Showing papers on "Low-pass filter published in 1973"

A unified approach to the design of optimum FIR linear-phase digital filters

Abstract: A method for designing finite-duration impulse-response (FIR) linear-phase digital filters is presented in which the four possible cases for such filters are treated in a unified approach. It is shown how to reduce each case to the proper form so that the Remez exchange algorithm can be used to compute the best approximation to the desired frequency response. The result is that a very flexible and fast technique is available for FIR linear-phase filter design.

Practical design rules for optimum finite impulse response low-pass digital filters

Abstract: Although a great deal is known about design techniques for optimum (in a minimax error sense) finite impulse response (FIR) low-pass digital filters, there have not been established any practical design rules for such filters. Thus, a user is unable to easily decide on the (approximate or exact) filter order required to meet his design specifications and must resort to tables or trial and error procedures. In this paper, such a set of design rules is given. In the case of very narrow bandwidth or very wide bandwidth filters, analytic relations between the filter parameters can be readily obtained. In all other cases, exceedingly good linear and nonlinear fits to the data can be obtained over somewhat restricted ranges of the parameters. These fitting procedures lead to a practical set of simple design rules for estimating filter order from the desired specifications.

Analysis of quantization errors in the direct form for finite impulse response digital filters

Abstract: An analysis of the three possible types of quantization effects in the direct form realization of finite impulse response (FIR) digital filters is presented. These quantization effects include roundoff noise, A-D noise, and filter frequency response errors due to coefficient quantization. Since the analysis of roundoff noise and A-D noise for the direct form is straightforward, this paper concentrates on an analysis of the effects of quantized coefficients on the resulting filter frequency response. Based on this analysis, statistical bounds on the error incurred in the frequency response of a filter due to coefficient quantization are developed and verified by extensive experimental data. Using these bounds, a procedure for applying known techniques for FIR filter design to the design of filters with finite word length coefficients is presented. On the whole, the direct form is shown to be a very attractive structure for realizing FIR filters.

Approximate design relationships for low-pass FIR digital filters

Abstract: In this paper, a set of simple, approximate relationships between FIR, linear phase, low-pass filter parameters is presented. Based on these relationships, it is shown how an existing, readily available, filter design program can be used to efficiently design low-pass filters that meet or exceed arbitrary input specifications.

Nth-root Stack Nonlinear Multichannel Filter

Abstract: A nonlinear multichannel filter is developed which appears to be particularly useful for enhancement of seismic refraction and teleseismic array data The basic filter involves the extraction of the Nth root of each element in the matrix forming the data set, where N is any positive integer, and the Nth power of the summation over the channels The filter is effective in reducing random noise, whereas identical signals which are in-phase on all channels are retained at the expense of some distortion The output from this nonlinear filter has far greater resolution in specifying phase velocity than any multichannel linear filter we have employed Examples of theoretical and actual field seismograms are presented after various forms of filtering to illustrate their effectiveness

Surge indicator for turbine engines

Abstract: A pair of pressure transducers, one having a high response capability and the other a low response capability, are positioned in a turbofan or turbojet engine flow passage. The output from the high response transducer is passed through a notch filter to isolate the frequencies of interest. The output from the low response transducer is passed through a low pass filter to provide the absolute steady-state pressure level. The ratio of the high frequency pressure signal within the frequency range of interest to the steady-state pressure signal is calculated and compared to a reference ratio signal. If the calculated pressure ratio signal exceeds the reference ratio signal, the engine is approaching a surge condition, and a warning signal is generated which can be used for rescheduling of the engine away from stall. The apparatus may be used both to anticipate surge and to sense the actual occurrence of a surge condition. The invention herein described was made in the course of or under a contract with the Department of the Air Force.

