Showing papers on "Band-stop filter published in 1977"

Adaptive noise canceling applied to sinusoidal interferences

Abstract: This paper investigates a new method for eliminating sinusoidal or other periodic interference corrupting a signal. This task is typically accomplished by explicitly measuring the frequency of the interference and implementing a notch filter at that frequency. The method proposed herein uses an adaptive filter to eliminate the interference. The procedure is called adaptive noise canceling and is applicable when an auxiliary reference input is available containing the interference alone. The reference input is filtered in such a way that it closely matches the interfering sinusoid, and is then subtracted from the primary input leaving the signal alone. The results of this research show that when a sum of sinusoids is applied to an adaptive filter, the filter converges to a dynamic solution in which the weights of the filter are time varying. This time-varying solution implements a tunable notch filter, with a notch located at each of the reference frequencies. When used in noise-canceling applications, this adaptive notch filter provides a simple alternative to other methods of tracking and eliminating sinusoidal interferences.

Sharpening the response of a symmetric nonrecursive filter by multiple use of the same filter

Abstract: When processing data by filters, we often find it necessary to improve the performance of the filter, either by increasing the out-of-band rejection (loss) or by decreasing the error in the passband, or both. A first approach is to process the data by repeated passes through the same filter. Each pass, while increasing the out-of-band loss, also increases the passband error, often to an undesirable level. It also increases the length (order) of the equivalent filter. How can we do a better job of filtering by suitably combining the results of several passes through the same filter? By "better" we mean both less passband error and greater out-of-band, or stopband, loss. This process is called filter sharpening. A simple, powerful method for doing this is described in detail, and its computational efficiency is compared to the best possible filter designs meeting the same specifications. The design method, based on the idea of the amplitude change function, is restricted to symmetric nonrecursive (finite impulse response) filters with piecewise constant pass- and stopbands. Several illustrative examples are given.

Sharpening the response of a symmetric nonrecursive filter by multiple use of the same filter

Abstract: When processing data by filters we often find it necessary to improve the performance of the filter, either by increasing the out-of-band rejection (loss) or by decreasing the error in the passband, or both. A first approach is to process the data by repeated passes through the same filter. Each pass, while increasing the out-of-band loss, also increases the passband error, often to an undesirable level. It also increases the length (order) of the equivalent filter. How can we do a better job of filtering by suitably combining the results of several passes through the same filter? By "better" we mean both less passband error and greater out-of-band, or stopband, loss. This process is called filter sharpening. A simple, powerful method for doing this is described in detail, and its computational efficiency is compared to the best possible filter designs meeting the same specifications. The design method, based on the idea of the amplitude change function, is restricted to symmetric nonrecursive (finite impulse response) filters with piecewise constant pass and stopbands. Several illustrative examples are given.

Design of microstrip spur-line band-stop filters

Abstract: The paper describes the design of a new class of microstrip bandstop filter. This filter is a very compact structure, it radiates power significantly less than conventional shunt-stub and coupled-line filters and is also less susceptible to the influence of other components and lines in its vicinity.

Combination optical low pass filter capable of phase and amplitude modulation

Abstract: A combination optical low pass filter adapted for use in a single or double tube color television camera is provided with a phase retarding pattern on a transparent substrate. The pattern includes optical elements having a transmissivity which varies with the wavelength of light passing there through so that the optical low pass filter functions as both a phase diffraction and an amplitude diffraction filter. Medium indexed filler material can further supplement the pattern to eliminate any light scattering.

Passive filter compensation network

Abstract: A compensation network for a passive filter which compensates for the variation of the loss attenuation of the filter dependent on ambient temperature variations consists of an L-section whose series arm is formed by a temperature dependent resistor and whose shunt arm is formed by another temperature dependent resistor in series with a resonant circuit. The resonance frequency value of the resonant circuit is chosen nearly equal to the band limit frequency of the filter.

