Sensitive RF Research Articles

RF-to-DC Characteristics of Direct Irradiated On-Chip Gallium Arsenide Schottky Diode and Antenna for Application in Proximity Communication System

We report the RF-to-DC characteristics of the integrated AlGaAs/GaAs Schottky diode and antenna under the direct injection and irradiation condition. The conversion efficiency up to 80% under direct injection of 1 GHz signal to the diode was achieved. It was found that the reduction of series resistance and parallel connection of diode and load tend to lead to the improvement of RF-to-DC conversion efficiency. Under direct irradiation from antenna-to-antenna method, the output voltage of 35 mV was still obtainable for the distance of 8 cm between both antennas in spite of large mismatch in the resonant frequency between the diode and the connected antenna. Higher output voltage in volt range is expected to be achievable for the well-matching condition. The proposed on-chip AlGaAs/GaAs HEMT Schottky diode and antenna seems to be a promising candidate to be used for application in proximity communication system as a wireless low power source as well as a highly sensitive RF detector.

Quantitative 23Na MRI of human knee cartilage using dual‐tuned 1H/23Na transceiver array radiofrequency coil at 7 tesla

To develop quantitative dual-tuned (DT) (1) H/(23) Na MRI of human knee cartilage in vivo at 7 Tesla (T). A sensitive (23) Na transceiver array RF coil was developed at 7T. B1 fields generated by the transceiver array coil were characterized and corrected in the (23) Na images. Point spread function (PSF) of the (23) Na images was measured, and the signal decrease due to partial-volume-effect was compensated in [(23) Na] quantification of knee cartilage. SNR and [(23) Na] in anterior femoral cartilage were measured from seven healthy subjects. SNR of (23) Na image with the transceiver array coil was higher than that of birdcage coil. SNR in the cartilage at 2-mm isotropic resolution was 26.80 ± 3.69 (n = 7). B1 transmission and reception fields produced by the DT coil at 7T were similar to each other. Effective full-width-half-maximum of (23) Na image was ∼5 mm at 2-mm resolution. Mean [(23) Na] was 288.13 ± 29.50 mM (n = 7) in the anterior femoral cartilage of normal subjects. We developed a new high-sensitivity (23) Na RF coil for knee MRI at 7T. Our (1) H/(23) Na MRI allowed quantitative measurement of [(23) Na] in knee cartilage by measuring PSF and cartilage thickness from (23) Na and (1) H image, respectively.

Magnetic sensing via ultrasonic excitation

We present ultrasonic techniques for magnetic measurements. Acoustically modulated magnetization is investigated with sensitive rf detection by narrowband loop antennas. Magnetization on the surface of ferromagnetic metals is temporally modulated with the rf frequency of the irradiated ultrasonic waves, and the near-field components emitted from the focal point of the ultrasonic beam are detected. Based on the principle of the acoustically stimulated electromagnetic (ASEM) response, magnetic sensing and tomography are demonstrated by ultrasonic scanning. We show that ASEM imaging combines good acoustic resolution with magnetic contrast. The sensitivity of this method is estimated to be about 6 G/Hz(1∕2) in our current setup.

Novel Design of a Highly Sensitive RF Strain Transducer for Passive and Remote Sensing in Two Dimensions

A novel design of a highly sensitive wireless passive RF strain transducer is presented based on a patch antenna loaded with an open loop that is capable of sensing strain independently in two directions. An original idea of utilizing a cantilever at the gap of the open loop significantly improves the sensitivity of resonant frequency shifts. The frequency shifts in two distinct resonant modes are detected based on two dominant orthogonal modes of the patch resonators. In measurements, the prototypes achieved a sensitivity of 2.35% frequency shift per 1% strain, more than twice that of existing strain transducers of the same class. In simulations, the new design achieved a theoretical sensitivity up to four times as high as existing designs of RF passive wireless strain transducers. The ground plane allows for the sensitivity of the sensor to be independent from the applied surface. An implementation example of the passive remote sensing system based on the proposed strain transducer is also discussed as a proof-of-concept case. Based on calculations, the interrogation method in the example shows a radar cross section fluctuation of 3.8 dB corresponding to the strain induced at the sensor.