A Time-Domain Analysis of the Power Factor for a Rectifier Filter System with Over- and Subcritical Inductance

Abstract: A simple switching network in form of a rectifier filter is analyzed with application of time-domain methods. Exact expressions are derived for currents in the filter network and the ac supply line as functions of the size of a series inductor in the L-type low pass filter connected to a resistive load. The power factor as viewed from the ac supply line is derived from the time-domain expressions for conditions of continuous and discontinuous current. A current form factor is introduced in form of the ratio irms/iav to provide a measure for the added burdens that are imposed on the circuit components due to current deformations inside the switching network. Normalized numerical results for average and rms currents, their form factor, output voltage, and power factor are presented as functions of the ratio of the series inductance L and the critical inductance Lc in form of graphs. The presented treatment of the problem provides insights and information which would require cumbersome work if approached with the conventional methods of frequency domain analysis. Rectifier-filer systems with series inductances L

Color television camera optical system

Abstract: An optical system in a single or double tube color television camera for minimizing color shading, moire patterns and color beats includes along the image optical axis a phase grating low pass filter, a color encoding filter and a lenticular lens member oriented relative to each other and the image scanning to minimize interference between the color encoding filter and striped patterns in the viewed object. The color encoding filter is striped, the low pass filter is either striped or has regularly longitudinally and transversely spaced phase retarding projections and the lenticular lens member includes transversely spaced longitudinal cylindrical lenses or regularly transversely and longitudinally spaced convex projections.

Thrust and flight path control decoupling system

Abstract: Signals representative of the acceleration of an aircraft due to the change in flight path angle are coupled through a high pass filter (washout circuit) to obtain a short term compensation signal (Vcomp) which signal is further coupled to a low pass filter which provides short term integration to obtain the compensation signal ( Delta Vcomp) representative of change in speed in response to change in flight path angle. These compensation signals (Vcomp) and ( Delta Vcomp) are summed respectively with signals representative of longitudinal acceleration (or thrust control damping) and air speed error (or thrust control reference) to inhibit short term thrust control of the aircraft due to small flight path angle perturbations thereby improving the manual path control handling quality and the stability of the aircraft by the automatic control system. A modification of the system is disclosed for avoiding sensor noise developed by the Alpha (angle of attack) vane sensor by utilization of other inputs to the system signal processing.

Color selective low pass filter

Abstract: A low pass filter adapted to be used in the taking lens system of a color television camera is provided with thin transparent film stripes of a fine pitch which have the effect of causing red and blue light of high spatial frequency passing therethrough to interfere and be eliminated and allowing only green light to pass therethrough from the low to high spatial frequency.

Waveguide filter for microwave heating apparatus

Abstract: A low pass filter is provided for rectangular waveguide of the type employed for launching electromagnetic energy in microwave ovens. The filter substantially rejects the transmission of harmonic frequencies of the fundamental generator operating frequency as well as adjacent energy oscillation modes. The structure described has a corrugated configuration with ribs defining alternate cavities and constrictions have cutoff frequency characteristics. Uniform and nonuniform spacing as well as dissimilar sections are discussed. The structure is electrically isolated from the broad waveguide walls to define two propagation paths for the electromagnetic energy. Lightweight conductive materials are suggested. The interrelationships of the alternating rib members provide a series of open and short circuit admittances which are analyzed to determine the desired electrical cutoff frequencies. Identical rib structure is defined on opposing sides of a reference plane extending along the longitudinal axis of the waveguide. Transforming means are utilized for impedance matching.

Adjustable electronic tunable filter with simulated inductor

Abstract: A stable tunable Q bandpass filter adapted to have its parameters dynamically varied over a wide range of bandwidths and center frequencies. The filter includes a capacitor and a network coupled in parallel across the capacitor. The network includes a plurality of operational amplifiers coupled to simulate an inductor without utilizing coils. A feature of the filter is that it can be readily made adaptive to track an input signal which may randomly vary in frequency. A further feature of the invention is to have one input of each operational amplifier coupled directly to ground to minimize stray capacitance. The performance characteristics of this filter are in part attained by the method of component layout and shielding techniques used, in addition to the method of circuit excitation.

