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As an important means in electrical impedance tomography (EIT), multi-frequency phase-sensitive demodulation (PSD) can be viewed as a matched filter for measurement signals and as an optimal linear filter in the case of Gaussian-type noise. However, the additive noise usually possesses impulsive noise characteristics, so it is a challenging task to reduce the impulsive noise in multi-frequency PSD effectively. In this paper, an approach for impulsive noise reduction in multi-frequency PSD of EIT is presented. Instead of linear filters, a singular value decomposition filter is employed as the pre-stage filtering module prior to PSD, which has advantages of zero phase shift, little distortion, and a high signal-to-noise ratio (SNR) in digital signal processing. Simulation and experimental results demonstrated that the proposed method can effectively eliminate the influence of impulsive noise in multi-frequency PSD, and it was capable of achieving a higher SNR and smaller demodulation error. [ABSTRACT FROM AUTHOR]

The three axis vector magnetic field measurement based on the interaction of a single elliptically polarized light beam with an atomic system is described. The magnetic field direction dependent atomic responses are extracted by the polarimetric detection in combination with laser frequency modulation and magnetic field modulation techniques. The magnetometer geometry offers additional critical requirements like compact size and large dynamic range for space application. Further, the three axis magnetic field is measured using only the reflected signal (one polarization component) from the polarimeter and thus can be easily expanded to make spatial array of detectors and/or high sensitivity field gradient measurement as required for biomedical application. [ABSTRACT FROM AUTHOR]

We have designed and built a modern versatile research-grade instrument for ultrasound pulse-echo probing of the elastic properties of a wide range of materials under laboratory conditions. The heart of the instrument lies in an AD8302 microchip: a gain and phase detector from Analog Devices, Inc. To construct the device, we have implemented a schematic that utilizes the homodyne principle for signal processing instead of the traditional superheterodyne approach. This design allows one to measure phase shifts with high precision and linearity over the entire range of 0°-360°. The system is simple in construction and usage; it makes ultrasound measurements easily accessible to a broad range of researchers. It was tested by measuring the temperature dependence of the ultrasound speed and attenuation in a KTa0.92Nb0.08O3 (KTN) single crystal at a frequency of ~40 MHz. The tests were performed in the vicinity of the ferroelectric transitions where the large variations of the speed and attenuation demand a detector with outstanding characteristics. The described detector has a wide dynamic range and allows for measuring in a single run over the whole temperature range of the ferroelectric transitions, rather than just in limited intervals available previously. Moreover, due to the wide dynamic range of the gain measurements and high sensitivity this instrument was able to reveal previously unresolvable features associated with the development of the ferroelectric transitions of KTN crystals. [ABSTRACT FROM AUTHOR]

By increasing the section thickness of a triangular flexible hinge, this study focuses on optimal selection of parameters of fillet triangle flexible hinges and flexible support. Based on Castigliano's second theorem, the flexibility expression of the fillet triangle flexible hingewas derived. Then, the case design is performed, and the comparison of three types of flexible hinges with this type of flexible hinge was carried out. The finite element models of fillet triangle flexible hinges and flexible support were built, and then the simulation results of performance parameters were calculated. Finally, the experiment platform was established to validate analysis results. The maximum error is less than 8%, which verifies the accuracy of the simulation process and equations derived; also the fundamental frequency fits the requirements of the system. The fillet triangle flexible hinge is proved to have the advantages of high precision and low flexibility. [ABSTRACT FROM AUTHOR]

When an optical chopper is used to modulate the light source, the rotating speed of the wheel may vary with time and subsequently cause jitter of the modulation frequency. The amplitude calculated from the modulated signal would be distorted when the frequency fluctuations occur. To precisely calculate the amplitude of the modulated light flux, we proposed a method to estimate the range of the frequency fluctuation in the measurement of the spectrum and then extract the amplitude based on the sum of power of the signal in the selected frequency range. Experiments were designed to test the feasibility of the proposed method and the results showed lower root means square error than the conventional way. [ABSTRACT FROM AUTHOR]

Two kinds of optical heterodyne saturation spectroscopies, namely, frequency modulation spec-troscopy (FMS) and modulation transfer spectroscopy (MTS), are demonstrated for locking a fiber laser to the transition lines of metastable 4He atoms around 1083 nm. The servo-loop error signals of FMS and MTS for stabilizing laser frequency are optimized by studying the dependence of the peak-to-peak amplitude and slope on the optical power of pump and probe beams. A comparison of the stabilization performances of FMS/MTS and polarization spectroscopy (PS) is presented, which shows that MTS exhibits relatively superior performance with the least laser frequency fluctuation due to its flat-background dispersive signal, originated from the four-wave mixing process. The Allan deviation of the stabilized laser frequency is 5.4 x 10-12 @100 s with MTS for data acquired in 1000 s, which is sufficiently applicable for fields like laser cooling, optical pumping, and optical magnetometry. [ABSTRACT FROM AUTHOR]

