Showing papers on "Voltage-controlled filter published in 1973"

Adaptable notch filter

Abstract: The present invention modifies a 2-stage Biquad notch filter by adding an inverter at the input to each stage and by replacing certain of the Biquad resistors with a set of resistors in the filter circuit. The notch width, notch depth, and notch center frequency of this modified Biquad filter may now be varied independently among a set of discrete values.

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.

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.

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.

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.

The Design and Application of Digital Filters

Abstract: The advantages of digital circuits and the frequency-response design procedures that are available for specifying the coefficients of the digital-filter transfer function are discussed. The hardware design of a digital filter is discussed, and an example is given of the application of the filter in a servo loop. Further application of the filter to generate a notch network characteristic is illustrated.

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.

Active notch filter and dual mode filter/oscillator

Abstract: A high-Q active notch filter includes only one operational amplifier employing resistive positive feedback and reactive negative feedback. The positive feedback resistance can be selectively changed to convert the notch filter to an oscillator operating at the notch frequency.

Filter network having negative feedback loops

Abstract: A filter network having an input terminal and an output terminal and a plurality of cascaded filter sections, each section having a designated frequency response, the bandpass or lowpass characteristics of each section being calculated by a formula, and including a plurality of negative feedback loops, one for each filter section except the first, each loop being coupled between the output terminal of the respective filter section and the input terminal of the filter network through a summing means, the gain of each feedback loop being calculated by a formula.

Computerized design and tuning of active filters

Abstract: The basic problem associated with the design of active filters is the sensitivity of filter characteristics to variations in filter component values. Utilization of a digital computer and the pole sensitivity concept provides a technique for filter design which incorporates sensitivity and filter turning as an integral step in the design procedure.

A real-time Fourier-plane filter

Abstract: An electrically addressed liquid-crystal Fourier-plane filter capable of real-time optical image processing is described. The filter consists of two parts, a wedge filter having 40 9° segments and a ring filter having 20 concentric rings in a 1-in diameter active area. Transmission of the filter in the off (transparent) state exceeds 50 percent. By using polarizing optics, contrast as high as 104:1 can be achieved at voltages compatible with FET switching technology. A phenomenological model for the dynamic scattering will be presented for this special case. The filter is designed to be operated from a computer and is addressed by a 7-b binary word that includes an ON or OFF command and selects any one of the 20 rings or 20 wedge pairs. The overall system uses addressable latches so that once an element is in a specified state, it will remain there until a change of state command is received. The drive for the liquid crystal filter is ±30 V peak at 60 Hz. These parameters give a rise time for the scattering of 20 ms and a decay time of 80-100 ms. A complete description of the operational parameters of the device will be given and the results of several filtering experiments discussed.

Circuit for use in connection with electric measured values indicators, especially electromechanical scales

Abstract: A measuring device, such as a weighing scale emits a direct current voltage signal which is supplied through a filter to a receiver. The filter suppresses alternating current carried by the voltage signal and thereby introduces a delay in the voltage signal supplied to the receiver. To speed up the signal, the signal is differentiated and the resulting pulse is employed to close a switch which is in parallel with the filter thereby substantially eliminating the delay caused by the filter.

Audio multifrequency signal receiver

Abstract: A multifrequency receiver first detects any frequency combination AiBj corresponding to the frequency combination code which is used, then invalidates that detection or not according to whether the frequency combination received was mixed with other voice frequencies or not. At the rest state, the input signals are applied to both an A-band-pass filter and a B-band-pass filter, which are respectively followed by two limiters then by two sets of selective filters tuned on the particular code frequencies A1 to A4 and B1 to B3. As soon as a possibly valid frequency combination AiBj is detected, a band stop filter rejecting only frequencies of bands A and B is connected to the receiver input and possible noise sources are disconnected from the receiver input. The output signals from the A-band-pass filter and the band stop filter are added and the resulting signals enter the first limiter. Similarly, the output signals from the B-band-pass filter and the band stop filter are added and the resulting signals enter the second limiter. If the previously detected frequency combination was pure, the selective filter will detect it again and the first detection is validated. If not, the energy of signals Ai and Bj at the limiter outputs will not be high enough to activate the selective filters and the detection is not confirmed.

Pole frequency stabilized active rc filter

Abstract: In an active RC filter exhibiting a predetermined signal transfer characteristic, a reference signal is applied to the input of the filter and examined for phase shift at the low-pass signal output of the filter. A signal proportional to the variation in phase of the reference signal is used to alter a voltage controlled resistor, which in turn varies the pole frequency of the filter. The reference signal is cancelled from the signal appearing at the bandpass output of the filter to remove it from the final desired output of the filter.

Noise margins of bandpass filters

Abstract: It is shown that if a bandpass filter is obtained from a prototype low-pass filter by the usual low-pass to bandpass transformation, the white-noise margin and the (1/f)^2 noise margin of the resulting filter will be identical to the white-noise margin of the prototype low-pass filter. A method for using this general result for evaluating some types of integrals that arise in noise calculations is indicated.

A frequency-locked adaptive filter

Abstract: A frequency-locked RC active ladder filter is described, which is frequency-locked to a sinusoidal control tone. The tunable ladder filter is used as a frequency discriminator by using two of the filter branch voltages to generate a control voltage. The control voltage is then used to tune or frequency-scale the ladder filter transfer function to lock the filter to the frequency of the control tone. A simple linear frequency-signal control system is developed to describe the frequency-locked performance of the adaptive filter. The frequency-locked filter can be designed so that the sinusoidal control tone is rejected at the filter output, making it possible to adapt the filter transfer function during transmission of a message. A practical frequency-locked filter is given in which a frequency-locked third order elliptic lowpass filter is designed. The performance of the practical filter is analysed using a simple linear control system model.

Experimental Investigation into a Commutative Filter

Abstract: : A commutative filter is a bandpass filter capable of very high Q's, large range of resonant frequency (0 to 2 megahertz that can be electronically tuned), comb-filter frequency response, small bandwidth (independent of resonant frequency), and extremely small size (can be constructed of integrated circuits since no inductors are required). The purpose is to present the general theory and evaluation of a commutative filter, in which presentation the effects of 'leaky' capacitors and load resistance are considered. An eight-channel filter is built, and laboratory and theoretical results are compared for the transfer function, bandwidth, quality factor, and sensitivity of the commutative filter. The commutative filter is also investigated from the viewpoint of enhancement of signal-to-noise ratio.