Showing papers on "Band-stop filter published in 1976"

Optical notch filter utilizing electric dipole resonance absorption

Abstract: An optical notch filter having a suspension of small absorbing particles which blocks a narrow band of frequencies. The small absorbent particles are suspended as a colloidal system which can have either a solid, liquid or gas as host material. The absorbing particles can be incorporated in a solid matrix as a transmission filter or used on the surface of a mirror as a reflection filter. The filter can be tuned by an externally applied electric field or by applying hydrostatic pressure.

A digital filter bank for spectral matching

Abstract: A new digital filter bank design is proposed for the processing of speech waveforms where spectral pattern matching techniques are applicable. Outputs in decibels from the 30 channels of the filter bank are computed every 12 ms. Care has been taken to select a time window and filter center frequency and bandwidth values that take into account the acoustic characteristics of speech. A distance metric is proposed for comparing a spectral frame with previously derived reference patterns. The metric incorporates procedures for crude speaker/microphone normalization, signal level normalization, background noise normalization, and procedures for emphasizing differences in the region of spectral peaks.

Phase-locked loop having a switched lowpass filter

Abstract: A phase-locked loop is disclosed having a phase comparator, a switched lowpass filter, a voltage controlled oscillator, a loss of lock detector, and a charging circuit. The switched lowpass filter has two different bandwidths: the first bandwidth has a wide frequency response for use during acquisition periods for rapid acquisition, and a second bandwidth has a very narrow bandwidth for use during normal tracking for tight control of the voltage controlled oscillator. A loss of lock detector controls the particular filter bandwidth state by causing certain filter components to be switched in and out according to the presence or absence of phase lock condition. A charging circuit, used in conjunction with the particular lowpass filter section not operative during acquisition, charges the energy storage components used in this section of the filter to preclude transients from occurring when these components are switched back into the lowpass filter.

Image processing system using incoherent radiation and spatial filter hologram

Abstract: A linear optical processing system in which a spatial filter in the form of a Fourier transform hologram is included with a lens for processing incoherent electromagnetic radiation. Such a system provides a given real, two-dimensional transfer function for spatially filtering incoherent radiation within a range of spatial frequencies 0 ≦ω≦Ω. The filter is made by forming a mask of a graph of a point spread function which is the inverse transform of the desired transfer function, and employing the mask in a coherent system to produce a Fourier transform hologram of the mask. Spatial filters are designed for providing the resultant systems with various transfer functions, including (a) a directional response for passing substantially higher spatial frequency components in a first direction than are passed in a second direction; (b) a selective attenuator response; (c) a notch filter response; (d) a correctional filter response; and (e) a linear minimum mean square error response. The systems are useful for recognizing specified patterns from a plurality of patterns and for determining the concentration of a species of particulate matter having a predetermined specific pattern within a sample of particles.

Automatically tunable notch filter and method for suppression of acoustical feedback

Abstract: Disclosed is an automatically tunable notch filter and method for suppression of acoustical feedback in an audio signal. The apparatus includes a selectively tunable notch filter having a center frequency which is variable over at least a substantial portion of the audio frequency spectrum. The apparatus receives an audio signal which is substantially non-periodic in the absence of acoustical feedback and substantially periodic with an instantaneous dominant frequency in the presence of the same. The duration of successive periods are monitored and compared by an up/down counter to determine whether the audio input signal is substantially periodic and to determine the instantaneous dominant frequency of such audio signal. Upon detection of an audio signal which is substantially periodic, the notch filter is tuned to the instantaneous dominant frequency so as to suppress the acoustical feedback.