The pulse-output Direct Digital Synthesis (DDS), in which the overflow signal of the phase accumulator is used for the pulse output, can be easily implemented due to its simple hardware architecture and low algorithm complexity. This paper introduces the fundamentals for generating Linear Frequency Modulation (LFM) pulse using pulse-output DDS technique. Error introducing mechanisms that affect the accuracy of signal's duration, initial phase, and frequency are studied. Extensive analysis of round-off error is given. A modified hardware architecture for LFM pulse generation with reduced round-off error is proposed. Experiment results are given, which shows that the proposed generator is promising in applications such as sonar transmitters. [ABSTRACT FROM AUTHOR]

Frequency modulation reflectometer has been developed to measure the plasma density profile of the Korea Superconducting Tokamak Advanced Research tokamak. Three reflectometers are operating in extraordinary polarization mode in the frequency range of Q band (33.6-54 GHz), V band (48-72 GHz), and W band (72-108 GHz) to measure the density up to 7 × 1019 m-3 when the toroidal magnetic field is 2 T on axis. The antenna is installed inside of the vacuum vessel. A new vacuum window is developed by using 50 μm thick mica film and 0.1 mm thick gold gasket. The filter bank of low pass filter, notch filter, and Faraday isolator is used to reject the electron cyclotron heating high power at attenuation of 60 dB. The full frequency band is swept in 20 μs. The mixer output is directly digitized with sampling rate of 100 MSamples/s. The phase is obtained by using wavelet transform. The whole hardware and software system is described in detail and the measured density profile is presented as a result. [ABSTRACT FROM AUTHOR]

A key issue for high-resolution frequency-modulation atomic force microscopy imaging in liquids is minimizing the frequency noise, which requires a detailed analysis of the corresponding noise contributions. In this paper, we present a detailed description for modifying a commercial atomic force microscope (Bruker MultiMode V with Nanoscope V controller), aiming at atomic-resolution frequency-modulation imaging in ambient and in liquid environment. Care was taken to maintain the AFMs original stability and ease of operation. The new system builds upon an optimized light source, a new photodiode and an entirely new amplifier. Moreover, we introduce a home-built liquid cell and sample holder as well as a temperature-stabilized isolation chamber dedicated to low-noise imaging in liquids. The success of these modifications is measured by the reduction in the deflection sensor noise density from initially 100 [formula] to around 10 [formula] after modification. The performance of our instrument is demonstrated by atomically resolved images of calcite taken under liquid conditions. [ABSTRACT FROM AUTHOR]

We have developed a wideband phase-locked loop (PLL) circuit with real-time phase correction for high-speed and accurate force measurements by frequency modulation atomic force microscopy (FM-AFM) in liquid. A high-speed operation of FM-AFM requires the use of a high frequency cantilever which, however, increases frequency-dependent phase delay caused by the signal delay within the cantilever excitation loop. Such phase delay leads to an error in the force measurements by FM-AFM especially with a low Q factor. Here, we present a method to compensate this phase delay in real time. Combined with a wideband PLL using a subtraction-based phase comparator, the method allows to perform an accurate and high-speed force measurement by FM-AFM. We demonstrate the improved performance by applying the developed PLL to three-dimensional force measurements at a mica/water interface. [ABSTRACT FROM AUTHOR]

We have developed a method for local potential measurements in liquid using frequency modulation atomic force microscopy. In this method, local potential is calculated from the first and second harmonic vibrations of a cantilever induced by applying an ac bias voltage between a tip and a sample. The use of an ac bias voltage with a relatively high frequency prevents uncontrolled electrochemical reactions and redistribution of ions and water. The nanoscale resolution of the method is demonstrated by imaging potential distribution of a dodecylamine thin film deposited on a graphite surface in 1 mM NaCl solution. [ABSTRACT FROM AUTHOR]