Least pth optimization of recursive digital filters

Abstract: The application of the Bandler-Charalambous method using extremely large values of p, typically 10,000 to the problem of choosing the coefficients of a recursive digital filter to meet arbitrary specifications on the magnitude characteristics, is described. The Fletcher (1970) method is used in conjunction with least pth optimization and is compared with the well-known Fletcher-Powell method. Some relevant design ideas, such as local optimality checking by perturbation, increasing the order complexity of the filter through growing filter sections, and meeting the stability requirements by using a pole inversion technique, have been implemented. A general description of a computer program package that uses these ideas, along with some illustrative examples are given.

Filter having frequency-dependent transmission properties for electric analog signals

Abstract: In one illustrative embodiment, a basic filter circuit for discrete signals is provided corresponding in filter function to an analog ladder network containing inductive and capacitive components. The reactive components of the analog filter structure are realized in the basic filter circuit as one-port and two-port elements having time delay, while non-reactive components of the analog filter are realized as time-delay-free one-port and plural-port elements. Interface means connect the port elements together in a configuration corresponding to the configuration of the analog filter, with sets of adder and multiplier means of the interface circuitry which couple successive sets of the port elements to each other being correlated with the connective relationship (whether series, series-parallel or parallel) between the corresponding components or branches of the analog filter and with the parameters of the analog components, so that the basic filter circuit essentially simulates the analog filter structure in its filter characteristics.

The predictability of certain optimum finite-impulse-response digital filters

Abstract: Some of the properties of optimal solutions to the finite-impulse-response low-pass filter design problem are discussed. These solutions are optimum in the sense of discrete Chebyshev approximation over a union of closed compact sets, i.e., the error of approximation exhibits at least (N + 3)/2 alternations (of equal amplitude) over the frequency ranges of interest, where N is the duration of the filter impulse response in samples. It has been shown that, in certain special cases, the solution can exhibit (N + 5)/2 alternations of equal amplitude. These solutions have been called extraripple filters because of the extra alternation that is present. How these extraripple solutions can, within bounds, be scaled to yield additional solutions, which are still optimal over new frequency ranges, is shown.' Thus an infinite number of optimal low-pass filters may be obtained directly from a finite number of extraripple solutions. An interpretation of the various types of optimal filters, in terms of locations of the zeros of the z - transform polynomial, is also given.

Harmonic and power factor compensation means and method for power systems controlled by a non-linear device

Abstract: The fundamental frequency power factor is improved and a practical reduction in significant harmonic current is made in electrical power systems where the power to the load is controlled by a non-linear device, such as a saturable reactor, by using series inductive-capacitive filters to filter the significant harmonic current to act as a shunt for harmonic current in conjunction with a supply line inductive reactance. The resonant frequency of each filter is less than the frequency of the harmonic current and the impedance of each said inductivecapacitive filter is a net inductive reactance. The capacitive element of the filter provides a leading power factor component to compensate for the lagging power factor component imposed on the supply system by the non-linear control device.

On the transition width of finite impulse-response digital filters

Abstract: Several properties of finite-duration impulse-response (FIR) digital filters designed to have the maximum possible number of ripples are discussed and illustrated with examples. Such filters have been called extraripple filters. Among the properties of such filters are as follows. 1) Extraripple low-pass filters with fixed passband ripple δ 1 and stopband ripple δ 2 achieve the local minimum of transition width in the class of linear phase filters with fixed impulse-response duration of N samples. 2) For the case δ 1 = δ 2 the minimum transition width is roughly independent of F p , the passband cutoff frequency. 3) For the case δ 2 1 , the minimum transition width decreases with increasing bandwidth. Several figures are included to show the relation between the transition width and bandwidth for low-pass filters.

