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A triple-section arm structure is proposed for designing a planar spiral antenna. All three sections are designed by combining logarithmic, rooted, and sine equations. The slowly outstretched and contractive structure is innovatively realized. According to the radiation characteristics of the spiral antenna, each section corresponds to different in-band enhancement effects. Numerical simulation in the frequency domain and experiments using two different baluns are carried out. The results show that the novel spiral topology could simultaneously achieve improved axial ratio, low cross-polarized gain, and excellent impedance matching throughout the whole band. The axial ratio is reduced by 1.5 dB at midfrequencies and more at low frequencies, comparing the proposed arm with a sinusoid-added equiangular spiral arm. Without applying the resistive loading method, a lower cut-off frequency of 750MHz is still realized both in impedance bandwidth and axial ratio bandwidth. The low cut-off frequency of the proposed arm is 30.2% lower than the conventional Equiangular spiral arm. Besides, the polarization isolation is significantly improved, especially at low frequencies. Therefore, the proposed miniaturized spiral arm structure could be a competitive form for designing spiral antennas. [ABSTRACT FROM AUTHOR]

Over the past decade, helicopter-borne transient electromagnetic (HTEM) systems have been rapidly developed. A new HTEM prototype (referred to as a CAS-HTEM) has been developed by the Chinese Academy of Sciences. In terms of hardware, the CAS-HTEM system uses an inflatable structure to carry the transmitting loop, which significantly reduces the weight of the system and makes it easier to transport. A dual gain receiver was innovated to extend the dynamic range of the system. In addition, an observation circuit for transmitting voltage waveform is introduced, so that the derivative waveform of transmitting current with higher SNR could be calculated. In terms of data processing, more reliable early time data could be obtained by band-limited effect removal; a weighted stacking algorithm is introduced to reduce the narrow band noise more effectively and increase the sensitivity of data to the anomaly location; a method based on τ-domain transform is used for late time signal processing. The results of the field test which was carried out in Inner Mongolia were found to be consistent with the drill data, which effectively verified the performance of this HTEM prototype. [ABSTRACT FROM AUTHOR]

Powerline noise is one of the most common contaminating types of the observed transient electromagnetic signal. For the conventional TEM method using the bipolar square wave as transmitter waveform, synchronous sampling is the main technology for powerline noise suppression, and notching filtering is sometimes used also. When the transmitter wave has been encoded based on pseudo-random binary sequence, it has proven difficult to achieve the effect by using the above-mentioned conventional methods for the suppression of powerline noise. This is due to the fact that the duration of each logic states of the pseudo-coded transmitter waveform is normally inconsistent. In this study, a method for the suppression of powerline noise is proposed, which is based on the independent component analysis method (ICA). In order to introduce the observation and processing details of this method more clearly, we attempt to apply this method to the time-domain electromagnetic method with coded source researched and developed by the institute of geology and geophysics of Chinese academy of sciences on the basis of the MTEM method. In terms of specific processes, the electrical measurements need to be simultaneously observed in the inline and crossline directions firstly, and then the data will be input into the processing procedures based on ICA method to realize the effective separation of the powerline noise and the useful signals. The processing results of the simulation data and field data show that the method proposed in this paper can suppress the powerline noise effectively, and the processing results build a good data foundation for the subsequent processing. [ABSTRACT FROM AUTHOR]

Precise characterization of reinforcing bars (rebars) in a concrete structure is of significant importance for construction quality control and post-disaster safety evaluation. This paper integrates ground-penetrating radar (GPR) and electromagnetic induction (EMI) methods for simultaneous estimation of rebar diameter and cover thickness. A prototype of GPR-EMI dual sensor is developed, and a calibration experiment is conducted to collect a standard EMI dataset corresponding to various rebar diameters and cover thicknesses. The handheld testing cart can synchronously collect both GPR and EMI data when moving on the concrete surface, from which a data processing algorithm is proposed to simultaneously estimate the rebar diameter and cover thickness. Firstly, by extracting the apex of the hyperbolic reflection from the rebar in the preprocessed GPR profile, the rebar position is determined and further used to extract the effective EMI curve. Then, the rebar diameter and cover thickness are simultaneously estimated from the minimum mean square error between the measured and calibrated EMI data under the constraint of the GPR-estimated cover thickness. A laboratory experiment is performed using four casted concrete specimens with 11 embedded steel rebars. The results show that the diameters of 10 rebars are correctly estimated out of the 11 rebars, and the maximum estimation error for the cover thickness is 6.7%. A field trial is carried out in a newly-constructed building, and the diameters of four tested rebars are all accurately estimated while the estimation errors of the cover thickness are less than 5%. It is concluded that the developed GPR-EMI dual sensor and the proposed algorithm can estimate the rebar diameter and cover thickness accurately by a single scan. [ABSTRACT FROM AUTHOR]

