Showing papers on "Butterworth filter published in 1990"

CMOS active filter design at very high frequencies

Abstract: A study of the limitations of active CMOS filters at high frequencies suggests automatic means to compensate imperfections in the filter response introduced by active devices. The effects of nonzero FET output conductance, limited frequency response and noise on the filter characteristics, and dynamic range are analyzed, particularly for filters with high Q components. These are used to demonstrate a 3- mu m CMOS realization of a fourth-order bandpass filter with a 250-kHz passband centered at 12.5 MHz. The filter demonstrates that the maximum frequency of filter operation is not as seriously limited by device f/sub T/ as was previously thought, but that automatic means may be used to tune out the imperfections introduced in the filter elements by the limited voltage gain and frequency response of transistors. >

Tunable bandpass filter system and filtering method

Abstract: A tunable bandpass filter system and filtering method wherein the bandwidth of a tunable bandpass filter is adjusted as a function of the fundamental frequency of an input signal. Either a constant bandwidth or a constant quality factor tunable bandpass filter may be used. The center frequency of the tunable bandpass filter is adjusted in direct proportion to the product of the fundamental frequency and the number of a selected harmonic thereof. In the case of a constant bandwidth filter the bandwidth is adjusted in direct proportion to the quotient of the fundamental frequency divided by a selected filter quality factor. In the case of a constant quality factor filter, the bandwidth is indirectly controlled by adjusting the center frequency while adjusting the quality factor in inverse proportion to the harmonic number. The output amplitude of the filter is measured to determine the amount of distortion in the input signal. The fundamental frequency may be determined by measuring the input signal.

Generalization of the linear matched filter concept to nonlinear matched filters

Abstract: A nonlinear matched filter based image correlator is investigated. The linear matched filter is expressed as a bandpass function containing the amplitude and phase of the Fourier transform of the reference signal. The bandpass filter function is then applied to a kth law nonlinear device to produce the nonlinear matched filter function. Analytical expressions for the nonlinear matched filter are provided. The effects of the nonlinear transfer characteristics on the correlation signals at the output plane are investigated. The correlation signals are determined in terms of the nonlinear characteristics used to transform the filter. We show that the nonlinear filter results in a sum of infinite harmonic terms. Each harmonic term is envelope modulated due to the nonlinear characteristics of the device, and phase modulated by m times the phase modulation of the linear filter function. The correct phase information of the filter is recovered for the first-order harmonic of the series. The envelope of each harmonic term is proportional to the kth power of the Fourier transform magnitude of the reference signal. We show that various types of filter such as the continuous phase-only filters can be produced simply by varying the severity of the nonlinearity. Nonlinear filters provide higher correlation peak intensity and a better defined correlation spot.

Digital decimation filter

Abstract: A narrow band digital decimation filter that can be implemented on a single integrated circuit has a high order decimation filter that receives input digital data at P sampling rate and produces output digital data at Q sampling rate, where P>Q The high order decimation filter can be a Hogenauer filter A finite impulse response (FIR) filter, containing only a single multiplier and only a single accumulator, receives the output from the high order decimation filter at the now lowered sampling rate, and cleans up the frequency response of the high order decimation filter to provide narrow band filtering

High frequency continuous time filter

Abstract: A high-frequency integrated circuit continuous time low pass filter. An on-chip oscillator is built into the filter. The filter frequency accuracy is established by trimming the frequency of the on-chip oscillator during wafer probe. The oscillator is off during normal operation of the filter. Therefore, the filter does not produce noise that will degrade the performance of the filter during normal operation. After trimming, the filter design is such that accuracy is maintained even during temperature and power supply changes. The adjustment can be made without the oscillator by direct measurement of the filter response.

Three-mirror fibre Fabry-Perot filters of optimal design

Abstract: A three-mirror, all-fibre, Fabry–Perot filter was built having a second-order near Butterworth bandpass response, which is characterised by a fiat top and steep skirts.

Filter system with controlled amplitude in stopband or passband

Abstract: A filter system which may be configured either as a stopband (notch) or passband filter having controlled gain and low ripple throughout its stopband or passband. The system includes a filter network which is operative for splitting an input signal spectrum into a pair of complimentary spectrum components and a coupler for proportionately combining these spectrum components in accordance with a prescribed gain factor. The filter network may either comprise a pair of complimentary band reject and band pass filters connected to a common input terminal or a diplexer including identical filters coupled between a splitter and a combiner.