Dynamic ground detector

Abstract: A dynamic ground detector for monitoring a two-wire power distribution system, where both sides are nominally isolated from ground. A sampling bridge is connected between the lines, each shunt section of the bridge comprising a resistor and a capacitor having impedances in quadriture at the line frequency; and the junctions of the shunt sections of the sampling bridge are alternatively sampled by a switching network. The switching network connected through a phase shifting network comprising an RC filter to a detector network comprising a fullwave rectifier with suitable indicator and alarm apparatus. The rate of sampling is a multiple, including unity, of the line frequency. Additional circuitry may also be provided for sensing the direction and phase of any fault current caused by a fault to ground from either or both of the lines, so that the fault current may be opposed by insertion of another current having the same phase and opposite direction.

Single sideband system for digitally processing a given number of channel signals

Abstract: A single sideband system for digitally processing a given number of analog channel signals, provided with: a digital filter to which filter coefficients are applied which characterized a lowpass filter having a cut-off frequency which is equal to half the bandwidth of the channel signals; a fast fourier transformer to which a number of carrier signal functions is applied, which number is at least equal to twice the number of channel signals and each of which represents a carrier frequency, each frequency being an even multiple of the cut-off frequency of the lowpass filter.

Finite impulse response digital filter with reduced storage

Abstract: An algorithm and instrumentation for a digital transversal filter utilizing a finite impulse response wherein storage is reduced by storing partial results of the convolution process for a number of output samples rather than storing input data.

Frequency shift keyed demodulator

Abstract: A frequency shift keyed (FSK) modem (modulator and/or demodulator) utilizing a phase lock loop which modem may operate at any of several predetermined frequencies and whose output for the modulator section, is combined with a frequency shift keyed signal and the result passed through a low pass filter to provide a frequency shift keyed output. The phase lock loop also supplies signals to the receive or demodulation section. The use of a multiple frequency phase lock loop negates the requirements for a plurality of bandpass filters of different frequencies to be used with the frequency shift keyed source.

Voltage-controlled filter

Abstract: An audio frequency filter having voltage controlled transfer functions providing one or more of low pass, bandpass and high pass characteristics, selective in terms of cut-off frequencies and center frequency, including in cascade a first operational amplifier, a first multiplier responsive to a multiplicative function, a second operational amplifier having a frequency selective transfer function, a second multiplier responsive to said multiplicative function and a third operational amplifier having a frequency selective transfer function, in which negative feedback is provided between the outputs of the second and third operational amplifiers and said first operational amplifier.

Selectable frequency bandpass filter

Abstract: A selectable frequency bandpass filter includes a set of electrical filters with each of which is associated a switch. By closing appropriate switches a subset of the set of electrical filters is connected in parallel between the input and the output of the selectable frequency filter. Each of the electrical filters has a frequency response which consists of a number of very narrow passbands separated by frequency regions of effectively zero transmission. By connecting the appropriate subset of electrical filters in parallel between the input and the output of the selectable frequency filter the overall frequency response is caused to have a dominant very narrow passband at one of a set of predetermined discrete frequencies. By varying the subset of electrical filters this dominant passband can be caused to occur at any of the set of predetermined frequencies. In a preferred embodiment each of the electrical filters is a properly designed surface wave delay line device.

Wheel brake antilock system with noise signal rejection circuit

Abstract: To prevent spurious response due to spurious signals in the electrical system of a vehicle, a group of threshold switches responding to different wheel speed rates of change are provided; in advance of a differentiator, a first low pass filter of a band pass range of about 45 Hz is connected, the output of the differentiator having a second low pass filter of a band pass range of about 18 Hz connected thereto, the threshold switches being connected either to the differentiator directly or behind the second low pass filter. Some of the threshold switches preferably have threshold shift inputs which are energized over time delay circuits to shift the threshold levels, after having responded, to prevent extended bleeding of pressurized brake fluid, for responsiveness to changes in road condition and even response of systems associated with different wheel axles.

