Electrical Filter Research Articles

Comb-based radiofrequency photonic filters with rapid tunability and high selectivity

Photonic technologies have received considerable attention for enhancement of radio-frequency (RF) electrical systems, including high-frequency analog signal transmission, control of phased arrays, analog-to-digital conversion, and signal processing. Although the potential of radio-frequency photonics for implementation of tunable electrical filters over broad RF bandwidths has been much discussed, realization of programmable filters with highly selective filter lineshapes and rapid reconfigurability has faced significant challenges. A new approach for RF photonic filters based on frequency combs offers a potential route to simultaneous high stopband attenuation, fast tunability, and bandwidth reconfiguration. In one configuration tuning of the RF passband frequency is demonstrated with unprecedented (~40 ns) speed by controlling the optical delay between combs. In a second, fixed filter configuration, cascaded four-wave mixing simultaneously broadens and smoothes comb spectra, resulting in Gaussian RF filter lineshapes exhibiting extremely high (>60 dB) main lobe to sidelobe suppression ratio and (>70 dB) stopband attenuation.

Provision of IR-UWB wireless and baseband wired services over a WDM-PON

A simple scheme to simultaneously generate an on-off keying or bi-phase modulation (BPM) impulse radio ultra wideband (IR-UWB) signal and a baseband wired signal in the optical domain using a dual-drive modulator is proposed and demonstrated. Although the two signals have spectral overlap in the optical spectrum, they are located at different frequency bands when converted to electrical signals at a photodetector (PD), which can be well separated by an electrical filter. An experiment is carried out. Eye diagrams, electrical spectra and BER measurements show that the co-channel interference between the UWB and the wired signals is small for a single-channel 36-km fiber link to provide 1.25-Gb/s UWB wireless and 1.25-Gb/s baseband wired services. The inter-channel interference is also small and negligible when the link is operated together with two other 1.25 Gb/s baseband wired links, which demonstrates that a conventional WDM-PON can be upgraded to provide additional UWB services without affecting the existing services by modifying the modulators in the center office and inserting UWB antennas in the optical network units.

Transfer function of multi-stage active filters: a solution based on Pascal's triangle and a general expression

A method is proposed to simplify analytical computations of the transfer function for electrical circuit filters, which are made from repetitive identical stages. A method based on the construction of Pascal's triangle is introduced and then a general solution from two initial conditions is provided for the repetitive identical stage. The present analysis is mainly of educational value, but the computing method is also found to be useful in ladder network designs, in applied systems such as transmission line networks and most likely in biological systems involving repetitive units.

Binary phase filters with a maximally-flat response

We investigate the binary phase filters of finite support that have a Fourier transform magnitude that is the broadest and flattest. We call this behavior maximally flat, after the electrical filters devised by Butterworth. Solutions are obtained by direct algebraic manipulation, and numerical results are given for filters of up to five elements. Their application for increasing depth of focus is described.

A frequency sextupling scheme for high-quality optical millimeter-wave signal generation without optical filter

Based on a laser with an integrated Mach-Zehnder modulator (IMZM), a novel frequency sextupling scheme is proposed for the millimeter-wave (mm-wave) signal generation in radio over fiber system. Theoretical analysis is presented to implement the two-tone optical frequency multiplication. The simulation results show that the high-quality frequency sextupling mm-wave signal can be generated without any optical or electrical filter. The optical sideband suppression ratio (OSSR) higher than 29 dB can be obtained without optical filter when the IMZM is consisted of three sub-MZMs with extinction of 30 dB. Furthermore, the influence of imperfect extinction ratio of the IMZM and nonideal phase difference between RF drive signal of two sub-MZMs of the IMZM on OSSR is investigated, and the quality of the generated optical mm-wave signal for two cases of the proposed scheme is also compared.

An electrical-filtered optical heterodyne technique for tuning speed measurement of DBR lasers

A new scheme based on the electrical-filtered optical heterodyne technique is proposed for measuring the tuning speed of tunable distributed Bragg reflector (DBR) lasers. In this scheme, a 10 GHz high-pass electrical filter is used and the wavelength switching time of a tunable DBR laser for different tuning frequencies corresponding exactly to different delay lines is measured. The switching time is measured to be nearly 300 ns and can be improved by reducing the equivalent resistance-capacitance (RC) time constant of the device. The distribution of the beat signal of the DBR laser in the time domain is also obtained, and is a good match with the waveforms measured using an optical filter.