Frequency modulation atomic force microscopy (FM-AFM) is rapidly evolving as the technique of choice in the pursuit of high resolution imaging of biological samples in ambient environments. The enhanced stability afforded by this dynamic AFM mode combined with quantitative analysis enables the study of complex biological systems, at the nanoscale, in their native physiological environment. The operational bandwidth and accuracy of constant amplitude FM-AFM in low Q environments is heavily dependent on the cantilever dynamics and the performance of the demodulation and feedback loops employed to oscillate the cantilever at its resonant frequency with a constant amplitude. Often researchers use ad hoc feedback gains or instrument default values that can result in an inability to quantify experimental data. Poor choice of gains or exceeding the operational bandwidth can result in imaging artifacts and damage to the tip and/or sample. To alleviate this situation we present here a methodology to determine feedback gains for the amplitude and frequency loops that are specific to the cantilever and its environment, which can serve as a reasonable “first guess,” thus making quantitative FM-AFM in low Q environments more accessible to the nonexpert. This technique is successfully demonstrated for the low Q systems of air (Q∼40) and water (Q∼1). In addition, we present FM-AFM images of MC3T3-E1 preosteoblast cells acquired using the gains calculated by this methodology demonstrating the effectiveness of this technique. [ABSTRACT FROM AUTHOR]

A complete electronic system that is suitable for use in megahertz frequency domain time-resolved fluorescence instruments based on mode-locked lasers is described. The circuit produces a 10 MHz signal, phase locked to the mode-locked laser pulse frequency, which is required by many commercial frequency synthesizers as the external reference signal. This device is particularly useful in conjunction with ultrafast gated intensified charge coupled device cameras capable of being frequency modulated for time-resolved fluorescence imaging. [ABSTRACT FROM AUTHOR]

Complementary to industrial cantilever based force sensors in scanning probe microscopy (SPM), symmetrical quartz crystal resonators (QCRs), e.g., tuning fork, trident tuning fork, and needle quartz sensors, are of great interest. A self-excitation scheme with QCR is particularly promising and allows the development of cheap SPM heads with excellent characteristics. We have developed a high performance electronic interface based on an amplitude controlled oscillator and a phase-locked loop frequency demodulator applicable for QCR with frequencies from 10 up to 10 MHz. The oscillation amplitude of the sensing tip can be set from thermal noise level up to amplitudes of a tenth of nanometers. The device is small, cheap, and highly sensitive in amplitude and frequency measurements. Important features of the design are grounded QCR, parasitic capacity compensation, bridge schematic, and high temperature stability. Characteristic experimental data of the device and its operation in combination with a commercial SPM and a homemade scanning near-field optical microscope are reported. By using the 1 MHz needle quartz resonator with a standard atomic force microscope tip attached, atomic scale resolution in ambient conditions is achieved. Furthermore, reproducible measurements on very soft materials (Langmuir-Blodgett layers) with a very stiff needle quartz (∼400 000 N/m) are possible. [ABSTRACT FROM AUTHOR]

We have developed a liquid-environment frequency modulation atomic force microscope (FM-AFM) with a low noise deflection sensor for a wide range of cantilevers with different dimensions. A simple yet accurate equation describing the theoretical limit of the optical beam deflection method in air and liquid is presented. Based on the equation, we have designed a low noise deflection sensor. Replaceable microscope objective lenses are utilized for providing a high magnification optical view (resolution: <3 μm) as well as for focusing a laser beam (laser spot size: ∼10 μm). Even for a broad range of cantilevers with lengths from 35 to 125 μm, the sensor provides deflection noise densities of less than 11 fm/√Hz in air and 16 fm/√Hz in water. In particular, a cantilever with a length of 50 μm gives the minimum deflection noise density of 5.7 fm/√Hz in air and 7.3 fm/√Hz in water. True atomic resolution of the developed FM-AFM is demonstrated by imaging mica in water. [ABSTRACT FROM AUTHOR]

A frequency modulation (FM) reflectometer is designed to linearly sweep the microwave frequency in order to diagnose the plasma density profile. The static characteristic curve of the frequency versus the control voltage can be easily measured by using a spectrum analyzer. However, it is not guaranteed that the characteristic curve will be the same when the frequency is swept. A technique is devised which measures the dynamic characteristics by using the reflectometer itself. The results show that the shape of the characteristic curve does not depend on the sweep rate but on the time delay between the control voltage of the function generator and the microwave frequency at the detector input. This time delay should be compensated. Based on this measured characteristic curve, the frequency sweep rate and the distance between the antenna and the vacuum chamber are experimentally determined. [ABSTRACT FROM AUTHOR]

The fast data acquisition system (FAS) has been developed for the motional Stark effect (MSE) diagnostic in many machines. The FAS digitizes the signals coming directly out of the photomultiplier tube. The software solution of digital lock-in technique to derive the MSE signals from the data of FAS is presented in this article. The calculated frequency and phase of the photoelastic modulators (PEMs) with this digital lock-in technique can reach so high precision that the first, second, and fourth harmonic frequency signals of PEMs can be perfectly recovered. [ABSTRACT FROM AUTHOR]