Ground penetrating radar (GPR), as a nondestructive testing tool, is suitable for estimating the thickness and permittivity of layers within the pavement. However, it would become problematic when the layer is thin with respect to the probing pulse width, in which case overlapping between the reflected pulses occurs. In order to deal with this problem, a hybrid method based on multilayer perceptrons (MLPs) and a local optimization algorithm is proposed. This method can be divided into two stages. In the first stage, the MLPs roughly estimate the thickness and the permittivity of the GPR signal. In the second stage, these roughly estimated values are used as the initial solution of the full-waveform inversion algorithm. The hybrid method and the conventional global optimization algorithm are respectively used to perform the full-waveform inversion of the simulated GPR data. Under the same inversion precision, the objective function needs to be calculated for 450 times and 30 times for the conventional method and the hybrid method, respectively. The hybrid method is also applied to a measured data, and the thickness estimation error is 1.2 mm. The results show the high efficiency and accuracy of such hybrid method to resolve the problem of estimating the thickness and permittivity of a “thin layer”. [ABSTRACT FROM AUTHOR]

Ultra-wideband (UWB) impulse radar plays an important role in contactless vital sign (VS) detection. The VS can be extracted remotely by acquiring the oscillations in the human chest. Unfortunately, it is usually challenging to identify VS due to the low signal-to-noise ratio (SNR) only based on the traditional fast Fourier transform (FFT) especially in complicated conditions. To extract VS accurately, this paper presents a new scheme by analyzing the skewness characteristic of the received UWB impulses, which are modulated by life activities. The distance from the human subject to the radar antenna can be calculated by performing the discrete short-time Fourier transform (DSFT) on skewness. The frequency of human respiratory movement can be estimated based on the developed ensemble empirical mode decomposition (EEMD)-based accumulation technique by canceling out the harmonics effectively. The performance of the developed detection method is tested with several experiments carried out in different environments. [ABSTRACT FROM AUTHOR]

A new iterative inversion algorithm is proposed to reconstruct the electrical conductivity profile in a stratified underground medium for the grounded electrical source airborne transient electromagnetic (GREATEM) system. In forward modeling, we simplify the mathematical expressions of the magnetic fields generated by a finite line source in the layered ground to semianalytical forms in order to save the computation time. The Fréchet derivative is derived for the electromagnetic response at the receivers due to a small perturbation of the conductivity in a certain layer underground. The initial expression of the Fréchet derivative has an expensive triple integral and contains the Bessel function in the integrand. It is simplified by partially eliminating the integration along the source line and deriving the analytical expression for the integration in the vertical direction inside the perturbed layer. In the inverse solution, we use the distorted Born iterative method (DBIM). This is the first time that the DBIM is applied to data measured by the GREATEM system. Besides, the forward and inverse procedures are carried out in the frequency domain and based on the Fréchet derivative of a line source. We demonstrate the validity of our forward model, Fréchet derivative, inverse model, and the precision as well as robustness of the inversion algorithm through numerical computation and comparisons. Finally, we apply the inversion algorithm to the measured data and compare the retrieved conductivity to the actual drilling data. [ABSTRACT FROM AUTHOR]

The presence of the primary field in the helicopter transient electromagnetic system makes the dynamic range of the response signal so large that it is difficult to observe the secondary field. Therefore, a bucking coil is usually introduced to eliminate the primary field. However, in a traditional design, the size of the bucking coil increases with the size of the system, which makes the bucking coil hard to install, and opposite magnetic moment is large in huge systems. In this paper, a new bucking coil design for a helicopter transient electromagnetic system is proposed. Compared with the traditional design, the bucking coil diameter, total weight and total magnetic moment in two designs are calculated. The results show that the bucking coil we designed is more than 8 times smaller and 5 times lighter than that in the traditional design, which is easier for installation. The bucking moment impact is reduced to 0.03% of the total magnetic moment when the diameter of the transmitting coil increases to 35m, which improves the efficiency of the system. Then we analyze the requirement of manufactory precision and installation accuracy for the bucking coil in our design to get the best bucking result. [ABSTRACT FROM AUTHOR]