SNR approach for performance evaluation of time-stretching photonic analogue to digital converter system

A semi-analytical simulation method (SASM) is proposed to evaluate the signal-to-noise ratio (SNR) of time stretched signals at the output of photonic analogue-to-digital converter (Ph-ADC) system. Analytical expressions of the signal at Ph-ADC output considering generic electrical signals applied to the electro-optic modulators of the Ph-ADC are derived. The contribution to the total variance of the received signal from the noise introduced by the electrical transmitter and receiver, and by the optical amplifier are derived analytically taking into account the pulsed nature of the optical signal. The proposed SASM shows excellent agreement of SNR estimates with the estimates provided by Monte Carlo simulation. This result is confirmed for variance dominantly imposed by the noise introduced by the electrical transmitter, by the optical amplifier and by the electrical receiver. A simplified approach is also proposed and compared with previous work. It is shown that mean power estimates obtained from this simplified approach are valid while the modulator is operating in the linear region and the signal is not affected by the frequency response of the electrical receiver filter. Additionally, it is concluded that the estimates of the noise variance due to the electrical transmitter are acceptable when a small signal analysis of noise along the Ph-ADC is valid.

Support Vector Machine for soft fault location in electrical circuits

The paper is concerned with the application of Support Vector Machine (SVM) to the fault location in the analog electrical circuits. The recognition of fault is based on the measurements of the accessible terminal voltage and current of the network at the set of frequencies. The SVM network is applied as the recognizing system and as the classifier. The important feature of the proposed solution is its high accuracy and great speed of operation. Once the network has been trained, the recognition of fault is achieved immediately, irrespective of the size of the circuit. Thus the solution is suited for real time applications for fault location in electrical circuits. The numerical results of recognition of faulty elements in two different structures of electrical filters are presented and discussed in the paper.

Investigation and comparison of digital backward propagation schemes for OFDM and single-carrier fiber-optic transmission systems

The performance of orthogonal frequency-division multiplexing (OFDM) and single-carrier (SC) fiber-optic transmission systems with digital backward propagation (DBP) are compared. Sixteen quadrature amplitude modulation (QAM) is used for both systems with the bit rate of 100 Gb/s. The results show that OFDM and SC with Nyquist pulses (SC-Nyquist) have a superior performance as compared to SC with raised-cosine pulses (SC-NRZ) when the DBP is used. We also studied the impact of electrical filter bandwidth and nonlinear phase/amplitude noise on the performance. As the filter bandwidth increases, the performance improves for the case of ideal BP for both OFDM and SC systems when the signal-to-noise ratio (SNR) is large. The analysis of nonlinear phase/amplitude noise revealed that it causes significant impairments for SC-NRZ systems and less impact on OFDM and SC-Nyquist systems, which explains the performance advantage of OFDM/SC-Nyquist over SC-NRZ when the DBP is used.

OSNR Monitoring in the Presence of First-Order PMD Using Polarization Diversity and DSP

We analytically study optical signal-to-noise ratio (OSNR) monitoring using polarization diversity and digital signal processing (DSP) in the presence of first-order polarization-mode dispersion (PMD). We derived conditions on the input signal polarization, principle states of polarization (PSP) and differential group delay (DGD) of the fiber, polarization beam splitter (PBS) axes and receiver electrical low-pass filter bandwidth such that the monitored OSNR is least affected by PMD and we propose the use of a polarization scrambler (PS) before the PBS to achieve such conditions in practice. In addition, two special polarization states for which the proposed technique can operate independent of first-order PMD are identified. The theoretical predictions are verified through simulations. The analytical insights developed represent a more general and unified framework of which various OSNR monitoring techniques based on polarization diversity proposed in the literature can be understood as special cases.