A new analog frequency modulation (FM) detector (demodulator) for dynamic force microscopy (DFM) is presented. The detector is designed for DFM by utilizing the FM detection method where the resonance frequency shift of the force sensor is kept constant to regulate the distance between a tip and a sample surface. The FM detector employs a phase-locked loop (PLL) circuit using a voltage-controlled crystal oscillator (VCXO) so that the thermal drift of the output signal is negligibly reduced. The PLL is used together with a frequency conversion (heterodyne) circuit allowing the FM detector to be used for a wide variety of force sensors with the resonance frequency ranging from 10 kHz to 10 MHz. The minimum detectable frequency shift was as small as 0.1 Hz at the detection bandwidth of 1 kHz. The detector can track a resonance frequency shift as large as 1 kHz. We also present some experimental results including the observations of the Si(111)-7×7 reconstructed surface and fullerene molecules deposited on the surface by DFM using this FM detector. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Determines numerical simulations of frequency modulation reflectometry to examine the effects of density fluctuations on phase measurements. Important roles played by fluctuation amplitude, fluctuation frequency, sweep time of a source and beat frequency of a mixer output; Reliability of the density profile measurement.

Describes a high-frequency modulation and detection system capable of measuring absorbance changes on the order of 10[sup-6] in optical pump-probe experiments using submilliwatt laser powers from a mode-locked ultrafast laser. Use of two base oscillators to realize a single modulation scheme; Shot-noise-limited performance offered by the system.

A nuclear frequency-modulation spectrometer (NFMS) for high-resolution gamma-ray spectroscopy is described in this article. As the name implies, this device operates by modulating the cross section for gamma-ray absorption. The automation of this spectrometer required the development of an interface to an Apple computer which provides a real-time data display. This interface also enables the Apple computer to control up to two Mössbauer spectrometers at once, with a real-time data display for each. A nuclear frequency-modulation spectrometer makes it possible to observe directly the phenomenon known as "rf sidebands' in Mössbauer spectroscopy, without interference from the "parent transitions." The high resolution of NFMS makes it possible to examine the "rf sidebands" for any fine structure. [ABSTRACT FROM AUTHOR]

Wavelet transform (WT) is widely used in signal processing. The frequency modulation reflectometer in the KSTAR applies this technique to get the phase information from the mixer output measurements. Since WT is a time consuming process, it is difficult to calculate the density profile in real time. The data analysis time, however, can be significantly reduced by the use of the Graphics Processing Unit (GPU), with its powerful computing capability, in WT. A bottle neck in the KSTAR data processing exists in the data input and output (IO) process between the CPU and its peripheral devices. In this paper, the details of the WT implementation assisted by a GPU in the KSTAR reflectometer are presented and the consequent performance improvement is reported. The real time density profile calculation from the reflectometer measurements is also discussed. [ABSTRACT FROM AUTHOR]

A phase compensation technique in the fiber optical quadrature passive demodulation scheme is presented. The phase error between the two interferometric signals is calculated first, and a new signal is generated which has an accurate phase difference of 90° with respect to another one. The experimental results show that the distortion in the demodulated signal is removed and the input signal is correctly recovered. The technique is successfully used in fiber optical interferometric sensors. [ABSTRACT FROM AUTHOR]

We have developed a wideband digital frequency detector for high-speed frequency modulation atomic force microscopy (FM-AFM). We used a subtraction-based phase comparator (PC) in a phase-locked loop circuit instead of a commonly used multiplication-based PC, which has enhanced the detection bandwidth to 100 kHz. The quantitative analysis of the noise performance revealed that the internal noise from the developed detector is small enough to provide the theoretically limited noise performance in FM-AFM experiments in liquid. FM-AFM imaging of mica in liquid was performed with the developed detector, showing its stability and applicability to true atomic-resolution imaging in liquid. [ABSTRACT FROM AUTHOR]

Semiconductor laser based, injection locking was performed for efficient excitation of subjects. The linewidth of the master laser was extended to 380 MHz by using a frequency modulation technique through a direct current modulation. By the injection locking technique with a broad-area semiconductor laser, the frequency modulated input was amplified to 610 mW, faithfully maintaining the broadened linewidth and the spectral shape. This result means that a compact high power laser system is possible with broad linewidth that is precisely controllable. [ABSTRACT FROM AUTHOR]