Synchronous Filter Research Articles

Straight-line soliton data transmission at 20 Gbit/s beyond Gordon-Haus limit

Soliton data signals at 20 Gbit/s have been successfully transmitted over 3000 km with the use of soliton control by synchronous modulation and optical filters. The installation of only one simple modulator makes it possible to extend the maximum transmission distance from 2300 km (without soliton control) to 3000 km which is clearly beyond the Gordon-Haus limit.

A high performance digital synchronous noise filter

A digital synchronous filter for rejecting periodic noise such as that arising from the mains power supply is described. With a stationary noise signal the filter achieves a rejection ratio of 127 dB. With the somewhat variable mains, rejection ratios obtained in practice are typically 60 dB or better. The rejection notches are very narrow and the passband response is flat, leaving the signal of interest essentially undistorted. The performance of a hardware implementation of the filter is examined in detail and alternative implementations are discussed.

Enhancement of neural synchronization in the anteroventral cochlear nucleus. II. Responses in the tuning curve tail

1. Discharges of neurons in the peripheral auditory system contain information about the temporal features of acoustic stimuli. Phase-locking of neurons in the anteroventral cochlear nucleus (AVCN) is usually reported to be less robust than in auditory nerve (AN) fibers, which provide their major input. In a companion paper we reported that some cells in AVCN of the cat show enhanced phase-locking compared with the AN when stimulated at the frequency to which they are most sensitive [characteristic frequency (CF)]. We called neurons "high-sync" when they showed vector strengths (R, a measure of phase-locking) > or = 0.9. Here we report phase-locking properties to stimuli at frequencies below CF. 2. Horseradish peroxidase-filled glass micropipettes or metal microelectrodes were inserted into the trapezoid body (TB), which is the large output tract of the AVCN. Acoustically driven fibers were classified on the basis of the shape of the poststimulus time (PST) histograms to short tone bursts at CF. We then presented low-frequency tones of increasing SPL and determined the maximum R value at 500 Hz (R500) for each fiber. Using the same experimental protocol we studied phase-locking in the ANs of two animals because maximal R values at the tuning curve tail have not been reported for AN fibers. 3. Although phase-locking in AN fibers is usually assumed to be independent of CF, we found that fibers with CF > 2 kHz tended to have higher R500 values than fibers with CF < or = 2 kHz. Moreover, R500 was > or = 0.9 in 20% (42 of 196) of the fibers studied and could be as high as 0.95. This population of fibers was defined as having "high-sync tails" and consisted almost entirely of fibers with low or medium spontaneous rate. 4. High-CF TB fibers stimulated at 500 Hz showed very high phase-locking. High-sync tails (R500 > or = 0.9) were found in 41 of 70 TB fibers. For a subset of these fibers (1/3 in total: 23 of 70) phase-locking was higher than is ever observed in the AN (R500 > or = 0.95); these fibers were defined as showing synchronization "enhancement." Virtually all fibers showing synchronization enhancement had primary-like-with-notch (PLN) PST histograms. Chopper and primary-like fibers showed high-sync tails for CFs > 3 kHz. 5. Synchronization filter functions were obtained for high-CF AN fibers by determining maximum synchronization for a range of stimuli below CF.(ABSTRACT TRUNCATED AT 400 WORDS)

Evaluation of the false alarm and detection probabilities in a spread spectrum system using a matched filter

A new method for the evaluation of the false alarm and detection probabilities in a direct sequence spread spectrum system using a matched filter for initial synchronization is presented. The probability of false alarms is evaluated by considering the number of consecutive secondary peaks, spaced by Tb (the bit duration), which exceed a predetermined value set at the threshold detector, in a time frame of a preassigned number of received bits. The probability of detection is similarly evaluated. Numerical results for a number of m-PN spreading sequences using low SNR values and various threshold values have been obtained. For a 127 length m-PN sequence, values of pfa of the order of 1 × 10−12 and of Pd of the order of 0-9 can be obtained at −8dB SNR.