Optical prisms can spread compound light spatially into a rainbow and have widespread applications in spectroscopy and imaging. Limited by the natural materials as well as technologies, there has been no natural counterpart of the optical prism that works in the Terahertz (THz) band so far. In this letter, a THz prism analogue based on metasurfaces working in the transmission diffraction mechanism is first proposed to generate the THz rainbow spectrum. The physics of different modes excited by the interaction between the incident wave and the metasurface is investigated in theory and simulation. A coherent enhancement method was developed to improve the mode competition of the rainbow spectrum over other unwanted leaky modes to guarantee the high transfer efficiency of the wavelength dependent transmission diffraction. The experimental results show that the prism analogue can spread the incident spectrum from 0.15 to 0.22THz in an angular scope of about 30.8° with comparatively high transferring efficiency. [ABSTRACT FROM AUTHOR]

In this paper, the methods and analysis for estimating the location of a three-dimensional (3-D) single source buried in lossy medium are presented with uniform circular array (UCA). The mathematical model of the signal in the lossy medium is proposed. Using information in the covariance matrix obtained by the sensors' outputs, equations of the source location (azimuth angle, elevation angle, and range) are obtained. Then, the phase and amplitude of the covariance matrix function are used to process the source localization in the lossy medium. By analyzing the characteristics of the proposed methods and the multiple signal classification (MUSIC) method, the computational complexity and the valid scope of these methods are given. From the results, whether the loss is known or not, we can choose the best method for processing the issues (localization in lossless medium or lossy medium). [ABSTRACT FROM AUTHOR]

An adaptive sharp boundary inversion scheme is developed to improve resolution with feasibility for transient electromagnetic (TEM) data inversion. By using weighted minimum gradient support (WMGS) constraint, this method focuses the resistivity change areas on layer boundary locations. Prior information describing roughness can be added into the constraint to improve resolution. Furthermore, even though no prior information about layer boundaries is available, it can still reconstruct models with geo-electrical interfaces. Synthetic models prove that this method has a better performance in presenting layer boundaries than smooth-model inversion. Field data of a TEM test line are inverted using this method, which makes the basement layer visualized easily. [ABSTRACT FROM AUTHOR]

Magnetic anomaly detection (MAD) is to find hidden ferromagnetic objects, and a hidden object is often described as a magnetostatic dipole. Many detection methods are based on the orthonormal basis functions when the target moves along a straight line relatively to the magnetometer. A new kind of parabolic trail orthonormal basis functions (PTOBF) method is proposed to detect the magnetic target when the trajectory of the target is parabola. The simulation experiment confirms that the proposed method can detect the magnetic anomaly signals in white Gaussian noise when SNR is -15.56 dB. The proposed method is sensitive to the characteristic time and curvature. High detection probability and simple implementation of proposed method make it attractive for the real-time applications. [ABSTRACT FROM AUTHOR]

Magnetic anomaly detection (MAD) is to find hidden ferromagnetic objects, and a hidden object is often described as a magnetostatic dipole. Many detection methods are based on the orthonormal basis functions when the target moves along a straight line relatively to the magnetometer. A new kind of parabolic trail orthonormal basis functions (PTOBF) method is proposed to detect the magnetic target when the trajectory of the target is parabola. The simulation experiment confirms that the proposed method can detect the magnetic anomaly signals in white Gaussian noise when SNR is -15.56 dB. The proposed method is sensitive to the characteristic time and curvature. High detection probability and simple implementation of proposed method make it attractive for the real-time applications. [ABSTRACT FROM AUTHOR]

In this paper, a portable frequency domain electromagnetic system CEM-2 is presented for shallow metal targets detection. This paper discusses the detection principle of frequency domain electromagnetic system, introduces hardware implementation, presents test results of each module, and gives the system's imaging results in field tests. Sinusoidal pulse width modulation technique is employed in this system to produce single-frequency or multi-frequencies synthetic electromagnetic signals with signal to noise ratio of about 85 dB. After integration, the CEM-2 system's in-phase noise level is about 90 ppm while the quadrature response is about 100 ppm. The experiment results of CEM-2 agree well with the simulation ones both from signatures and amplitudes. The experiment for detecting targets of different sizes and materials conducted in field indicates that CEM-2 system can be used to distinguish metallic and ferrous objects. [ABSTRACT FROM AUTHOR]