Fast on-wafer electrical, mechanical, and electromechanical characterization of piezoresistive cantilever force sensors

Validation of a technological process requires an intensive characterization of the performance of the resulting devices, circuits, or systems. The technology for the fabrication of micro and nanoelectromechanical systems (MEMS and NEMS) is evolving rapidly, with new kind of device concepts for applications like sensing or harvesting are being proposed and demonstrated. However, the characterization tools and methods for these new devices are still not fully developed. Here, we present an on-wafer, highly precise, and rapid characterization method to measure the mechanical, electrical, and electromechanical properties of piezoresistive cantilevers. The setup is based on a combination of probe-card and atomic force microscopy technology, it allows accessing many devices across a wafer and it can be applied to a broad range of MEMS and NEMS. Using this setup we have characterized the performance of multiple submicron thick piezoresistive cantilever force sensors. For the best design we have obtained a force sensitivity Re(F) = 158μV/nN, a noise of 5.8 μV (1 Hz-1 kHz) and a minimum detectable force of 37 pN with a relative standard deviation of σ(r) ≈ 8%. This small value of σ(r), together with a high fabrication yield >95%, validates our fabrication technology. These devices are intended to be used as bio-molecular detectors for the measurement of intermolecular forces between ligand and receptor molecule pairs.

Virtual Ground Fence: A Simple Method for Protection against High Frequency Simultaneous Switching Noise

When integrating sensitive RF analog devices with complex VLSI digital components, simultaneously switching drivers cause supply voltage fluctuations which can propagate both horizontally and vertically between the power/ground planes. The same voltage source on a printed circuit board can be shared to increase power efficiency and reduce space used. In order to accomplish this, on board filtering is needed to isolate the noise between these two types of devices for proper operation. Hence, accurate estimation and improvement of the performance of power/ground planes is critical in a mixed-signal system. We present a new method to minimize the noise transfer at high frequencies to the power distribution system, called the Virtual Ground Fence. At its basic level, the Virtual Ground Fence consists of quarter-wave transmission-line stubs that act as short circuits between power and ground planes at their design frequency. We will present various configurations of Virtual Ground Fence for different coupling scenarios.

A high sensitivity ultralow temperature RF conductance and noise measurement setup

We report on the realization of a high sensitivity RF noise measurement scheme to study small current fluctuations of mesoscopic systems at milli-Kelvin temperatures. The setup relies on the combination of an interferometric amplification scheme and a quarter-wave impedance transformer, allowing the measurement of noise power spectral densities with gigahertz bandwidth up to five orders of magnitude below the amplifier noise floor. We simultaneously measure the high frequency conductance of the sample by derivating a portion of the signal to a microwave homodyne detection. We describe the principle of the setup, as well as its implementation and calibration. Finally, we show that our setup allows to fully characterize a subnanosecond on-demand single electron source. More generally, its sensitivity and bandwidth make it suitable for applications manipulating single charges at GHz frequencies.

Erratum to: A high sensitivity and wide dynamic range zero-if RF receiver for cognitive radio application

Erratum to: A high sensitivity and wide dynamic range zero-if RF receiver for cognitive radio application

A high sensitivity and wide dynamic range zero-IF RF receiver for cognitive radio application

This paper presents an RF receiver of zero-Intermediate Frequency (IF) architecture for Cognitive Radio (CR) communication systems. Zero-IF architecture reduce the image reject filter and IF filter, so it is excellent in low cost, compact volume, and low power dissipation. The receiver employs three digital attenuator and a high gain, high linearity low noise amplifier to achieve wide dynamic range of 70 dB and high receiving sensitivity of −81 dBm. A fully balanced I/Q demodulator and a differential Local Oscillator (LO) chips are used to minimize the negative effects caused by second-order distortion and LO leakage. In order to select an 8 MHz-channel from 14 continuous ones located in UHF band (694–806 MHz) accurately, approach of channel selectivity circuits is proposed. The RF receiver has been designed, fabricated, and test. The measured result shows that the noise figure is 3.4 dB, and the error vector magnitude is 7.5% when the input power is −81 dBm.

An Experimental Study on High-Frequency Substrate Noise Isolation in BiCMOS Technology

In this letter, four substrate noise isolation structures in standard 0.18-mum SiGe bipolar CMOS technology were investigated using S-parameter measurements. The experimental and simulated results on different isolation structures, such as triple-well p-n junction isolated walls, deep trench isolation, and double P+ guard-ring structures, are presented. Each element in the equivalent circuits has been calculated or fitted based on the parasitic resistance, capacitance, and physical dimensions using the device simulator MEDICI and the measured results of the test patterns. The proposed structure B significantly reduced substrate noise below -70 dB up to 20 GHz. The proposed structure C with an extra triple-well junction achieved the best isolation at the lower frequency range, in which |S21| was less than -71 dB from 50 MHz to 10.05 GHz, and -56 dB from 10.05 to 20.05 GHz. The measured results showed an excellent agreement with the calculations. Structure B is good enough and is recommended for a general-purpose RF circuit design, whereas structure C can be used in a highly sensitive RF circuit block below 10 GHz.