A novel IF frequency stabilization scheme for coherent optical FDM systems

An IF frequency stabilization scheme for coherent multichannel systems was demonstrated. The transmitter laser diodes are frequency-modulated with sinusoidal signals for channel spacing stabilization. A frequency-selective IF servo-system consisting of a phase-locked loop and a synchronous filter was used to make the local oscillator track the additional sinusoidal modulation on the transmitter frequency so that the modulation amplitude in the IF domain was greatly reduced. In the demonstration experiment with frequency-shift-keying (FSK) transmission, it was reduced to 5 MHz/sub p-p/ and no system performance degradation due to this residual modulation was observed. The circuit also gives a channel identification signal. >

Noninvasive techniques for assessing the effect of environmental stressors on visual function

Visual acuity, a standard measure of visual function, is affected by a variety of physical and toxic stressors. In anesthetized animals, the acuity may be determined by the visual-evoked potential (VEP). We compared two evoked potential methods: the steady-state VEP, elicited with a fixed stimulus, and the sweep VEP, elicited by a dynamic stimulus. The steady-state VEP is a conventional electrophysiological technique for determining acuity, although its signal-to-noise ratio declines at high spatial frequencies and introduces a source of error for acuity determinations. Moreover, it is unsuited for repeated or rapid measurements. The sweep VEP is elicited by a dynamic stimulus which sweeps through a series of spatial frequencies. Response amplitude is measured with a sensitive, synchronous filter. The sweep VEP can be completed in as little as 10 s and may be used for repeated measures. On the other hand, waveform data is lost with the sweep VEP. The two techniques yield fairly similar acuity measures, generally agreeing within a factor of two.

Excitonic transitions in GaAs/GaxAl1−xAs quantum wells observed by photoreflectance spectroscopy: Comparison with a first-principles theory

Excitonic transitions in GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As multiple quantum wells are observed using photoreflectance spectra obtained with a novel double-monochrometer spectrometer. The first pass of the monochrometer produces the infrared probe beam while the second pass is used as a synchronous band-pass filter, resulting in a better signal-to-noise ratio than the low-pass filter conventionally used, and allowing simultaneous collection of the photoluminescence spectrum. The photoreflectance data are analyzed using a multiband effective-mass theory which includes valence-band mixing and assumes a first-derivative modulation of the built-in field as the dominant modulation mechanism. The entire photoreflectance spectrum is simultaneously fit using a first-principles theory in which only the exciton linewidth and value of the built-in electric field are not determined by the theory. Full advantage is thus taken of the signal intensity information, in contrast to comparisons currently encountered in the literature where positions of excitons are usually compared to values predicted by a finite-square-well calculation, without valence-band mixing effects. The theory is shown to have much success in explaining the observed photoreflectance signal from a 200-A\r{} multiple quantum well at 77 K, including the qualitative line shape for any given exciton.

Synchronous rejector filter

Synchronous rejector filter

Low-noise In-Sb photodetector preamp for the infrared

A low-noise, cryogenically cooled preamplifier has been designed and built for indium antimonide detectors used in IR astronomy. General noise and frequency response equations for the transimpedance amplifier are derived. The effects of finite open-loop gain and bandwidth and of photodetector parameters on circuit performance are shown. A current-injecting background cancellation scheme based on optical feedback and synchronous filtering is described. Total electrical NEP figures as low as 10/SUP -16/ W (at 5 /spl mu/m) and background rejection ratios higher than 3000:1 have been measured using a 0.5 mm diameter detector.

A Novel Spread-Spectrum Receiver Synchronization Scheme Using a SAW-Tapped Delay Line

In spread-spectrum communication systems, a basic task which has to be performed is the synchronization of the receiver pseudonoise (PN) signal with the pseudonoise signal contained in the input. In this paper, a synchronization system employing a surface acoustic wave tapped delay line (SAW TDL) matched filter for both initial synchronization (acquisition) and tracking is presented. The periodic correlation peaks at the SAW TDL output repeatedly correct the epoch of the local PN signal clock phase and the code generator initial condition to their correct values. As the correlation impulses are distorted and attenuated due to the effects of message modulation, Doppler frequency shifts and unavoidable interferences, a modulation canceller and a differentiating device are employed to improve synchronization performance. Formulas for estimating the performance of a system incorporating these ideas are given. A hardware implementation of the suggested system has been built and tested. Experimental results obtained with the system are presented.

