Low-frequency Electrical Signal Research Articles

Calibration-free scheme for frequency response measurement of a Mach–Zehnder modulator with low-frequency detection

This paper presents a novel, to the best of our knowledge, calibration-free scheme for measuring the frequency response of Mach–Zehnder modulation (MZM) using low-frequency detection. An auxiliary MZM1 modulated by a MHz-level electrical signal f1 is utilized to generate a double-sideband optical signal, which is subsequently directed into the device under test (MZM2). The MZM2 is biased at its minimum point and driven by a frequency-swept microwave signal f m . After photoelectric converting, a fixed low-frequency electrical signal f L carrying the frequency response information of the MZM2 is generated. By sweeping f m while extracting the amplitude of f L , the frequency response of the MZM2 free from the response of the photodetector (PD) is thus determined. In the experiment, the frequency response of a commercial MZM is characterized in the frequency range from 0.1 GHz to 20 GHz with a resolution of 50 MHz. The measurement result is consistent with the vector network analyzer swept method. In addition, the new method eliminates errors introduced by the PD and reduces the requirement for a high-bandwidth PD.

Low-frequency capacitor with hopping electrical conductivity of the working substance (on the example of a-Si:H)

We propose a structural and electrical schemes of a capacitor based on a 3 μm thick a-Si:H (amorphous hydrogenated silicon) layer separated from the metal plates by 0.3 μm thick dielectric layers of SiO2 (silicon dioxide). We consider room temperatures (T ≈ 300 K) when in the absence of illumination for a-Si:H the hopping mechanism of electron migration via point defects of the structure prevails. For such a capacitor, the dependencies of the capacitance on the frequency of the measuring signal ω/2π in the range from 0.1 to 300 Hz are calculated for the a-Si:H layer with stationary hopping electrical conductivity σdc ≈ 1 ∙ 10−10 (Ohm ∙ cm)−1. It is assumed that there is no end-to-end electron transfer between the a-Si:H layer, dielectric layers and capacitor plates in the small-signal mode of capacitance measurement. It is shown that the real part of the capacitance of the capacitor decreases with increasing angular frequency ω, and the imaginary part is negative and depends non-monotonically on ω. The decrease in the real part of the device capacitance to the geometric capacitance of the series-connected oxide layers and the a-Si:H layer with increasing ω is due to a decrease in the electrical resistance of the capacitor. As a result, with increasing ω, the imaginary part of the capacitance is shunted by the hopping electrical conductivity of the capacitor. The phase shift for a sinusoidal electrical signal supplied to the capacitor is determined depending on the frequency ω/2π in the range of 0.1–300 Hz for the values of electrical conductivities of the hydrogenated amorphous silicon layer σdc ≈ 1 ∙ 10−11, 1 ∙ 10−10, and 1 ∙ 10−9 (Ohm ∙ cm)−1 at the temperature 300 K. With an increase in the electrical conductivity σdc of the a-Si:H layer, the minimum absolute value of the phase shift angle (≈65°) shifts to the high- frequency region (from 1 to 100 Hz). The proposed low-frequency capacitor can find application in electrical circuits for detecting low-frequency electrical signals for the purposes of biomedicine.

Visually evoked local field potential changes in the caudate nucleus are remarkably more frequent in awake, behaving cats than in anaesthetized animals.

Previous results show that halothane gas anaesthesia has a suppressive effect on the visually evoked single-cell activities in the feline caudate nucleus (CN). In this study, we asked whether the low-frequency neuronal signals, the local field potentials (LFP) are also suppressed in the CN of anaesthetized animals.To answer this question, we compared the LFPs recorded from the CN of two halothane-anaesthetized (1.0%), paralyzed, and two awake, behaving cats during static and dynamic visual stimulation. The behaving animals were trained to perform a visual fixation task.Our results denoted a lower proportion of significant power changes to visual stimulation in the CN ofthe anesthetized cats in each frequency range (from delta to beta) of the LFPs, except gamma. Thesedifferences in power changes were more obvious in static visual stimulation, but still, remarkable differences were found in dynamic stimulation, too. The largest differences were found in the alpha and beta frequency bands for static stimulation. Concerning dynamic stimulation, the differences were the biggest in the theta, alpha and beta bands.Similar to the single-cell activities, remarkable differences were found between the visually evoked LFP changes in the CN of the anaesthetized, paralyzed and awake, behaving cats. The halothane gas anaesthesia and the immobilization suppressed the significant LFP power alterations in the CN to both static and dynamic stimulation. These results suggest the priority of the application of behaving animals even in the analysis of the visually evoked low-frequency electric signals, the LFPs recorded from the CN.

