designed to resolve structural details down to the level of 10-100 microns in samples ranging from less than 1mm to several centimeters in size. Until recently, major advancements in MRM have focused on hardware and software developments allowing the detection of radio-frequency signals originating from very small volume elements within the sample. Such high-resolution images have facilitated the early detection of diseased tissue by focusing on sub-millimeter structural changes induced in the tissue. To sensitize the MRM technique to pathologic tissue changes, investigators have developed techniques, such as chemical shift imaging to detect pre-cancerous changes in tissue metabolism and MR relaxometry to detect changes in tissue composition during the earliest stages of degeneration for diseases such as osteoarthritis or multiple sclerosis. However, such non-specific measurements can only serve as surrogate measures of disease progression and potential measures of treatment efficacy. As disease diagnosis moves from the anatomic to the molecular stage, scientists will require imaging techniques that can detect molecular events deep inside the human body. To meet this goal, MR scientists are working to improve imaging resolutions in vivo and they are developing molecular probes that can dramatically amplify the MR signal in response to specific and highly localized molecular events. This article will identify current trends in the MRM field aimed at meeting the challenges imposed by molecular imaging and areas for future development in this highly promising imaging field.

Recent advances in image reconstruction, coil sensitivity calibration, and coil array design for SMASH and generalized parallel MRI.

Parallel magnetic resonance imaging (MRI) techniques use spatial information from arrays of radiofrequency (RF) detector coils to accelerate imaging. A number of parallel MRI techniques have been described in recent years, and numerous clinical applications are currently being explored. The advent of practical parallel imaging presents various challenges for image reconstruction and RF system design. Recent advances in tailored SiMultaneous Acquisition of Spatial Harmonics (SMASH) image reconstructions are summarized. These advances enable robust SMASH imaging in arbitrary image planes with a wide range of coil array geometries. A generalized formalism is described which may be used to understand the relations between SMASH and SENSE, to derive typical implementations of each as special cases, and to form hybrid techniques combining some of the advantages of both. Accurate knowledge of coil sensitivities is crucial for parallel MRI, and errors in calibration represent one of the most common and the most pernicious sources of error in parallel image reconstructions. As one example, motion of the patient and/or the coil array between the sensitivity reference scan and the accelerated acquisition can lead to calibration errors and reconstruction artifacts. Self-calibrating parallel MRI approaches that address this problem by eliminating the need for external sensitivity references are reviewed. The ultimate achievable signal-to-noise ratio (SNR) for parallel MRI studies is closely tied to the geometry and sensitivity patterns of the coil arrays used for spatial encoding. Several parallel imaging array designs that depart from the traditional model of overlapped adjacent loop elements are described.

A generalized approach to parallel magnetic resonance imaging.

Parallel magnetic resonance (MR) imaging uses spatial encoding from multiple radiofrequency detector coils to supplement the encoding supplied by magnetic field gradients, and thereby to accelerate MR image acquisitions beyond previous limits. A generalized formulation for parallel MR imaging is derived, demonstrating the relationship between existing techniques such as SMASH and SENSE, and suggesting new algorithms with improved performance. Hybrid approaches combining features of both SMASH-like and SENSE-like image reconstructions are constructed, and numerical conditioning techniques are described which can improve the practical robustness of parallel image reconstructions. Incorporation of numerical conditioning directly into parallel reconstructions using the generalized approach also removes a cumbersome and potentially error-prone sensitivity calibration step involving division of two distinct in vivo reference images. Hybrid approaches in combination with numerical conditioning are shown to extend the range of accelerations over which high-quality parallel images may be obtained.

300 MHz continuous wave electron paramagnetic resonance spectrometer for small animal in vivo imaging

Design and construction of an electron paramagnetic resonance (EPR) spectrometer, operating in the continuous wave mode in the radio frequency (rf) region, and capable of performing spectroscopy and in vivo imaging of paramagnetic spin probes is described. A resonant frequency of 300 MHz was chosen to provide the required sensitivity at nontoxic levels of commonly used spin probes and penetration of the rf in small animals. Three major components, the magnet, the radio frequency signal detection bridge, and the data acquisition module are described in this article. Integration of a rapid scan capability to reduce imaging time is also described. Two- and three-dimensional EPR images of the spin probe distribution in phantom objects as well as from in vivo experiments are reported. From the EPR images, morphology of some internal organs could be recognized. EPR images of the spin probe distribution in mice suggest differences in perfusion of the spin probe between normal and tumor regions. Addition of a spectral dimension to spatial images should enable differentiation of oxygen status in normal and pathological conditions.

Multi-instrument coincident detection of sprites

This paper describes the instrumentation used for sprite detection during two campaigns near Darwin, Australia, in 1997 and 1998. The sprites (large electrical luminous discharges above thunderstorms) are detected by both ‘direct’ (optical) and ‘indirect’ (radio frequency) methods. The optical detectors consist of (i) two intensified video capture systems that are capable of capturing and storing only video frames containing sprites, and (ii) up to four photometers whose outputs are monitored so that only events defined as significant are captured. The radio-frequency detectors consist of (i) four-channel sferic detectors operating in the VLF region and (ii) one or more AbsPAL VLF phase and amplitude loggers. All detectors use GPS receivers for timing reference, which allows post-capture comparison of all events with high accuracy.

Integrating microfabricated fluidic systems and NMR spectroscopy.