Input filtering apparatus for loudspeakers

Abstract: Improved input filtering apparatus for loudspeaker systems, and loudspeaker systems including such apparatus. The bass roll-off point of input filtering apparatus for loudspeakers is automatically adjusted to an optimum in accordance with the amplitude and spectral content of the audio signal being received at the time, thereby attenuating components in the signal liable to cause "bottoming" of the loudspeaker. The input filter apparatus comprises a first high-pass filter having a breakpoint frequency at or near the lower threshold of the audio frequency range, a second variable bandwidth, high-pass filter connected to receive the first high-pass filter output, a feedback circuit responsive to the second filter output to provide a feedback signal as a linear function thereof with a gain which is substantially inversely proportional to the square of the frequency, and circuit means including threshold means connected with the second filter and responsive to the said feedback signal to relatively rapidly increase the breakpoint frequency of the second filter in response to values of the feedback signal which exceed a threshold and to relatively showly reduce that breakpoint frequency in response to values of the feedback signal which do not exceed the said threshold. Bass equalization circuit means having a frequency response at the low end of the audio spectrum which is substantiallythe inverse of the frequency response of said loudspeaker means are provided between the output of the second filter and the loudspeaker drive circuit.

Filter having an automatically controlled variable cut-off frequency

Abstract: A device for measuring the frequency or period of an input signal independent of different frequency noise signals which may be superimposed on the input signal includes a filter circuit having a variable cut-off frequency. The output level of the filter circuit is detected by a level detector, the output of which electrically controls a variable element such as a variable resistor, a variable capacitance diode or the like which is included in the filter circuit. This controls the cut-off frequency of the filter circuit, so as to maintain its output level at a given level. In this manner, an input signal frequency is converted into a corresponding level, utilizing a declining portion adjacent to the cut-off frequency of the frequency response of the filter circuit. The converted level is utilized to vary the cut-off frequency of the filter circuit to maintain its output level constant, thereby automatically changing the cut-off frequency in accordance with the input signal frequency.

On Understanding the Matched Filter in the Frequency Domain

Abstract: A matched filter is viewed as a cascade of a spectral-phase matched filter and a spectral-amplitude shading filter. The former provides the peak output in time, and the latter guarantees that that peak has maximum signal-to-noise ratio (SNR), or the latter may alternatively be chosen to provide better time resolution.

Multi-channel R.F. transducer with channel selector coupled to selected channel filter

Abstract: A voltage variable filter circuit useful, for example, in a radio frequency transmitting device and having a center frequency which is changeable with changes in an applied voltage employs one or more voltage variable reactive elements in the filter in conjunction with circuitry for changing the direct current voltage across such reactive elements to change the reactance thereof and therefore also change the center frequency of the filter. To extend the range of filter variation, additional reactance may be selectively introduced into the filter circuit. In the environment of a multi-channel radio frequency transmitting device, a selector switch simultaneously selects one radio frequency channel for transmission and a unique voltage source for application to the voltage variable reactance so that the filter is appropriately tuned for the selected channel.

A reprogrammable filter bank using charge-coupled device discrete analog-signal processing

Abstract: A filter bank having up to 32 channels is formed by multiplexing a recursive two-pole delay line filter Thirty-two sets of filter constants are stored and addressed digitally while multiple delays are provided in analog form by a charge-coupled device (CCD) delay line The interface between the digitally stored constant and the delayed analog signal occurs in a four quadrant multiplying D/A where the delayed analog signal is weighted by the constant Sampling and clamping at the CCD interfaces insures accurate dipolar analog operation The filter hardware is described, the filter constants are derived, and the performance is documented

Filter circuit having an acoustic surface-wave filter device

Abstract: A filter circuit includes an acoustic surface-wave filter device having an input transducer comprised of a set of interleaved electrodes disposed on a first portion of a body of piezo-electric material adapted to propagate acoustic surface waves for producing an acoustic surface-wave signal in response to an input signal applied to the input transducer, and an output transducer comprised of a set of similar interleaved electrodes disposed on a second portion of the piezo-electric body spaced a selected distance from the first portion for receiving the acoustic surface-wave signals so as to produce a corresponding output signal having a selected frequency response. The frequency response (e.g., bandwidth) of the acoustic surface-wave filter device is controlled by selectively changing the number of conductor elements constituting the interleaved electrodes of the input and output transducers. An amplifier drives the acoustic surface-wave filter device, the gain and output impedance of the amplifier being changed as the frequency response of the acoustic surface-wave filter device changes, so as to compensate for corresponding changes in the insertion loss and input impedance of the filter device.