Estimating the Performance of Direct-Detection DPSK in Optical Networking Environments Using Eigenfunction Expansion Techniques

In-band crosstalk, due to multiple interferers, has been identified as one of the most severe impairments in optical transparent networks, especially in the ones with a large number of nodes and a high wavelength density. Due to its robustness to in-band crosstalk differential phase-shift keying (DPSK) emerges as an attractive modulation scheme to be used in such environments. This paper proposes a rigorous formulation to estimate the performance of direct-detection DPSK receivers using an eigenfunction expansion technique in the time domain. The method takes into account both the in-band crosstalk, due to an arbitrary number of interfering terms, and the amplified spontaneous emission noise and is able to deal with any combination of optical and electrical filter shapes. Using this method the accuracy of an approximation based on the wideband optical filtering assumption was evaluated and shown that the approximation, although not providing reliable results for the error probabilities, can be used with confidence to compute power penalties due to in-band crosstalk. Furthermore, the crosstalk tolerance of DPSK over on-off keying was quantified and shown that this tolerance is reduced when the number of interferers increases.

Evolution of Analog Electric Circuits using a New Genetic Algorithm Approach

Evolvable hardware is a hardware that depends on evolutionary algorithms (EAs) for performing electrical circuit synthesis and evolving its electrical circuit architecture, furthermore it depends on EAs for making the necessary adaptations to this architecture while working on line, and electrical circuit synthesis is recognized as an act of human intelligence. This paper presents a new approach for solving the electrical circuit synthesis problem using genetic algorithms as an automated design technique, the proposed approach offers a new coding style for the chromosome representing the electric circuit, and also minimizes the chromosome size in an attempt to solve the scalability problem associated with evolvable hardware. This approach is tested upon the synthesis of low pass electrical filter and has proved to be efficient and capable of handling more complex circuit design tasks with minor future changes.

A digital interface for gyroscopes controlling the primary and secondary mode using bandpass sigma–delta modulation

This paper demonstrates a micro-electro-mechanical gyroscope system with extensive use of sigma–delta (ΣΔ) modulation in both, primary and secondary modes. Both control schemes are implemented digitally on a field programmable gate array (FPGA). The primary loop has a bandpass ΣΔ digital-to-analog converter (DAC) driving the primary mass into resonance using a two-level driver. With this strategy of replacing the discrete DAC of the primary oscillation control with a bandpass ΣΔ-DAC, the analog circuit complexity is enormously reduced. The Coriolis rate signal is converted into a bit stream with a new excess loop delay (ELD) compensated micro-electro-mechanical ΣΔ modulator (ΣΔM) incorporating the gyroscope in the loop with a second-order electrical bandpass filter (BPF). To investigate the ELD effect, this electro-mechanical ΣΔM is implemented on the FPGA emulating continuous-time (CT) behavior. Measurements show stable modulators, with an ELD of nearly one clock period of the sampled system, achieving in-band noise (IBN) below −60 dBFS. The full-scale (FS) is measured to 1019°/s. This paper demonstrates that the stability of ΣΔ modulators with large ELD can be ensured with the new ELD compensation technique.

A novel frequency sextupling scheme for optical mm-wave generation utilizing an integrated dual-parallel Mach-Zehnder modulator

A novel scheme is proposed for frequency sextupling mm-wave generation based on a laser and an integrated dual-parallel Mach-Zehnder modulator (MZM) without optical filter. Theoretical analysis is presented to suppress the undesired optical sidebands for the high quality generation of frequency sextupling mm-wave signal. The performance of the proposed scheme is evaluated by simulations. Utilizing the integrated MZM consisted of two sub-MZMs with extinction ratio of 30 dB, the optical sideband suppression ratio (OSSR) is as high as 29.9 dB and the radio frequency spurious suppression ratio (RFSSR) exceeds 24 dB without any optical or electrical filter. The impact of the nonideal RF driven voltage and phase difference of RF driven signal applied to two sub-MZMs of the integrated MZM on OSSR and RFSSR is discussed and analyzed. After transmission over fiber, the generated optical mm-wave signal demonstrates good performance. Furthermore, the performance of two cases for the proposed scheme is also compared.