The Scalar Triangulation and Ranging (STAR) method, which is based upon the unique properties of magnetic gradient contraction, is a high real-time ferromagnetic target localization method. Only one measurement point is required in the STAR method and it is not sensitive to changes in sensing platform orientation. However, the localization accuracy of the method is limited by the asphericity errors and the inaccurate value of position leads to larger errors in the estimation of magnetic moment. To improve the localization accuracy, a modified STAR method is proposed. In the proposed method, the asphericity errors of the traditional STAR method are compensated with an iterative algorithm. The proposed method has a fast convergence rate which meets the requirement of high real-time localization. Simulations and field experiments have been done to evaluate the performance of the proposed method. The results indicate that target parameters estimated by the modified STAR method are more accurate than the traditional STAR method. [ABSTRACT FROM AUTHOR]

The thickness estimation of the top surface layer and surface layer, as well as the detection of road defects, are of great importance to the quality conditions of asphalt pavement. Although ground penetrating radar (GPR) methods have been widely used in non-destructive detection of pavements, the thickness estimation of the thin top surface layer is still a difficult problem due to the limitations of GPR resolution and the similar permittivity of asphalt sub-layers. Besides, the detection of some road defects, including inadequate compaction and delamination at interfaces, require further practical study. In this paper, a newly-developed vehicle-mounted GPR detection system is introduced. We used a horizontal high-pass filter and a modified layer localization method to extract the underground layers. Besides, according to lab experiments and simulation analysis, we proposed theoretical methods for detecting the degree of compaction and delamination at the interface, respectively. Moreover, a field test was carried out and the estimated results showed a satisfactory accuracy of the system and methods. [ABSTRACT FROM AUTHOR]

This paper analyzes the extremely low frequency electromagnetic wave excited by a horizontal electric dipole immersed in the sea. Analytical solutions in the air from HED underwater are deduced using a three-layer model. The effect of the sea-air interface is studied along two perpendicular directions. The electric field is inversely proportional to the square of r while the magnetic field is inversely proportional to the cube of r along the interface. By decomposing the total response into direct, up-going and down-going components, contributions of each component are discussed, indicating that interference cancellation effect occurs among the arrival electromagnetic signals from multi-paths at specific offsets and frequencies. [ABSTRACT FROM AUTHOR]

An efficient algorithm based on high-order cumulant is addressed for the scenarios where both far-field and near-field narrow-band signals may exist synchronously. The first matrix built by four-order cumulant is utilized to estimate the two dimensional direction-of-arrivals (DOAs) using the orthogonal projection matrix of the signal subspace and the virtual steering matrix. Then, the second matrix built by four-order cumulant is decomposed to get the noise subspace using the eigen decomposition. Meanwhile, a virtual steering matrix is used to distinguish far-field signals (FFSs) from near-field signals (NFSs). And one-dimensional MUSIC algorithm is used to estimate the range of the NFSs. Compared to the TSMUSIC, the proposed algorithm can provide high resolution for the DOAs. In addition, there is higher accuracy for the DOA of NFS in the proposed algorithm than that in TSMUSIC and in TSMD. Simulation results are carried out to certify the performance of the proposed algorithm. [ABSTRACT FROM AUTHOR]

A novel exponentially tapered slot antenna with ultrawide bandwidth and small late-time ring is presented. A microstrip-to-coplanar-stripline (CPS) transition of Chebyshev multisection type is used as feeding transmission. The exponentially tapered slot, which is considered as electric-type radiator (ER), is combined with a magnetic-type radiator (MR) to achieve ultrawideband performance and small size. The MR is completed by a lumped resistor bridged between the loop arm and the transmission ground. The measured impedance bandwidth (VSWR <; 2) of optimized antenna is 0.65 ~ 5.9 GHz, which agrees well with the simulated results. The measured gain is almost 3 dBi in the whole band, and the front-to-back ratio is about 15 dB. [ABSTRACT FROM PUBLISHER]