Comparison of the Unbalance Responses of Jeffcott Rotors With Shaft Bow and Shaft Runout

The unbalance response of a Jeffcott rotor with shaft bow and/or runout was theoretically and experimentally studied. Bow refers to a rotor which is warped; bow is a function of running speed. Runout refers to electrical or mechanical asymmetrics of the shaft and is not dynamical. Included in the theoretical model is the capability of low-speed response compensation, such that the response at low speed can be vectorially subtracted from the total response at any rotational speed. Responses of rotors with equal amounts of bow or runout are shown to be significantly different in both Bode and Nyquist forms. Using low speed compensation is shown to “correct” the unbalance response of a rotor with runout to an ideal (unbowed - no runout) case. The amplitude response plot of a bowed rotor is not corrected to the ideal response plot by the use of such compensation; however, the shape of the phase response plot closely resembles the ideal case for most cases. A small scale lightly damped Jeffcott rotor rig was also tested. The magnitude and angular position of the shaft bow were parametrically varied. The vibration data from the rotor tests were plotted using a synchronous tracking filter by two methods: both not using and using low speed compensation. Experimental data agree excellently with predictions for a bowed rotor for all cases and differences less than 8 percent were usually found.

Noise cancellation using tracking filter

First Page

Lock-in amplifier using a sampled-data synchronous filter

A lock-in amplifier is constructed using a synchronous filter instead of a phase sensitive detector. The synchronous filter based on a signal sampling method offers an extremely narrow bandpass characteristic whose centre frequency can be easily controlled by its drive frequency. Since the filter makes smaller responses for harmonics of the signal at the tuned frequency than a phase sensitive detector, the error due to the harmonics becomes less and required performances of a prefilter are relaxed. By this lock-in amplifier the magnitude of a low-level signal affected by noise can be measured accurately without annoying phase adjustments which are required in a conventional lock-in amplifier.

Time-Notch Filtering

A method is described for measuring low-level time-truncated signals by using narrow-bandwidth synchronous time-notch filters. These filter systems, intended for use with ordinary narrowband receivers or field intensity meters, provide a means of isolating time-truncated signals of interest from adjacent interfering signals. In addition, the important property of near optimum sensitivity (approaching that of a matched filter) is discussed, and the laboratory results of a low-frequency time-notch filter covering the 500-Hz to 2-MHz range are given.

Synchronous Analysis for Diagnostics of Rolling Bearings in the Turbojet Engine

Designed in the Aviation Institute, the K-15 turbojet engine has got rolling bearings, which answers with frequency5.87×ωrthe unbalance excitation. The signal with such frequency indicates a fault of the outer race of the rolling bearing. A set of the digital synchronous filters was used for the K-15 vibration spectrum analysis. A procedure of filtration was performed by the computer software. The sychronous summation of the measured signals was carried out before the spectrum analysis. Two cases were considered: the engine with a small force due to unbalance (a small angular velocity of the rotor), and the engine with a big unbalance force (high angular velocity). In the first case, the outer race frequency was not observed, despite the existence of the vibration amplitude (caused by unknown disturbances) with such frequency before the synchronous summation. In the second case, the outer race frequency after synchronous summation has enlarged amplitude while other spectrum components in its vicinity have been damped. It underlines the usefulness of the synchronous analysis in the vibration diagnostics of the rotating machinery.

Analysis and Synthesis of AC Compensation Filters

A state-space approach is used to derive a general analysis and design method for ac compensation networks including both synchronous filters and demodulating networks. The approach is based on a general analysis of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</tex> port <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</tex> networks, containing an arbitrary number of periodically operated switches, subjected to a suppressed carrier modulated input. The results can be simply extended to include passive and active <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RLC</tex> networks, which can be described by state equations.

Leakage Rejection in Beam-Switched CW Radars

This paper is directed to the problem of transmitter-receiver leakage rejection in CW and FMCW radars which employ stepped-scanning antennas. An IF carrier-elimination filter has been devised and analyzed which provides leakage-rejection capabilities limited only by leakage variations during stationary beam positions. Leakage rejection is provided by coherently reducing the signal to zero IF, rejecting the leakage components by a combination synchronous switch and high-pass filter, and remodulation onto a second IF carrier. The filter operates in a linear manner. Positive and negative senses of Doppler signals are retained. The spectrum of the input signal is faithfully reproduced in the output with the exception of a sharp notch at the input carrier frequency. In one configuration, the minimum notch bandwidth is limited by the on-time of each beam, and is approximately equal to one-half the reciprocal of this on-time. A more complex arrangement removes this restriction, permitting a minimum notch width which is limited only by the cutoff frequency of a high-pass RC filter. The simpler configuration has been used with the AN/APN-118 Doppler Navigation Radar, a system which was developed under contract with the U. S. Signal Corps, Fort Monmouth, N. J., by the Sperry Gyroscope Company.