The performance of a dissipative electro-optomechanical system using the Caldirola–Kanai Hamiltonian approach

By combining all optical, electronic, and mechanical capabilities, electro-optomechanical systems, along with the use of mechanical displacement, the conversion of low-frequency electrical signals into much higher-frequency optical signals is possible. This process provides several capabilities that have very important applications in various fields, including classical, as well as quantum communications. In this regard, the inevitable effect of dissipation on the performance indicators of such systems is one of the important issues affecting its efficiency. One approach for investigating the effect of dissipation is based on a time-dependent Hamiltonian has been known as the Caldirola-Kanai (CK) Hamiltonian, wherein the exponentially increasing mass, i.e. enters the mechanical oscillator system. Utilizing this approach shows that, the performance of the system under the influence of dissipation with CK Hamiltonian formalism, in which the dissipation terms are embedded in the Hamiltonian, are more logical, analytical, and reliable than considering the effect of dissipation with the phenomenological method (including the dissipation effects in the equations of motion by hand), which has been recently used in Bagci et al. (Nature, 507, 81, 2014). In our method, it is also possible to monitor the performance of the system via the adjustment of dissipation components, i.e., resistance and reactance (and CK damping parameter ) in the electrical (mechanical membrane) part of the system.

UWB-Modulated Microwave Imaging for Human Brain Functional Monitoring.

Morphological microwave imaging has shown interesting results on reconstructing biological objects inside the human body, and these parameters represent their actual biological condition, but not their biological activity. In this paper, we propose a novel microwave technique to locate the low-frequency (f≃1 kHz) -modulated signals produced by a microtag mimicking an action potential and proved it in a cylindrical phantom of the brain region. A set of two combined UWB microwave applicators, operating in the 0.5 to 2.5 GHz frequency band and producing a nsec interrogation pulse, is able to focus its radiated field into a small region of the brain containing the microtag with a modulated photodiode. The illuminating UWB microwave field was first modulated by the low-frequency (f≃1 kHz) electrical signal produced by the photodiode, inducing modulated microwave currents into the microtag that reradiating back towards the focusing applicators. At the receiving end, the low-frequency (f≃1 kHz) -modulated signal was first extracted from the full set of the backscattered signals, then focused into the region of interest and spatially represented in the corresponding region of the brain, resulting in a spatial resolution of the images in the order of 10 mm.

Analysis of Information Transmission Characteristics Based on Adaptive Ground Electrode Current Field

The information transmission mechanism of the ground electrode current field uses a very low-frequency electrical signal, which is applied to the two electrodes driven into the soil layer or the collapsed body of the tunnel to form a current field in the rock layer or soil layer. Signal detection is created via the strong penetration of wireless information transmission. This research focuses on various electromagnetic effects, such as polarization, magnetization, and the transmission of electromagnetic waves under the influence of different media, such as rock, sand, reinforced concrete, and air voids. The influence of these adaptive electromagnetic effects on the transmission of electromagnetic waves is mainly reflected in the reflection, refraction, and attenuation of electromagnetic wave signals. The inhomogeneity of the earth medium, the influence of topographic features, and multi-path transmission all cause signal distortion, fading, or changes in the direction of electromagnetic wave propagation. By studying the three physical quantities of magnetic permeability, permittivity, and conductivity, the electromagnetic characteristics of the earth medium are described to research the information transmission characteristics of the earth electrode current field.

Photonics-Based Simultaneous DFS and AOA Measurement System without Direction Ambiguity

A novel scheme that can simultaneously measure the Doppler frequency shift (DFS) and angle of arrival (AOA) of microwave signals based on a single photonic system is proposed. At the signal receiving unit (SRU), two echo signals and the reference signal are modulated by a Sagnac loop structure and sent to the central station (CS) for processing. At the CS, two low-frequency electrical signals are generated after polarization control and photoelectric conversion. The DFS without direction ambiguity and wide AOA measurement can be real-time acquired by monitoring the frequency and power of the two low-frequency electrical signals. In the simulation, an unambiguous DFS measurement with errors of ±3 × 10-3 Hz and a -90° to 90° AOA measurement range with errors of less than ±0.5° are successfully realized simultaneously. It is compact and cost-effective, as well as has enhanced system stability and improved robustness for modern electronic warfare systems.