FPGA Based GPS Receiver Design Considerations

A project to build a GPS receiver using an FPGA for base-band processing began in 2004. The new receiver platform uses a commonly available RF front end ASIC to convert the GPS signals to a suitable IF. The digital design for baseband processing is normally a reasonably straight forward task. However, because the received GPS signals are at such low levels this presents some challenges. One of the main considerations is to avoid contamination of the incoming signals with interference that can be generated from the digital electronics when using an FPGA. In this paper we describe the hardware design process with a focus on avoiding interference while still allowing complex FPGA logic to operate alongside sensitive GPS RF signal processing.

Evaluation of the turn variation of spiral RF surface coils for MR microscopic imaging and spectroscopy

The purpose of this study was to enhance the technical improvement so as to develop an advanced sensitive RF surface coil for investigating the sensitivities of the multi-spiral surface coils, and we eventually wanted to achieve high resolution of the microscopic MR images and MR spectra. The magnetic field inhomogeneity and the shape of a surface coil were statistically estimated by simulation of the magnetic field distribution. On the basis of the experimental results with single, three- and five-turned spiral RF surface coils in 1.5 T, we found that the three-turned coil had the highest sensitivity. The present study showed that the sensitivity of the RF surface coil was improved by increasing the number of spiral coil turns, and also the SNR of the RF surface coil was dependent upon the number of spiral coil turns. However, we found, rather strikingly, that the sensitivity of excessive turns of the coils was decreased due to the rise of the coil’s impedance. Thus, the present results demonstrated that the sensitivity was not proportional to the number of a spiral RF coil’s turns, and the number of spiral coil turns should be optimized for obtaining the highest sensitivity and SNR.

SU‐FF‐I‐63: Evaluation of the Turn Variation of Spiral RF Surface Coils for MR Microscopic Imaging and Spectroscopy

Purpose: The purpose of this study was to enhance the technical improvement which can develop the advanced sensitive RF surface coil to investigate the sensitivities of the multi‐spiral surface coils and eventually achieve the high resolution of microscopic MR images and MR spectra. Method and Materials: The simulation of a coil's magnetic field was processed in MATLAB. Biot‐Savart law provides information about the relation between the amplitude and the position. The experiment was processed in the Oxford magnet (1.5T), Copley gradient (25mT/m) coil and Analogic RF amplitude system. The diameter of a coil was fixed at 4cm, and the turn of a coil was increased with a spiral turning method. Results: 1. Simulation of magnetic field. The magnetic field strength was increased close to the wire because of a ring‐shape wire coil. This simulation was performed assuming no obstruction in the current and domain. Thus, the shape and the field distribution of a RF surface coil were able to be visually analyzed. 2. Coils' performance. The ROI with approximately 3cm was identified by a water phantom in 1.5T. The three and five turned spiral coils were developed and their Q‐factor and bandwidth were estimated. Conclusion: The present study showed that the sensitivity of RF surface coil was improved by the increasing the number of a spiral coil's turn, and also the SNR of RF surface coil was dependent upon the number of a spiral coil's turn. However, the sensitivity was not proportional to the number of a spiral RF coil's turn. There is an optimal vale in a spiral RF coil turn and sensitivity. In order to obtain a high sensitivity as varying the spiral RF coil's performance, it is important to find the optimal number of turn. This study provides an efficient approach to designing high‐field RF coils.

Modulation depth enhancement for highly sensitive electro-optic RF near-field measurement

A modulation depth enhancement technique for highly sensitive electro-optic (EO) sensing of RF near-fields is presented. This technique increases EO signal intensity without increasing total power received by a photodiode (PD). Therefore, sensitivity limitation owing to saturation of the PD can be avoided. Sensitivity improvement of 20 dB compared to the conventional configuration is successfully achieved.

High sensitivity RF coil system for visual fMRI studies at 3T

High sensitivity RF coil system for visual fMRI studies at 3T

One micrometer resolution NMR microscopy.

We obtained a magnetic resonance image of 1 microm resolution and 75 microm(3) voxel volume for a phantom filled with hydrocarbon oil within an hour at 14.1 T. For this work, a specially designed probe with a high sensitivity RF coil and gradient coils generating over 1000 G/cm was built. The optimal pulse sequence was analyzed in consideration of the bandwidth, diffusion coefficients, and T(1) and T(2) relaxations of the medium. The system was applied to the in vivo imaging of a geranium leaf stem to get the images of 2 microm resolution and 200 microm(3) voxel volume.