Television security system

Abstract: A system for subscription TV security which is comprised of a scramble signal generator which adds a modulated RF carrier signal to the TV channel transmission. Preferably the frequency of the scramble signal generator is selected to fall within the band of frequencies of the TV channel between the video carrier and the audio carrier. The TV channel is transmitted to receivers encoded with the scramble signal carrier providing false and misleading information, preventing the receiver from reconstituting a coherent picture or sound while this scramble signal is present. The transmitted signal is decoded, (i.e. scramble signal removed), restoring regular reception by filtering with a sharply tuned high-Q notch filter attached to the drop line going to a TV receiver. By careful selection of the band width and attenuation of the filter and pre-emphasis of the channel band, degradation of the video information is prevented.

A general second-order digital filter with controlled rounding to exclude limit cycles for constant input signals

Abstract: In this paper a new general second-order digital filter section is proposed. One of the two quantizers used in the filter applies controlled rounding which has the effect that limit cycles are completely eliminated under constant-input conditions. Furthermore, the configuration of the filter is such that multipliers can be saved in case the filter is used as an all-pass filter or as a filter with zeros of transmission on the unit circle in the z domain.

Comprehensive feedback elimination system employing notch filter

Abstract: A feedback elimination apparatus for a sound system which employs a notch filter assembly comprising a plurality of active notch filters, each capable of producing a narrow notch in the frequency characteristic of the system. Means are provided for temporarily switching each notch filter to an oscillating mode to produce a reference signal at notch frequency which, for adjustment purposes, is zero beat with an acoustic feedback signal in the system, so the feedback signal is canceled when the notch filter is returned to its normal mode. A compressor is temporarily interposed in the sound system for limiting and controlling the level of the acoustic feedback signal to facilitate separation of acoustic feedback signals of different frequency and to prevent overload. An equalizer is also interposed in the system and which, in addition to performing its equalization function, is capable of producing a relatively wide notch in the frequency characteristic of the system for canceling the points of acoustical feedback grouped in frequency.

Selective interference filter

Abstract: Filtering means for biophysical recording apparatus for removing interference from a signal source which includes a band rejection or notch filter and means for inserting the filter into the signal circuit when the signals have a certain characteristic and for removing the filter when the signals have another characteristic.

Digital filter circuit

Abstract: Apparatus in a digital filter circuit for substantially eliminating limit cycle noise comprises means for randomly inhibiting the rounding signal in at least one of the multipliers comprising the digital filter circuit.

Electrically tunable bandpass filter

Abstract: A bandpass filter is described for filtering a television signal. The filter has a passband which may be selectively centered for filtering signals in one of two channels of the television receiver. The television channel at which the filter passband is centered is selectable by applying a voltage to a control terminal. The filter frequency response for each channel selected may be independently controlled and a linear phase response is realized for the selected channel minimizing distortion.

Feedback elimination system employing notch filter

Abstract: Feedback elimination apparatus for a sound system which includes a notch filter assembly comprising a plurality of active notch filters connected in parallel with one another. Each of the notch filters is formed of a loop amplifier circuit including an operational amplifier having a resonant feedback network so arranged that in the normal mode an auxiliary signal peaked at the resonant frequency is applied out-of-phase to the system, thereby to establish a notch in the frequency characteristic curve of the system. An improved loop circuit is used to produce a notch which is sharp and well defined, with auxiliary circuitry for aligning the notch with a feedback signal. In the preferred embodiment means are provided for temporarily switching the notch filter into an oscillating mode in which it oscillates at notch frequency to produce an audible reference signal. A manual control in the notch filter smoothly varies the frequency of the resonant network so that the reference signal is swept over the audio range to establish zero beat with a "first" acoustic feedback signal. Thus when the notch filter is restored to its normal mode the notch is in alignment with the feedback signal, the depth of the notch being subsequently increased as necessary to the point where the feedback signal is cancelled. The system gain is then incrementally advanced to produce additional feedback signals and the procedure is repeated for each of the notch filters until each acoustic feedback signal has been disposed of.