A Fitted empirical demodulation for low frequency electrical signal evaluation

Empirical Demodulation (ED) is a technique used to build a discrete signal, called an empirical envelope, from a modulated time sequence. For example, in three-phase induction motors, this envelope can carry fault frequency data that allows the machine health status to be evaluated during a spectral analysis. However, due to mathematical reasons, the method is very sensitive to the amplitude oscillations within the signal. When these oscillations are unwanted, as in the presence of measurement noise, the results can be strongly affected. This work proposes an iterative and adjustable version of the ED that considerably reduces its sensitivity to the presence of high frequency noise, thus eliminating the need for signal pre-filtering. To prove the effectiveness of the Fitted Empirical Demodulation, the authors applied the new tool in motor current signals for analysis of the rotor bars conditions.

Helmholtz Coils Based WPT Coupling Analysis of Temporal Interference Electrical Stimulation System

Electrical nerve stimulation (ENS) is clinically important in treating neurological diseases. This paper proposes a novel temporally interfering wireless power transfer (WPT) system, based on Helmholtz coils, to address energy depletion and the miniaturization of wireless power transfer systems for implantable devices. Compared to conventional WPT systems, this paper uses Helmholtz coils with a centrosymmetric structure as the transmitting coils. A more uniform and stable magnetic field was obtained through structural improvements. It also improves the problem that changes in the receiving coil’s position affect the transmission power’s stability. Based on the principle of temporal interference (TI), two transmitting coils with a slight frequency difference generate a superposition of magnetic fields on the receiving coil and then induce a low-frequency electrical signal on it. The electrical stimulation system applies stimulation parameters of a specific intensity and frequency directly to the target nerve with electrodes connected to it. This eliminates the need for the conventional high-frequency signal to low-frequency signal processing circuitry and reduces the device’s size. In this paper, numerical calculations and an experimental verification of the proposed system are carried out. The magnetic field distribution and the receiving coil current waveform of the system were tested to verify the effectiveness and stability of the proposed design. The experimental results showed that the proposed wireless power transfer system can generate electrical signals of the desired waveform in the receiving coil. Its frequency of 10 Hz and amplitude of 42.4 mA meet the requirements for the electrical stimulation of the sciatic nerve.

Photonic Measurement for Doppler Frequency Shift and Angle of Arrival Based on a Dual-Polarization Dual-Drive Mach–Zehnder Modulator

A novel photonic-assisted system for realizing the simultaneous measurement of Doppler frequency shift (DFS) and angle of arrival (AOA) is proposed. It is a simplified structure that is based on a dual-polarization dual-drive Mach–Zehnder modulator (DPol-DDMZM). The DFS direction can be accurately determined by comparing the phase relationship between the upper and lower low-frequency signal waveforms. An amplitude comparison function (ACF), which constructs a one-to-one mapping between the power ratio and the phase difference of echo microwave signals, can reduce the AOA measurement errors. The simulation results show that the simulated ACFs agree well with the theoretical ACFs, and the measurement errors of AOA are less than ±2.8° in a range of 0° to 78°. Moreover, the DFS can be realized by analyzing the spectrum of the low-frequency electrical signal with a measurement error of less than ±0.05 Hz. The system structure is compact and cost-effective, which provides an alternative solution for modern radar and electronic warfare receivers.

Tunable microwave frequency comb generation using a Si3N4-MDR based actively mode-locked OEO.

Microwave frequency combs (MFCs) have important applications in communication and sensing owing to their characteristics of large number of comb lines, wide frequency range, and high precision of comb spacing. In many applications, MFCs are required to emit signals with tunable center frequency and variable comb spacing to accommodate different operating frequency bands and accuracies. Here, we demonstrate a tunable MFC by injecting a low-frequency electrical signal into a tunable optoelectronic oscillator (OEO). Tuning of MFC's center frequency and comb spacing are realized, allowing a frequency tuning range from 1 to 22 GHz and 50 comb lines within a 5 MHz bandwidth obtained in the MFC generator. In addition, the introduction of the silicon nitride micro-disk resonator (Si3N4-MDR) in the system paves the way for the integration of MFC generator.

An Efficient Point-Matching Method-of-Moments for 2D and 3D Electrical Impedance Tomography Using Radial Basis Functions.