Presynaptic Miniature Gabaergic Currents in Developing Interneurons

Miniature synaptic currents have long been known to represent random transmitter release under resting conditions, but much remains to be learned about their nature and function in central synapses. In this work, we describe a new class of miniature currents ("preminis") that arise by the autocrine activation of axonal receptors following random vesicular release. Preminis are prominent in gabaergic synapses made by cerebellar interneurons during the development of the molecular layer. Unlike ordinary miniature postsynaptic currents in the same cells, premini frequencies are strongly enhanced by subthreshold depolarization, suggesting that the membrane depolarization they produce belongs to a feedback loop regulating neurotransmitter release. Thus, preminis could guide the formation of the interneuron network by enhancing neurotransmitter release at recently formed synaptic contacts.

A WDM-OFDM-PON architecture with centralized lightwave and PolSK-modulated multicast overlay

We propose and demonstrate a novel wavelength-division-multiplexing orthogonal-frequency-division-multiplexing passive-optical-network (WDM-OFDM-PON) architecture with centralized lightwave sources and polarization shift keying (PolSK) multicast overlay. The 10-Gb/s 16QAM-OFDM point to point (P2P) signal, 2.5-Gb/s multicast PolSK signal and 2.5-Gb/s on-off keying (OOK) upstream signal are experimentally demonstrated. After transmission over 25km standard single mode fiber (SMF), 1.5dB crosstalk between the downstream signals is eliminated by employing a low pass electrical filter at the PolSK receiver. The power penalty of the upstream OOK signal at BER of 10(-9) is less than 0.1dB.

高速光OFDM信号预失真用于多低带宽接收机系统的无失真接收

A cost-effective method of multi-receiver optical orthogonal frequency-division multiplexing (OOFDM) system is proposed to reduce the requirement for the speed of the receiver and the size of fast Fourier transform (FFT). The sampling rate of analog-to-digital converters (ADCs) can be reduced to 1/N of the original signal bandwidth in an N-receiver system. Aided by signal predistortion at the transmitter, aliasing-free signal can be retrieved independently and directly at the low speed receivers. A back-to-back experimental result is given for a two-receiver system. The effect of the electrical filters added before the ADC is studied and the analysis for filters optimization is given.

Electrical Prism: A High Quality Factor Filter for Millimeter-Wave and Terahertz Frequencies

A 2-D electrical filter is introduced that is compatible with today's conventional integrated circuit processes. The rich 2-D propagation properties of the medium are used to introduce a novel high quality factor filter called an electrical prism. The proposed filter shows a quality factor much larger than the quality factor of the individual components at high millimeter-wave and terahertz frequencies. This structure also provides a negative effective index in a low-pass LC lattice. Based on this idea, we show filters with quality factors of 130 at 230 GHz and 420 at 460 GHz consisting of elements with the quality factor of 10 and 20, respectively. The effect of component loss on the filter quality factor is discussed in this paper. The negative effective index and the filter behavior of the lattice is verified by measuring a prototype on a CMOS process at 32-40 GHz. There is good agreement among the theory, simulation, and experimental results.

Semi-analytical approach for performance evaluation of direct-detection OFDM optical communication systems

A semi-analytical method to evaluate the bit error ratio (BER) in direct-detection (DD) optical fibre transmission systems employing orthogonal frequency division multiplexing (OFDM) and optically pre-amplified receivers is proposed. The method considers a Gaussian approach for the signal at the equalizer output and allows evaluating accurately the BER of each OFDM subcarrier for the receiver structure considered and for practical optical and electrical filters shapes, being a powerful tool to perform the optimization of these systems. The results obtained by the proposed method have shown excellent agreement with Monte Carlo estimates for DD-OFDM ultra-wideband radio signals and for two different DD-OFDM signals proposed for long-haul systems.

Impact of ADC Sampling Speed and Resolution on Uncompensated Long-Haul 111-Gb/s WDM PM-QPSK Systems

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We analyzed the effect of analog-to-digital conversion on the performance of polarization-multiplexed quadrature phase-shift keying at 111 Gb/s. We address analog-to-digital converter sampling speed, resolution, and clipping, taking into account both linear and nonlinear propagation effects over a 20-span uncompensated wavelength-division-multiplexing long-haul test-link with 50-GHz channel spacing, using either standard single-mode fiber or nonzero dispersion-shifted fiber. We assumed realistic optical and electrical filters. We found that penalty can be limited within 0.5 and 0.8 dB, if at least 5 bits of resolution and 1.6 or 1.5 samples per symbol are used, respectively. </para>