Filtering technique based on high-frequency plant modeling for high-gain control

Abstract: This invention is an improvement in aircraft control systems that utilize feedback motion sensors to generate a control signal to control the aircraft. The improvement consists essentially of a complementary filter comprising a simplified model of the aircraft, a high-pass filter, a low-pass filter and a summing amplifier. The control signal is applied to the simplified model of the aircraft which ateempts to compute the vehicle response to the signal. This computed response is then fed into the high-pass filter to eliminate long-term errors in the calculated response, with the result that a good estimate of the high frequency content of the aircraft motion is obtained. In order to obtain a good estimate of the low frequency content of the motion a rate gyro signal is fed through the low-pass filter that eliminates all of the offending noise. The outputs from the high-pass and low-pass filters are summed by the summing amplifier to produce an estimated rate which is then used as a motion feedback signal. The use of the complementary filter permits a substantial increase in frequency band-width due to the simultaneous reduction in noise amplificaton and control limit-cycle tendencies.

Lens element incorporating narrow bandpass filter and usable in dual mode

Abstract: An optical system utilizing a lens element having a spherical surface incorporating a narrow bandpass filter, with the spherical surface being of such configuration that converging rays are normal to its surface, thereby permitting the narrow bandpass filter to operate with minimum bandwith, thus maximizing the rejection of unwanted background radiation. The surface of the lens element opposite the surface having the narrow bandpass filter may incorporate a dichroic filter, enabling the lens element to operate in a dual mode, with the dichroic filter reflecting entering visible light energy but being transparent to laser light energy, and the narrow bandpass filter thereafter eliminating wavelengths outside a given laser spectrum. As an example, my novel lens element may be used as a part of a catadioptric objective system sensitive to both visible and laser light frequencies.

Double-split-electrode transversal filter

Abstract: A novel structure for onchip tap-weight implementation in integrated c.c.d.-transversal-filter applications is presented. The new ‘double-split-electrodersquo; structure provides improved filter performance over the common-split-electrode technique by reducing the common-mode signal, sensing electrode capacitance and noise pickup.

Band-pass/band-stop filter for telecommunication system

Abstract: A filter for a telecommunication system, designed to suppress a signal frequency F a and to transmit an adjoining frequency F b without significant attenuation, comprises a resonant cavity of prismatic shape longitudinally traversed by a conductive coil wound on a hollow dielectric core into which an adjusting screw is inserted. One end of the coil is grounded at the cavity wall while its other end is left unconnected and forms with the adjusting screw a variable capacitance C d with distributed constants; a lumped-constant tuning capacitor C 1 is connected between ground and an input lead connected to an intermediate tap on the coil, the inductance L 2 " of that lead and the inductance L 2 ' of the coil portion between the tap and ground forming an inductive voltage divider bridged by the lumped-constant capacitor C 1 . The remainder of the coil, lying between the tap and the unconnected end, forms an inductance L 1 which together with the distributed-constant capacitance C d shunts the portion L 2 ' of the inductive divider and forms a series-resonant circuit tuned to frequency F a ; the combination of voltage divider L 2 ', L 2 " and lumped-constant capacitor C 1 constitutes a parallel-resonant circuit tuned to frequency F b .

Waveguide filter employing dielectric resonators

Abstract: A waveguide bandstop filter is described. Two ceramic dielectric disc resonators each having a resonant frequency f 0 and separated by 3/4λgo, where λgo is the wavelength at f 0 , are each disposed in individual apertures in the waveguide wall. The substantial portion of each resonator is exterior to the waveguide cavity and surrounded by a metallic housing which isolates each resonator from each other. The resonators are oriented so that a coupling arrangement exists between the electromagnetic energy propagating through the waveguide and each resonator so that a resonant mode is excited therein at frequency f 0 .