The inverse problem of computing conductivity distributions in 2D and 3D objects interrogated by low-frequency electrical signals, which is called Electrical Impedance Tomography (EIT), is treated using a Method-of-Moment technique. A Point-Matching-Method-of-Moment technique is used to formulate a global integral equation solver. Radial Basis Functions are adopted to express the conductivity distribution. Single-step quadratic-norm ( L2) and iterative total variation ( L1) regularization techniques are exploited to solve the inverse problem. Simulation and experimental tests on a circular reconstruction domain show satisfactory performance in deriving conductivity distribution, achieving a Correlation Coefficient ( CC) up to 0.863 for 70 dB voltage SNR and 0.842 for 40 dB voltage SNR. The proposed methodology with L2-norm regularization provided better results than traditional iterative Gauss-Newton's approach, whereas with L1-norm regularization it showed promising performance. Moreover, 3D reconstructions on a cylindrical cavity demonstrated superior results near the electrodes' planes compared to those of the conventional linearized approach. Finally, application to EIT medical data for dynamic lung imaging successfully revealed the breath-cycle conductivity changes. The results show that the proposed method can be effective for both 2D and 3D EIT and applicable to many applications. Strong conductivity variations are successfully tackled with a very good Correlation Coefficient. In contrast to conventional EIT solutions based on weak-form and linearization on small conductivity changes, the proposed method requires only one step to converge with L2-norm regularization. The proposed method with L1-norm regularization also achieves good reconstruction quality with a low number of iterations.

A Filterless Photonic Approach for DFS and AOA Measurement Using a Push-Pull DPol-MZM

We report a novel simple photonic approach for measuring the Doppler frequency shift (DFS) and the angle of arrival (AOA) of a microwave signal in a radar detection system. The echo signals combined with a reference signal are applied to a dual-polarization Mach-Zehnder modulator (DPol-MZM), and a low-frequency electrical signal is generated after beating between the two signals in a low-speed photodetector. The DFS without direction ambiguity can be calculated by the value of the low-frequency signal. A phase difference is introduced between two echo signals which are sent to two sub-MZMs of the DPol-MZM respectively. The AOA is determined by the peak power of the down-convert signals, and a simple calibration is used to remove the amplitude of the echo signals and the reference signal by switching the output polarization states of the system. Compared with previously reported works, the proposed method carries out a comprehensive and multifunction measurement system based on a filterless and simple structure. Experimental results demonstrate the DFS errors within ± 0.5 Hz for a 15 GHz echo signal with a frequency offset of ± 100 kHz, and AOA measurement errors are within ± 1° for a range of 16° – 82°

All-Optical Pulsed Signal Doppler Frequency Shift Measurement System

A simple and all-optical Doppler frequency shift (DFS) measurement system is presented. It is based on a dual-drive Mach Zehnder modulator (DDMZM) driven by a transmitted signal and an echo signal received by an antenna. A low-frequency sawtooth wave is applied to the DDMZM DC port to frequency shift the carrier and sidebands generated by the transmitted signal. Beating of a frequency-shifted transmitted signal sideband and an echo signal sideband at the photodetector produces a low-frequency electrical signal. The DFS, and consequently the speed and moving direction of a target, can be obtained from the frequency of this low-frequency electrical signal. The DDMZM used in the proposed DFS measurement system does not need to be operated at a specific point in the transfer function and hence it has no bias drift problem. The proposed DFS measurement technique can be used in both CW and pulsed radar systems. Experimental results demonstrate the proposed DFS measurement system has a wide operating frequency range of 10 GHz to 19.95 GHz, small DFS measurement error of less than ±0.6 Hz and long-term stable performance. Results also demonstrate, for the first time, DFS measurement of a pulsed signal using a microwave photonic technique.

Dual-function measurement system for Doppler-frequency-shift and angle-of-arrival of microwave signals

A photonic method for simultaneously measuring the Doppler frequency shift (DFS) and angle of arrival (AOA) of microwave signal is proposed and verified by simulation. The echo signals are applied to a dual-parallel dual-drive Mach–Zehnder modulator for modulating the lightwave with carrier-suppressed double-sideband pattern. The lightwave carrying the echo signals is coupled with the lightwave carrying the local oscillator signal, and their upper and lower sidebands are separated by an interleaver. Then, the optical signals are converted into low-frequency electrical signals by two identical photodiodes, and the parameters of DFS and AOA are obtained by processing two low-frequency photocurrents. In the simulation, the measurements of the DFSs in the range of ± 1 MHz with errors < ± 9 × 10 − 4 Hz and the AOAs from 1.82 deg to 90 deg with errors <0.9 deg are realized, and the orientation of DFS can also be distinguished by comparing the phase difference of two electrical signals.