A varactor-tuned, active microwave band-pass filter

Abstract: A microwave tunable high-Q active bandpass filter was developed using a varactor diode for tuning and a MESFET to provide negative resistance for increasing the tank circuit Q-value. Tuning ranges of 500 MHz for the one-pole filter and 430 MHz for the two-pole filter were achieved with a center frequency of 10 GHz. A 3-dB bandwidth of 20 MHz for the one-pole filter and 80 MHz for the two-pole filter was obtained. The passband insertion loss is typically 0+or-1 dB. >

Design and DSP-chip implementation of a novel bilinear-LDI digital Jaumann filter

Abstract: The design and implementation, on a digital signal processing (DSP) chip, of a novel high-quality recursive digital filter is presented. The proposed digital filter is derived from an equally resistively terminated lossless Jaumann two-port network by using the bilinear-LDI (bilinear lossless discrete integrator) design technique. It has the important practical property that all the inductor-based states can be computed simultaneously and all the capacitor-based states can be computed simultaneously, thereby permitting a fast parallel-processing speed which is virtually independent of the order of the filter. This digital filter can be made minimal in the number of multipliers, requiring n multipliers for the realization of lowpass and bandpass filters, and n+1 multipliers for the realization of highpass and bandstop filters, where n is the order of the continuous-time prototype reference filter. It is shown that when implemented using modern DSP chips, such a filter exhibits very high-quality performance characteristics. >

A very small MMIC variable filter based on a new active filter design concept

Abstract: A novel MMIC (monolithic microwave IC) active filter is proposed for realizing a small-size variable bandpass filter with high selectivity at microwave frequencies. A second-order bandpass filter based on the proposed design concept occupies a 1.2*1.3 mm/sup 3/ area on the GaAs substrate. It is controllable for both quality factor Q, varying from 20 to 200, and a center frequency wider than 100 MHz at the 1.3 GHz band. A fourth-order bandpass filter was also fabricated by cascading two of the filter chips together. This is expected to be a key device in the future development of frequency-variable equipment. The design concept, fabrication, and measured performance of the filter are described. >

A CMOS switched-current filter technique

Abstract: Basic design techniques and considerations for switched-current (SI) circuits are presented, and experimental results from integrated filters are given. By means of analogies to switched-capacitor circuits, a five-pole lowpass Chebyshev SI filter has been integrated in a 2- mu m N-well, double-metal CMOS technology. The average die area is about 200 mil/sup 2/ per SI pole. The current mirror gain factors were derived by means of signal flow-graph techniques starting with the RLC prototype. A doubly terminated five-pole Chebyshev filter was designed for a 0.1-dB ripple bandwidth of 5 kHz with a sampling frequency of 128 kHz. The measured response is shown. The noise floor is about 70 dB down with respect to the passband. A three-pole elliptic SI filter has also been integrated to illustrate the realization of transmission zeros with SI filter techniques. >

A realization of a GaAs FET microwave active filter

Abstract: A methodology for microwave active filter design has been presented by this author (see ibid., vol.37, no.9, p.1418-24, 1989). The present work serves to verify aspects of this design approach. A practical implementation of a network discussed in that work is given. First, an ideal preliminary design for a second-order bandpass filter is obtained. Next, the nonideal effects of the active devices and the lossy reactive elements are introduced, and then optimization is used to obtain the final design. Finally, the filter is constructed and tested. The measured response is found to agree quite closely with the design specifications. The filter section is constructed in hybrid form with lumped components, and uses two NEC 900100 GaAs FETs. The prototype accurately realizes a second-degree bandpass response with a center frequency of 6 GHz and a pole q of 2.5. >

The effects of IF bandpass mismatch errors on adaptive cancellation

Abstract: The effects of IF bandpass mismatch errors on adaptive cancellers are investigated. Frequency mismatch errors occur because of errors in the synthesis process of the bandpass filters which are designed to be identical and are in each input channel. Tapped-delay line transversal filters can be used to compensate for these frequency mismatches and thus improve cancellation performance. A pole/zero error model of the filters is developed whereby closed-form solutions of the maximum achievable average cancellation are obtained. This cancellation is a function of the order of the ideally matched frequency filters, the number of time-delay taps in the compensating transversal filter, the bandwidth-tapped time-delay product, and the constraints on these parameters. A design procedure is outlined for optimizing the canceller with respect to these parameters and their constraints; specifically, results are presented for Butterworth-type input filters. It is shown that an arbitrarily low output noise residue cannot be achieved by arbitrarily increasing the number of time-delay taps. >