A notch filter employing current differencing operational amplifier

Abstract: A realization of a notch filter based on analogue computing techniques but employing current, differencing operational amplifiers is presented. The realization is canonic with respect to the number of operational amplifiers and passive components except for two bias resistors. An attempt is made to assess theoretically the performance of the filter and compare this with measured performance. A comparison is also made with a gyrator-derived version over which it is assessed to have superior notch rejection under the circumstances considered.

Surface acoustic wave filter

Abstract: A surface acoustic wave device useful as a notch filter or a narrow-band transmission filter, the device including a periodic metallic grating disposed on a suitable surface acoustic wave supporting substrate in a beam path defined by an input transducer, the grating elements each exhibiting energy storage discontinuities at the extremities thereof.

Filter which tracks changing frequency of input signal

Abstract: A signal-frequency tracking band-pass filter useful for passing an input signal which changes rapidly in frequency. The tracking filter includes a voltage-controlled band pass filter, and a frequency-to-voltage converter for translating the output frequency of the filter to a voltage for controlling the pass band center frequency of the filter. The Q of the filter is controlled by a feedback circuit which makes the Q vary inversely with output signal amplitude. The Q is high for weak signals that are hard to lock on to, and the Q is low for strong signals to facilitate tracking of signals which change rapidly in frequency. The filter is especially useful for inclusion in internal combustion engine diagnostic equipment.

Low pass active filter apparatus

Abstract: The subject of the disclosure is low-pass filter apparatus for use in filtering out noise in a signal channel for propagating PAM signals. A sample and hold circuit which operates in synchronism with the presence of PAM pulses is provided as the input section. The output of the sample and hold circuit is applied to a four-pole, low-pass, RC active filter network. An adjustment is made to the active filter network such that the filter network's low frequency complex pole pair is changed to provide compensation for an undesired roll-off of band pass in the intended in-band region due to the frequency response characteristics of the sample and hold circuit. The resultant apparatus provides a significant reduction in active filter gain required for a given PAM duty cycle. It also provides increased noise rejection capability in the out-of-band region of operation.

Communication network protection system

Abstract: An improved system and method for protecting communications systems using in-band signaling from malfunction which may produce serious circuit failures such as false operation of the disconnect circuit. Typical practice of the invention involves detection of those main signal sounds that are similar to and imitate the in-band control tones and using the detected sounds to control a switching circuit which inserts a band reject filter into the system so as to reduce the amplitude of such sounds before they are fed to the communication system utilizing in-band signaling. Under normal conditions such sounds are not present or are of insufficient amplitude or are present for too short a time period to cause false operation. Accordingly, the band reject filter is not normally active and there is no degradation of frequency response. Even during those periods when the filter must be inserted in the line to provide protection, the bandwidth of the filter is narrow so that disturbance of the frequency response is minimized.

SAW chirp filter performance above 1 GHz

Abstract: The performance of a practical compact SAW dispersive filter with a bandwidth in excess of 500 MHz and centered at 1.3 GHz is described. The linear FM chirp filter was fabricated and serially reproduced by electron-beam microfabrication. The effect of an externally implemented weighting function on the time domain performance is discussed. The filter has been found suitable for pulse compression in a high-resolution monopulse radar. Replication of filter sets with identical characteristics is discussed.

Discrete transversal filter

Abstract: Digital transversal filter for realizing a lowpass filter with a cut-off frequency f g and an impulse response h 1 (i), a lowpass filter having a cut-off frequency 2f g which has a transfer characteristic which is in phase with the first-mentioned lowpass filter, as well as a lowpass filter with a quadrature transfer characteristic of the in-phase filter.

Frequency sampling filter

Abstract: In accordance with the present invention, there is provided an economical and compact frequency sampling filter comprising a comb filter and resonators, in which the difference between the output of a frequency sampling filter of the bandpass type or the low-pass type, or the bandstop type or the high-pass type and the output of a circuit having the same gain as that at each sampling point in the pass band of said filter and having a similarly varying phase to that in the pass band is obtained.