Progress in low noise cooling performance of a pulse-tube cooler for HT-SQUID operation

Previously it was shown that Joule-Thomson- and pulse tube coolers (PTC) are attractive candidates for low noise cooling of HT-SQUIDs. Tests of a highly sensitive HT-SQUID that was mounted directly on the cold tips of these coolers revealed discrete peaks in the flux noise spectrum which were attributed to pressure wave-induced vibrations of the coolers. A first additional noise reduction for the PTC was achieved by using Ti-V-Al tubes instead of stainless steel for the regenerator and pulse tube. Further reduction was obtained by mounting the sensor on a separate platform that was thermally connected to the cold tip by use of a flexible copper link. The remaining amplitude of axial vibrations at the cold platform was measured to be about 0.5 /spl mu/m as compared to 6.0 /spl mu/m without vibration compensation. We report on the tests of different methods of vibration compensation for the PTC, using a sensitive rf HT-SQUID magnetometer with coplanar resonator showing an intrinsic noise of 45 fT//spl radic/Hz at frequencies above 100 Hz and 110 fT//spl radic/Hz at 10 Hz. Measurements of the flux noise spectrum of the HT-SQUID using the new PTC cold head will be presented.

A HTS rf SQUID vector magnetometer for geophysical exploration

We report on a highly sensitive rf SQUID vector (x, y, z) magnetometer system operating at 77 K and suitable for electromagnetic geophysical exploration. We achieved stable long-term operation of all three channels during field trials outside magnetic shielding in urban and rural environments. With sensors on 10/spl times/10 mm/sup 2/ substrates, the white noise level in earth's field was 40 fT//spl radic/Hz above 100-300 Hz, in all three channels. The new system meets the high requirements for slew rate (>2 mT/s), dynamic range (>130 dB) and bandwidth (20 kHz) which must be fulfilled in geophysical measurements, especially the TEM (transient electromagnetic) method. We also present results of geophysical TEM field trials over various test sites. We obtained good reproducibility of SQUID data and a good correlation between output signals of reference induction coils and the derivative of SQUID signals. Results from TEM measurements indicate that for late time signals, originating from deeper geological structures, the SQUID magnetometer provides a better signal-to-noise ratio than an induction coil.

MRI-guided treatment of degenerative spinal diseases

SummaryIn the treatment of degenerative spinal diseases, CT-guided local injections, RF or ethanol denervation of facet joints and laser decompression of herniated intervertebral disks offer a precisely aimed and effective option in cases of back pain and sciatica. As CT involves significant exposure of the patient to ionising radiation, we evaluated the feasibility of MR-guided therapy. MRI offers a variety of advantages, such as excellent soft-tissue contrast, high flow and temperature sensitivity, and arbitrary slice orientation within a 3D data set. The recently-introduced low-field open magnets, though of compromising imaging capabilities, have considerably facilitated access to the patient. The use of instruments and ancillary equipment, however, is made difficult by the high magnetic field and sensitive RF system of the MRI scanner. The techniques of MR-guided injection and using MR-compatible cannulae were developed and tested with ex vivo specimens of the spine and animal phantoms. 184 patients were treated with periradicular corticoid injections, six patients with laser decompression, 21 patients with neurolysis of facet joints and 16 patients underwent chemical lumbar sympathectomy. MR-guidance is feasible and the early clinical results are promising and comparable to those of CT-guided pain therapy. Further technical improvement of MR technology are required to facilitate the procedures.

Population study of the importance of rheumatoid factor isotypes in adults.

Blood samples collected from 13,858 randomly selected subjects participating in a health survey in Iceland from 1974 to 1983 were tested for rheumatoid factor. Samples that were positive in a sensitive RF screening test were analysed further by the Rose-Waaler technique and an isotype specific enzyme linked immunosorbent assay (ELISA). In 1987 the 173 available participants who were RF positive and 156 matched RF negative controls were evaluated clinically for rheumatoid diseases. RF levels and isotype patterns were more persistent in the patients with rheumatoid arthritis (RA) than in RF positive subjects who did not have overt RA. The prevalence of RA was only 19% in the participants who were RF positive in 1987. Forty per cent of the participants who had a persistent (four to 13 years) increase of IgA RF combined with either IgM or IgG RF were diagnosed as having RA. A positive correlation was found between RF levels and various manifestations of RA. This association was stronger for the IgA and IgG RF isotypes than for IgM RF. Excluding RF positivity as a diagnostic parameter, RA was diagnosed in 33 of the participants and 20 (61%) of these patients had increased levels of IgM and IgA RF. Patients with RA with bone erosions in their hands had higher levels of IgA RF than patients without erosions, but an association was not found between bone erosions and other RF isotypes. None of the RF negative participants who were symptom free when the original blood sample was taken developed RA during the four to 13 year follow up period. In contrast, five symptom free RF positive participants developed RA during this period. These five patients had all had increased levels of at least two RF isotypes before the onset of their symptoms. It is concluded that the IgA and IgG RF isotypes have a closer association with the clinical parameters of RA than IgM RF. Furthermore, increases in RF can precede clinical manifestations of RA and this applies in particular to the IgA and IgG RF isotypes.