Plug-in filter network for separating a communication frequency into discrete frequency channels

Abstract: A filter network for separating a transmitted communication frequency into discrete channels. A separator, exemplified by a triplexer, includes a pluggable mid-range high Q bandpass filter and a diplexer of low Q bandpass filters. Circulators receive signals reflected from the high Q filter and pass them to the low Q filters. A combiner circuit takes the attenuated signal from the low Q filters and sends it through a circulator to a high Q plug in filter where the signal passes through a circulator where it is combined with the signal from the high Q filter of the separator as output.

Input stage for a CTD low-pass filter

Abstract: An input stage for a transversal charge transfer filter, which suppresses parasitic image frequencies. It comprises two parallel channels to which the input signal is applied simultaneously. Sampling of the signal is performed in each channel with a periodicity Te, and a relative phase-shift of Te/2. The two samples are then added in order to eliminate components having periodicities in the neighborhood of Te. The present invention relates to low-pass filters using the phenomenon of electrical charge transfer occurring in a semiconductor. It relates more particularly to an input stage for this kind of CTD (Charge Transfer Device) filter, which attenuates parasitic frequencies. The design of CTD low-pass filters, often referred to as transversal filters, poses a classic problem due to the fact that the input stage is a sampling stage. In other words, when the input signal contains a component of frequency Fs close to the sampling frequency Fe, by a process of frequency mixing, two parasitic signals at the image frequencies Fe + Fs and Fe - Fs are created. Since we are concerned with a low-pass filter, the top image frequency (Fe + Fs) is automatically eliminated but the bottom image frequency (Fe - Fs) can be transmitted by the filter and appear in the output signal if the condition Fe - Fs < Fc is realised, Fc being the cut-off frequency of the low-pass filter. A well-known solution to this problem is to add to the transversal filter an associated band-stop filter for suppressing from the input signal any components whose frequencies are located between Fe - Fc and Fe + Fc. This kind of associated filter is outside the CTD filter and is generally made with the help of RC networks. The fact that it is outside the low-pass filter is a drawback especially from the point of view of bulk. A second drawback is due to the fact that the attenuation in the band which is to be stopped, is inadequate: it does not generally exceed a ratio of 4. The object of the present invention is to integrate into the input stage of a CTD transversal filter a band-stop filter designed by the application of a so-called cosine filtering method. According to the invention, there is provided an input stage for a CTD low-pass filter, comprising a semiconductor substrate, an insulating layer deposited upon the substrate, said substrate comprising at least two insulated channels, each channel receiving said input signal and comprising sampling means, said sampling means comprising a diode formed in said substrate and two sampling gates, arranged upon said insulating layer, the samples being constituted by electrical charges capable of moving through said substrate, the sampling performed in the first channel exhibiting a phase advance in relation to the sampling performed in the second channel, substantially equal to half the sampling period, said stage further comprising electrodes arranged on each of said channels after said sampling gates, in order to delay the sample formed in the first channel by a time substantially equal to said phase advance, said channels being united by a single summing electrode, extending simultaneously over the two channels and additing the samples formed in them .

Tone signal detecting circuit

Abstract: A tone signal detecting circuit for use in a paging receiver or the like employs an N-path filter in place of an active filter to facilitate circuit integration. The circuit includes a clock signal generating circuit responsive to a first control signal for selecting and reading out one of plural clock signals of a frequency equal to N (N being an integer equal to or greater than 2) times the frequency of the plural tone signals. The N-path filter has a prescribed rise and fall time and selects and responds to a tone signal in accordance with a clock signal from the clock signal generating circuit. A low-pass filter having prescribed frequency characteristics removes harmonic signals of the tone signals contained in the output of the N-path filter. A tone designating circuit responsive to a second control signal is connected to the low-pass filter and changes its frequency characteristics. A detector is connected to the output of the low-pass filter, and control circuits are responsive to the output of the detector to generate the first and second control signals.

Leaky-Wave Antenna and Band-Reject Filter for Millimeter-Wave Integrated Circuits

Abstract: Grating structures fabricated in inverted strip dielectric waveguide have for the first time been used as millimeter-wave leaky-wave antennas and band-reject filters. Experimental results agree reasonably well with theoretical predictions.