Electronic musical instrument with plural channels providing different phase shift

Abstract: The invention disclosed provides an electric musical instrument having an audio frequency signal source, an output from which the audio frequency signal passes to a plurality of parallel transmitting channels each of which comprises a delay line series circuit, a lowpass filter, an amplifier and loud speaker. Switching oscillators are disposed to provide 2 gate pulse trains, one running in opposite phase with regard the other, to the delay line series curcuit. Low frequency generators are disposed for generating a sine shaped sub-audio frequency signal to one of the switching oscillators while remaining units of the switching oscillators are connected to phase shifting networks. A second output from the audio frequency signal source connects to the amplifier of the last of the plurality of transmitting channels. The resultant combination of elements produces an ensemble effect.

On nonlinear filters involving transformation of the time variable

Abstract: A new type of nonlinear filter, called the E -filter, is introduced that involves a transformation of the independent variable of the input function. It is shown how an E -filter can be designed to filter out superimposed "noise" on a signal, leaving the large peaks of the signal unattenuated. Unlike a Iow-pass linear filter, the low-pass E -filter is almost frequency independent and so does not affect the amplitudes of large sharp peaks of the signal. It is shown that the E -filter can be realized in real time and that a wide class of E -filters have a filtering action which is independent of the dc level of the input signal.

Voltage controlled non-linear filter

Abstract: A voltage controlled non-linear filter for high and low frequency signals comprises means for separating and dissipating the high frequency out-of-band energy and passing the low freqeuncy in-band energy. The filtering in a low-pass filter is achieved without phase shift both in- and out-of-band by use of conjugate high- and low-pass filters located before and after the dissipating means, respectively. A hard clipper is used to perform the dissipating function, and its reference clipping level is provided so that the maximum amplitude of the in-band signal arriving at the input to the clipper is slightly less than the clipping level. The output of the low-pass filter may be fed back to set the clipping level so that this level can track the in-band signal amplitude. The clipping level may also be controlled by a bias control voltage. A sensing resistor between the clipper and the means to set the clipping level senses the high frequency information and thus, the non-linear filter can serve as a high-pass filter with no phase shift when only the low-pass filter is used after the dissipating means.

Voltage-controlled low-pass filter

Abstract: An active low-pass filter whose cutoff frequency is a function of the input voltage to the filter comprising a series connection from the input to the filter to ground which consists of: (1) an input resistor r, (2) a capacitor C, and (3) a grounded resistor R. A series connection across the capacitor C comprises: (a) a feedback amplifier having two input leads, one of which is grounded, the other lead being connected to that side of the capacitor nearest ground; and (b) a feedback resistor Rho , one end being connected to the output of the feedback amplifier, the other end being connected to the ungrounded side of the capacitor. The feedback amplifier may be an operational amplifier. The output of the filter is at the junction of the resistors r and Rho and the capacitor C.

Waveform averaging rc circuit

Abstract: A waveform averaging circuit composed of resistance, capacitance and diodes yields an output which is similar to an inductor-capacitor filter in rectified outputs of a transistor switching regulator. The LC filter is employed in a load output while the RC filter is employed in a feedback sense circuit for control of the regulator. The RC circuit provides a cost and weight reduction and provides less phase shift than would result from the employment of an LC filter in its stead.

Band pass filter and phase control circuit eliminating phase differences between input and output

Abstract: A band pass filter circuit for a band pass filter comprises a phase detector connected to the band pass filter for determining the difference in phase between input and output signals of the band pass filter A voltage controlled oscillator coupled to the phase detector has a frequency of oscillation controlled by detection output signals of the phase detector A first frequency converter is connected to the input of the band pass filter Input signals are supplied to the first frequency converter A second frequency converter is connected to the output of the band pass filter and to the voltage controlled oscillator and provides an output signal A phase control circuit connected between the voltage controlled oscillator and the first frequency converter varies the phase of output signals of the voltage controlled oscillator and controls the frequency conversion of the first frequency converter in accordance therewith