Photonic-Assisted Filter-Free Microwave Doppler Frequency Shift Measurement Using a Fixed Low-Frequency Reference Signal

A filter-free photonic approach for measuring microwave Doppler frequency shift (DFS) based on two cascaded electro-optic modulators (EOMs) and a fixed low-frequency reference signal is proposed. An optical carrier is carrier-suppressed single-sideband modulated by the transmitted signal and the reference signal at EOM-I. The output from EOM-I is then single-sideband modulated by the echo signal at EOM-II. Subsequently, the output from EOM-II is injected into a photodetector to implement optical-to-electrical conversion. Both the value and direction of the DFS can be obtained by analyzing the frequency of the generated low-frequency electrical signal. The proposed approach has a compact structure and no optical filtering is needed. An experiment is performed to verify the proposed approach. For different microwave frequencies from 7 to 16 GHz, DFS from −100 to 100 kHz is measured, with a measurement error of less than ±0.05 Hz. The stability of the system is also verified. Within 30 minutes, the change of measurement error is less than ±0.05 Hz.

Simple photonics-based system for Doppler frequency shift and angle of arrival measurement.

A novel photonic approach for simultaneously measuring both the Doppler frequency shift (DFS) and the angle of arrival (AOA) of a microwave signal in a radar system is presented. It has the same structure as a fiber optic link consisting of a laser, an optical modulator and a photodetector. The incoming microwave signal and a reference signal are applied to the optical modulator. Beating of the echo and reference signal sidebands at the photodetector generates a low-frequency electrical signal. The DFS and the AOA can be determined from the frequency and the power of the low-frequency electrical signal measured on an electrical spectrum analyzer. The system has a very simple structure and is low-cost. It has a wide operating frequency range and a robust performance. Experimental results demonstrate a DFS measurement at around 15 GHz with errors of less than ±0.2 Hz, and a 0° to 90° AOA measurement with less than ±1° errors.

Use of optical Kerr medium for parametric generation of very low frequency electrical signal

Optical self-focusing and defocusing, optical switching activities and optical signal processing can be successfully done by using optical Kerr materials. Due to the nonlinear property of Kerr medium, this medium can be used to convert the frequency of the applied light signal passing through the medium. Here, in this paper, the authors propose a method of using optical Kerr medium for parametric generation of very low frequency electrical signal.

A PM-based approach for Doppler frequency shift measurement and direction discrimination

A novel photonic approach for wideband Doppler frequency Shift (DFS) measurement and direction discrimination based on phase modulators (PMs) and optical frequency shift module is proposed and experimentally demonstrated. In the proposed scheme, a light emitted from the laser is split into two paths, one of which is modulated by the transmitted signal through the first PM and the other of which passes through the carrier frequency shifter and then is modulated by the echo signal through the second PM. After combining two optical signals and filtering out the negative sidebands, the value and direction of DFS can be obtained by comparing two peak frequencies in the low-frequency electrical signals generated by a low-speed photodetector. Proof-of-concept experiments with DFSs from -1 MHz to 1 MHz at an optical carrier frequency shift of 40 MHz are implemented, where the measurement errors are kept within ±0.7 Hz.

Angle-of-Arrival Estimation of Microwave Signals Based on Optical Phase Scanning

A photonic method to estimate the angle-of-arrival (AOA) of microwave signals based on optical phase scanning is proposed and experimentally demonstrated. At remote antenna unit (RAU), one received high-frequency microwave signal is directly applied to a phase modulator (PM), and another received high-frequency microwave signal coupled with a low-frequency large-voltage sawtooth-wave signal is applied to another PM. The sawtooth-wave signal is used to scan the phase of the optical sideband from 0 to 2π. After transmission over a segment of fiber link, the AOA parameter can be estimated by processing the obtained low-frequency electrical signals at central office (CO). Such scheme can be applicable to either single-tone or wideband microwave signals. Experimental results show that the AOA measurement errors are less than 2.27° and 4.45° for a single-tone signal at 10 GHz and a 2-GHz broadband signal with the center frequency of 18 GHz, respectively.