Dynamic transient response filter

Abstract: A filtering system for use in an FM modulating system. The filtering system provides frequency limiting of an input signal and does not overshoot amplitude limitations thereof. This filtering system includes a first filter and a second filter, separated by an overshoot compensator. The compensator modifies the output of the first filter in such a manner that, when the modified signal is filtered by the second filter, an output signal will be provided which does not overshoot a preselected amplitude. The overshoot compensator includes a threshold clipper which separates out any overshoot component generated by the first filter, an amplifier which amplifies the overshoot component, and a subtractor circuit which subtracts the amplified overshoot component from the output of the first filter to provide the modified signal. The second filter is of a type which provides a constant delay for all frequencies within the passband on the filtering system.

Simple electronically tunable 2nd-order active filter

Abstract: A new current-tunable 2nd-order active filter is described which is based on the variation of the transconductance of a bipolar transistor with bias current. The circuit can give a lowpass or bandpass 2nd-order response or can be cascaded to give a high-order all-pole tunable filter. Measurements have indicated that a tuning range of at least four decades is possible with a dynamic range of 75 dB.

Voice scrambler using syllabic masking

Abstract: Voice signals are combined with a masking signal that changes frequencies between discrete frequencies at a selected rate and in a random manner. The combined voice spectrum and masking signal is frequency translated by heterodyning the combined signals and a carrier signal, with the carrier signal varying between discrete frequencies at a selected rate that is faster than the selected rate of the masking signal. The scrambled output is coupled to a utilization means through a communication link such as telephone lines or radio frequency waves. The utilization means includes an unscrambler in which the scrambled input is heterodyned with a carrier signal that is synchronized with the carrier signal in the scrambler and has the same discrete frequencies and at the same time as the carrier signal in the scrambler. The combined voice spectrum and masking signal is thus unscrambled and is applied in series to a mixer, a notch filter and another mixer for removing the mask signal. The local oscillator signal for each mixer is the mask signal translated to a higher frequency and synchronized with the mask signal in the scrambler unit.

Electronically controllable filter

Abstract: An active filter, capable of being electronically controlled over a 50:1 or greater frequency range, while maintaining a fixed filter characteristic and a constant, nearzero DC offset voltage. The filter utilizes passive element-multipliers, either capacitor-multiplier or resistor-multiplier circuits, as the frequency determining networks, depending upon whether the filter is a low-pass or a high-pass filter. Tuning is accomplished by varying the gain of a variable gain amplifier connected in series with the capacitive or resistive element of the multiplier networks. Also disclosed is an implementation of the variable gain amplifier which permits the filter's cut-off frequency to be controlled digitally. The filter may be designed to have any desired type of filter characteristic. Several filter sections may be connected in cascade to provide any desired number of filter poles.

D.D.A. notch filter

Abstract: A filter, based on digital-differential-analyser-type elements, for mains-interference rejection is described.

Apparatus for extracting the fundamental frequency from a complex audio wave form

Abstract: Apparatus for extracting the fundamental frequency from a complex audio wave form and for producing a pulse representation of said fundamental frequency includes a low pass filter having a frequency response characteristic which tends to exclude harmonics together with a control circuit including amplifier means, peak detector means and voltage to current converter means for supplying a control signal to the low pass filter thereby to vary the cutoff frequency of the filter and to maintain substantially constant amplitude of the fundmental wave form together with voltage comparator means for comparing the peaks of an alternating current quantity dependent upon the output of said filter with a direct current quantity proportional to the output of the filter, said comparator means thereby producing a pulse per cycle of said fundamental frequency.

Variable bandwidth pass-band filter

Abstract: In a switched multipath filter network, the passband is varied by varying the duty cycle of the switches which commutate the constituent filters. When no filter is operative, the output connection of the filters is interrupted to prevent the unfiltered signal passing through, but the network output is smoothed to provide sample and hold action.