The philosophy of miniature total analysis systems (mu-TAS) hinges on the integration of multiple chemical processing steps and the means of analyzing their results on the same miniaturized system. We have constructed chip-based capillary electrophoresis (CE) devices equipped with an integrated planar radio-frequency detector coil used for nuclear magnetic resonance spectroscopy (NMR). Separations were accomplished in the devices, but satisfactory NMR spectra could only be obtained from samples of high concentration. The relative sensitivity is explained and the scaling law dichotomy of CE and NMR explored.

Radio Frequency Detectors Based on Microelectrodes

Microelectrodes commonly used in electrochemistry display a rectification effect that has escaped current notice. The application of microelectrodes as radio frequency detectors dates to 1899. Rectification at 60 Hz and 1 MHz is demonstrated

Fourier-transform hyperfine spectroscopy

A new pulsed electron nuclear double resonance technique for the measurement of hyperfine interactions is proposed. The experiment is based on burning transient holes into an inhomogeneously broadened EPR line by microwave and radio frequency pulses and detection of the hole pattern via an FID. In the spectrum obtained, each group of equivalent nuclei is represented by one peak at the hyperfine frequency, independent of the nuclear spin quantum number.

Radiofrequency detector for gas chromatography

A radiofrequency detector is described that is designed as a parallel-plate 63Ni electron-capture detector working with pure argon as carrier gas and with a radiofrequency voltage superimposed on a d.c. voltage. The principle of operation is that the radiofrequency voltage increases the mean electron energy within the detector, decreases the drift velocity and, owing to an increase in the recombination rate, the ionization current drops. In the presence of sample molecules the reverse process takes place: the drift velocity increases, the recombination rate decreases and the ionization current increases. Positive substance peaks occur. Detection limits down to 0.1 ppm for organic substances and 1.0 ppm for inorganic gases (carbon dioxide) are obtainable.

Flow measurement by ‘‘Doppler shift’’ nuclear magnetic resonance

A nuclear magnetic resonance (NMR) 1D flow measurement technique is described and demonstrated experimentally. The technique uses a ‘‘meander coil’’ in the NMR probe for the radio frequency (rf) excitation and detection. The meander coil gives rise to highly inhomogeneous B1 field, whose direction varies linearly as a function of spatial coordinates. The detected NMR signal acquires a frequency shift which is proportional to the speed of the nuclear spin. In a sense, this frequency shift is similar to the Doppler shift. The flow speed can be measured by one free induction decay (FID) without applying B0 field gradient.

Light-emitting diode as radio-frequency detector: development of RFI-free electromagnetic field sensor

A unique RFI-free electromagnetic field sensing technique using an LED as an RF detector is described. A double heterojunction (AlGaAs/GaAs) LED is used as an RF rectifier coupled to an EM probe; it delivers proportional light output with significant efficiency even at low (rectified) drive currents. DC biasing of the LED to the threshold of conduction can be achieved by an optically powered photodiode pair. Use of fibre-optic links from the detector to the receiver and in the optically powered bias arrangement eliminates the possibilities of RF1. Presented here are the sensitivity and frequency response of a test probe measured over 3–30 MHz under near-field conditions.

Transference numbers for magnesium chloride at 25°C

Transference numbers for aqueous magnesium chloride at 25°C have been measured by the moving-boundary method at concentrations up to 0.5 molar. The boundary movement was followed by using a newly designed radiofrequency detector by means of which the boundary velocity was determined by its passage past wire electrodes wrapped around the external surface of the moving-boundary tube. Extrapolation to infinite dilution gives T+/0 (MgCl2, 25°C) = 0.4114 which when combined with the known limiting conductance for the chloride ion gives the following equivalent conductances: Λ0 (MgCl2, 25°C) = 129.78 and l.0 (Mg2+, 25°C) = 53.39. Both of these values are independent of conductance measurements on MgCl2 but are shown to be consistent with the latest conductance data for MgCl2.

A radio-frequency moving-boundary detector

A new type of moving-boundary detector is described in which boundaries are detected at platinum ring electrodes situated on the outside of a moving-boundary tube. These rings are part of a bridge circuit operating at radio frequencies. Impedance changes in the region of the rings are converted by a phase-sensitive detector to a voltage output which, when operating in the differential mode, takes the form of a sharp peak as a boundary passes through the rings. Boundary displacements of less then 20 μm can be detected, and a precision of better than 0.02% is obtained in the measurement of boundary velocities in typical transference measurements. The fact that only three electrical leads are required in the detecting circuit gives this detector a decided advantage for high-pressure work. Equally important is the elimination of electrode polarization problems associated with other types of electrical detectors.

An Improved Sensitive Nonselective Radiofrequency Detector for Gas Chromatography

SUMMARY A dielectric-constant-response radiofrequency detector for gas chromatography is described. The detector is extremely sensitive to organic vapors; it should display general response to compounds which have a dielectric constant different than the carrier gas. Limits of detection have been significantly decreased when compared to previously suggested designs for radio-frequency detectors. The detector is physically small and readily compatible with commercially available chromatographs. A theory for the signal response of the detector is proposed.

Radio Receiver Design. Part I, Radio Frequency Amplification and Detection

<i>Radio Receiver Design. Part I, Radio Frequency Amplification and Detection</i>

Radio Receiver Design, Part I. Radio Frequency Amplification and Detection

Radio Receiver Design, Part I. Radio Frequency Amplification and Detection