In this letter, according to the two design principles of uniform distribution and less element shadowing for effective aperture, a novel two-dimensional (2-D) sparse ultrawideband (UWB) multiple-input-multiple-output (MIMO) antenna array topology is optimized and proposed for short-range high-resolution imaging. During the optimization, a “grid method” (GM) is proposed to judge the uniformity of effective aperture, and of which the maximum shadowing number (MSN) is obtained by the projection slice method. Compared to other two different arrays having the same number of transceivers, the proposed optimal array has a better control over the sidelobe level (SL) both in the interference region (IR) and non-interference region (NIR) of the simulated radiation patterns, from which the peak sidelobe levels (PSLs) of the optimal array obtained are lower than the compared ones over 2 dB. Experimental imaging results for a mannequin target also verify the outstanding imaging capability of the proposed array. Therefore, with a small number of effective elements, the proposed array topology has potential applications for high-efficiency UWB short-range imaging. [ABSTRACT FROM PUBLISHER]

We report the surface exploration by the lunar rover Yutu that landed on the young lava flow in the northeastern part of the Mare Imbrium, which is the largest basin on the nearside of the Moon and is filled with several basalt units estimated to date from 3.5 to 2.0 Ga. The onboard lunar penetrating radar conducted a 114-m-long profile, which measured a thickness of ~5 m of the lunar regolith layer and detected three underlying basalt units at depths of 195, 215, and 345 m. The radar measurements suggest underestimation of the global lunar regolith thickness by other methods and reveal a vast volume of the last volcano eruption. The in situ spectral reflectance and elemental analysis of the lunar soil at the landing site suggest that the young basalt could be derived from an ilmenite- rich mantle reservoir and then assimilated by 10-20% of the last residual melt of the lunar magma ocean. [ABSTRACT FROM AUTHOR]

A narrowband, high sensitivity, and low-power consumption induction magnetometer (IM) receiving sensor is designed, built, and tested for through-the-Earth (TTE) communications in this paper. The optimization method is described mathematically and applied for building an IM (central frequency 3180 Hz and 3-dB bandwidth 200 Hz). In this paper, we move the resonant frequency of the coils to the central frequency and optimize the parameters of the IM to minimize the equivalent input magnetic noise level to 7 fT/√Hz within 3-dB bandwidth. The IM dimensions are Φ 40 mm × 350 mm and the power consumption is <;4.2 mW. The feathers of the IM make it a portable receiving sensor for TTE communications. [ABSTRACT FROM PUBLISHER]

In this paper, we present a novel development of compact micropower ultrawideband (UWB) radar by integrating ultrashort balanced transmitter, sampling receiver, and antennas into a single package. The developed transmitter produces a pair of Gaussian pulses with 1 MHz of PRF, 400 ps of duration, and 8.3 V for peak-to-peak amplitude. The receiver down-converts microwave signals with a conversion gain of 3 dB over a 4.5-GHz bandwidth. The integrated UWB micropower radar system allows for the short-range detection with high range resolution and accuracy, thus enhance effectively the capability of target detection and classification. The high performances of such radar system for detecting objects are demonstrated through the real-field tests of various target scenes. [ABSTRACT FROM PUBLISHER]

In this paper, the defocus of a 2-D image reconstruction of targets illuminated by Gaussian beams in terahertz (THz) single-frequency holography was studied. An analytical point-spread function was derived to quantitatively evaluate the defocus effect, considering the deviation of the restoration distance from the real distance of the target. It is found that the cross-range image defocuses in a similar way to the diffusion of a Gaussian beam from its beam waist, which limits the depth of focus for image restoration. To improve the restored images for targets with varying range distances, an adaptive focusing algorithm based on the minimum-entropy method was proposed for single-frequency holography. Simulation results with fairly good agreement were performed to verify the theoretical results and the proposed algorithm. Proof-of-principle experiments in the 0.2-THz band were also performed based on a monostatic prototype imager with a Gaussian beam transceiver. The experimental results confirm the effectiveness of the adaptive focusing reconstruction algorithm proposed in this paper. [ABSTRACT FROM PUBLISHER]

Induction magnetometer (IM) is the transducer for measuring the magnetic field's variations, which is widely used in space research, geophysical exploration, and so on. Among all kinds of IMs, IMs of 0.1 mHz to 1 Hz with negative feedback flux and chopper stabilized amplifiers are especially popular used for the magnetotelluric method in geophysical explorations. In this paper, the equivalent input magnetic noise (EIMN) of IMs of 0.1 mHz to 1 Hz is discussed and modeled. Seven main noise sources are described, numerical evaluated, and experimental modeled separately. At last, an experimental IM is built and its EIMN is measured low to 0.1 mHz to confirm the theoretical model. [ABSTRACT FROM PUBLISHER]