CMOS VHF transconductance-C lowpass filter

Abstract: Experimental results of a VHF CMOS transconductance-C lowpass filter are described. The filter is built with transconductors as published earlier. The cutoff frequency can be tuned from 22 to 98 MHz and the measured filter response is very close to the ideal response

Digital filter with dynamically variable filter characteristics

Abstract: Filter coefficients corresponding to a plurality of discrete cutoff frequencies are stored in advance in a memory and a filter coefficient corresponding to a specified cutoff frequency is generated by performing an interpolation referring to the filter coefficients stored in the memory, thereby saving the capacity of the memory. When a resonance index is to be specified in addition to the cutoff frequency, filter coefficients corresponding to two difference resonance indexes are acquired by the interpolation, then a new filter coefficient is acquired by executing the interpolation referring to these two filter coefficients.

Pole-zero decision feedback equalization for the digital subscriber loop

Abstract: A decision feedback equalizer (DFE) containing a feedback filter with both poles and zeros is proposed for high-speed digital communications over the subscriber loop. The feedback filter is composed of two sections; a relatively short finite-impulse-response (FIR) filter that cancels the initial part of the channel impulse response, which may contain rapid variations due to bridge taps, and a pole-zero filter than cancels the smoothly decaying tail of the impulse response. It is shown that the feedback filter can be adapted using existing infinite-impulse-response (IIR) adaptation techniques. Numerical results using a simulated subscriber loop impulse response show that a feedback filter with two poles adapted via a simplified gradient algorithm can offer significant improvement in mean squared error (MSE) (i.e., 3 to 4 dB at a signal-to-noise ratio of 20 dB) relative to a conventional DFE of the same complexity. >

Integrated filter circuit

Abstract: An integrated filter (10) having a controllable frequency response includes a pair of cascaded differential transconductance amplifiers (12, 14) with the differential outputs of the first one of the pair being coupled to the differential inputs of the second one via coupling resistors (32, 36) and the differential inputs of the first one being coupled to inputs of the filter via input coupling resistors (20, 22). The differential outputs of the second transconductance amplifier are coupled both to outputs of the filter and are fed back (50, 52 and 54, 56) to the respective differential inputs of the two amplifiers to provide multiple inputs such that the damping factor of the filter is controllable for adjusting the filter frequency transfer function by varying the ratio of the coupling resistor value to the feedback resistor value while the damping factor is independent of the natural resonant frequency of the filter. Series coupled integrating capacitors (26, 28 or 38, 40) are coupled across the differential outputs of both pairs of amplifiers with the interconnection therebetween return to ground.

Linear phase low pass filter

Abstract: An electrical signal filter composed of a pair of cascaded filter sections in which the group delays of the two filter sections are matched so that their non-linear effects cancel in the passband, resulting in an overall linear phase. In addition, the attenuation slope after the first null in one filter section's stopband response is complemented by the other filter's stopband attenuation. This prevents the overall stopband attenuation from falling to extremely low values as with some filters, which, in turn, limits the time domain overshoot. The passband group delay and stopband attenuation characteristics are controlled by appropriate selection of the passband ripple and filter order number parameters of the filter. In a preferred embodiment, the two filter sections are of the elliptic and Chebyschev type. The invention can be implemented as analog-component active filter circuit or as a digital filter.