A New Band-Splitting Filter for Guided-Millimeter-Wave Transmission Systems

Abstract: A new Michelson-interferometer (MI) hybrid having a miter angle is developed for use as a millimeter-wave band-splitting filter. The construction and operating principle of the filter are described. The design method and the experimental results are also presented. This filter has low branching loss, yet keeps very wide band characteristics. For the 4W-120-GHz-frequency-range filter with 35/spl deg/ miter angle, the branching loss is 0.68-1.56dB. This is about 40 percent lower than that of the conventional MI filter. The input VSWR is less than 1.29 and the guard bandwidth is less than 250 MHz. This filter can be used for the 40-120-GHz guided-millimeter-wave transmission systems.

70 MHz surface-acoustic-wave resonator notch filter

Abstract: A 70 MHz notch filter employing a surface-acoustic-wave resonator in a hybrid-coil circuit to produce a notch depth of 50 dB has been developed. The notch frequency is tunable over a 40 kHz range and the passband attenuation is approximately 7 dB.

Aural-visual ratio control apparatus

Abstract: This disclosure is concerned with obtaining aural-visual ratio control with novel intercarrier detection cooperative with a high-Q notch filter feeding a strip amplifier or heterodyne processor.

Electronic filter with active elements

Abstract: An electronic filter having active components for eliminating A.C. signals, and for producing constant D.C. voltage at its output, wherein the filter can be effectively used between the D.C. power supply and the B+ terminal for an audio amplifier, especially one having vacuum tubes for its major active elements. The electronic filter monitors its output voltage to control the conductivity of a transistor which maintains the output voltage constant. Bias current and voltage requirements for the filter elements are generated internally from the filter input, and are also referenced to the filter output to provide a constant D.C. output over load variations.

MFMBO filter response

Abstract: One of the new filters to emerge in recent years has a magnitude response which is maximally flat beyond the origin. The filter is a generalization of the Butterworth filter. This letter presents a study of this filter including its magnitude, delay, and step responses. A filter design nomograph is also presented. These results are compared with other classical filter types. It is shown that this filter provides an attractive alternative to Butterworth, Chebyshev, Papoulis, and Halpern filters.

Signal bandpass filter apparatus

Abstract: A signal bandpass filter apparatus is disclosed wherein a direct short circuit between input and output is prevented and the loss of the ground reference of the filter is detectable and fail-safe in operation.

Multisection filter using inflected amplitude change function to sharpen its band-edge responses

Abstract: A signal to be filtered is subjected to a known filter function a sufficient plurality of times and with appropriate processing to produce in a plot of the total filtering response versus the response of one such function at least one inflection in a portion of the plot corresponding to a transition band between a passband and a stopband of the filter function. Various embodiments of the overall filtering function are shown which are applicable to both recursive and nonrecursive filters and to filters having either single or multiple sets of passband and stopband combinations per filter function.

An active filter with a distributed RGC line

Abstract: A new positive feedback active filter using a RGC line is proposed The filter has zero Q-sensitivity of the dominant pole with respect to gain variation In comparison with other positive feedback filters it is characterized by small second-order sensitivity and having gain less than 1

RC Passive Notch Filters

Abstract: A critical study of various types of passive notch filters is made. The distributed parameter notch filters are found to be superior in performance as compared to the lumped RC and lumped-distributed types. The frequency response curves indicate that tapering in distributed notch filters improves the notch selectivity.

Waveguide transition section assembly - joins coaxial line to rectangular guide and has filter comprising specified circuit and resonant circuit

Abstract: The waveguide transition section is for joining a coaxial line to a rectangular waveguide. It has a filter circuit between the waveguide (H) and the coaxial line (K). The filter circuit consists of the series combination of a quarter wave line (L1) and an inductor (L), and a parallel resonant circuit (P) consisting of a terminal absorber (Ab) (bandstop filter) and a bandpass filter formed by a stepped section of waveguide inside the transition. The coaxial line inner conductor (2) extends into this section. The coaxial line is coupled in on the broad face of the guide.

Surface acoustic wave notch filter

Abstract: A surface acoustic wave device including a synchronous reflection array disposed in a delay line between the input and output transducers, the synchronous reflection array having a synchronous reflection frequency that distorts the beam profile of propagating surface acoustic wave energy in the delay line so that when the beam profile is amplitude averaged at the output transducer, the transducer output signal has a relatively deep notch in amplitude at the synchronous reflection frequency.