Evaluating wide-band performance of ground-penetrating radar (GPR) antennas is a challenging task due to the complex electromagnetic coupling effects between the GPR antennas and the platform. Typically, the efficiency of conventional methods utilized to deal with the problem is quite low. However, there are various frequency sweeping techniques which can achieve fast analysis of antennas over a broadband. Accordingly, this paper describes an efficient hybrid scheme, based upon the electric field integral equation (EFIE), for modeling a GPR antenna which is mounted on an electrically large platform. In this paper, an antenna with two half-elliptical-shape arms, which is an improvement of bowtie antennas, is presented with resistances loaded in the terminal. A shallow rectangular conducting backed cavity is attached to the antenna. On the other hand, the best uniform rational approximation technique is applied to analyze the wide-band property of the antenna since it can avoid repeatedly solving the integral equation at each single frequency point. The main scheme of the frequency sweeping method is as follows: 1) determining the Chebyshev nodes within a given frequency range; 2) computing the equivalent surface currents of antennas at the frequency points corresponding to those Chebyshev nodes; 3) calculating the wide-band response of surface currents according to the Chebyshev series. Finally, the Maehly approximation is utilized to improve the accuracy by matching the Chebyshev series to a rational function. In the hybrid scheme, the adaptive integral method (AIM) is applied to accelerate matrix-vector products as the IE algorithm and the impedance matrix is stored in a sparse form to facilitate analysis of large antenna-platform system. It is easy to combine the best uniform rational approximation technique with AIM but also other IE methods which are suited for modeling radiation of antennas. This hybrid method greatly extends the range of conventional numerical modeling for GPR antenna system. Furthermore, the efficiency and capability of the presented algorithm can be validated by the designed GPR antenna. [ABSTRACT FROM AUTHOR]

A generalized folded transformation procedure is presented for the space with arbitrary shapes. General expressions for the constitute parameters of complementary media are deduced, which can be readily applied to design complementary media based transformation optics devices (CMTOD) with arbitrary shapes. It's no longer limited to the situation when the inner and outer boundaries of the CMTOD are conformal or similar shapes, and can be available for the non-conformal situations. Three kinds of CMTOD are designed and studied, which involves a super-lens, an external cloak that hides object outside the cloaking shell, and an illusion optics device that transforms one object to another. Full-wave simulations are carried out to validate the proposed approach. The generalization introduced here makes a step forward for the flexible design of CMTOD with arbitrary geometries. [ABSTRACT FROM AUTHOR]

A novel ultra-wideband (UWB) monocycle pulse generator with good performance is designed and demonstrated in this paper. It contains a power supply circuit, a pulse drive circuit, a unique pulse forming circuit, and a novel monopolar-to-monocycle pulse transition circuit. The drive circuit employs wideband bipolar junction transistors (BJTs) and linear power amplifier transistor to produce a high amplitude drive pulse, and the pulse forming circuit uses the transition characteristics of step recovery diode (SRD) effectively to produce a negative narrow pulse. At last, the monocycle pulse forming circuit utilizes a novel inductance L shortcircuited stub to generate the monocycle pulse directly. Measurement results show that the waveform of the generated monocycle pulses is over 76V in peak-to-peak amplitude and 3.2 ns in pulse full-width. These characteristics of the monocycle pulse are advantageous for obtaining long detection range and high resolution, when it is applied to ultra-wideband radar applications. [ABSTRACT FROM AUTHOR]

We describe the architecture of a time-to-digital converter (TDC), specially intended to measure the delay resolution of a programmable delay line (PDL). The configuration, which consists of a ring oscillator, a frequency divider (FD), and a period measurement circuit (PMC), is implemented in a field programmable gate array (FPGA) device. The ring oscillator realized in loop containing a PDL and a look-up table (LUT) generates periodic oscillatory pulses. The FD amplifies the oscillatory period from nanosecond range to microsecond range. The time-to-digital conversion is based on counting the number of clock cycles between two consecutive pulses of the FD by the PMC. Experiments have been conducted to verify the performance of the TDC. The achieved relative errors for four PDLs are within 0.50%-1.21% and the TDC has an equivalent resolution of about 0.4 ps. [ABSTRACT FROM AUTHOR]