A pole sharing technique for linear phase switched-capacitor filter banks

Abstract: A pole-sharing technique was used for implementing bandpass filter banks using the nonminimum-phase Lerner approximation without placing additional speed requirements on the operational amplifiers and without resorting to complicated clocking strategies; only a biphasic clock was used. Substantial hardware saving has been achieved. The filter synthesis is shown to be micropower-compatible and parasitic-insensitive; phase reversal was easily accommodated while retaining all desirable features. A uniformly and nonuniformly spaced filter bank for typical speech applications have been simulated with SWITCAP and three channels of the uniformly spaced filter bank have been constructed on a breadboard using discrete components. The results obtained agreed with design specifications, yielding a good approximation to linear phase in the passband and sharp attenuation in the stopband. Single-parameter and statistical multiparameter sensitivity analyses are presented. >

Logarithmic filter banks

Abstract: A technique for designing and approximating digital filter banks whose center frequencies are spaced logarithmically is provided. The resonator with the purely imaginary transfer function on the unit circle is treated as the basic element of the bank, and the frequency interpolating filter bank structure is applied. Different pole-zero structures are studied, and an infinite filter bank is explored from the theoretical point of view. The connection to a variant of least-mean-square adaptive filters is described. The results show that these filter banks have good frequency response and allow control of crossover gain, and they are useful for voice and music analysis and tone receivers. >

Preparing several TV signals for transmission - involves frequency conversion and multiplexing with additional digital filter preceding channel bandpass filter with same clock rate

Abstract: A conventional digital frequency converter consists of an anti-aliasing analog filter (AAF), a digitiser (A/D), a channel band pass filter (CHBF), a digital modulator, a complex interpolation filter raising the sampling rate by a given factor (L), and an adder (+) of corresp. outputs from other branches of the circuit. The adder inserts a digital filter (RBF) having real coeffts., scanned at the same rate (fA) with at least half of its coeffts. equal to zero. USE/ADVANTAGE - For broadband cable TV distribution networks, with complexity of the analog portion greatly reduced.

Non-resonating noise filter

Abstract: It is noted that a commercially available noise filter does not always represent the attenuation characteristics shown in the catalog, but sometimes magnifies the noise voltage when the filter is applied between the AC power cable and the equipment. This degradation mechanism is clarified here by considering the resonance phenomena involved in the input/output impedances and their phase angles, taking the circuits connected to the filter input/output into consideration. The filter has been realized on the basis of a non-resonating filter comprising a high-loss inductor and capacitor. The no-degradation attenuation characteristics of the filter can be improved by realizing a dull series resonance point and a small phase angle in use of high loss inductor and capacitor. >

Implementation Of Non-linear Filters For Iterative Penalized Maximum Likelihood Image Reconstruction

Abstract: Six low-pass linear filters applied in frequency space were implemented for iterative penalized maximum-likelihood (ML) single photon emission computed tomography (SPECT) image reconstruction. The filters implemented were the Shepp-Logan filter, the Butterworth filter, the Gaussian filter, the Hann filter, the Parzen filter, and the Lagrange filter. The low-pass filtering was applied in frequency space to projection data for the initial estimate and to the difference of projection data and reprojected data for higher-order approximations. The projection data were acquired experimentally from a chest phantom consisting of nonuniform attenuating media. All the filters could effectively remove the noise and edge artifacts associated with the ML approach if the frequency cutoff was properly chosen. The improved performance of the Parzen and Lagrange filters relative to the others was observed. The best image. by viewing its profiles in terms of noise-smoothing, edge-sharpening, and contrast, was obtained by Parzen filter. However, the Lagrange filter has the potential to consider the characteristics of the detector response function. >

Method of synthesising sub-band filters with quantised coefficients and filter structures obtained by this method

Abstract: The method consists in determining (28), by iteration, the quantised coefficients k of the filter by an approximation of the ideal response of the filter sought by a filter with transfer function H0, in calculating (19) the frequency response of the filter H0 determined by the coefficients of each iteration, in comparing the frequency response obtained at each iteration to that of the ideal filter sought, and in keeping the coefficients of the iteration for which the error U(k) between the frequency response obtained and that of the ideal filter is a minimum. Application: compatible coding of television images.

Failsafe bandpass filter/decoder

Abstract: A failsafe bandpass filter/decoder system comprising: an active bandpass filter which comprises a high-pass filter section having selected resistors which can only decrease in value during a failure mode and, in series with the high-pass filter section, a low-pass filter section having selected resistors which can only increase in value during a failure mode, whereby under any failure mode, the pass band of the filter will be reduced; and a level detector having an upper threshold voltage trip point and a lower threshold voltage trip point, both trip points having to be exceeded by the input wave form to produce a proper (square wave) output.

A 4-40 GHz wide bandwidth, magnetically tuned bandpass filter

Abstract: In recent years, as microwave test equipment evolved, the need for a coaxial input-output bandpass filter that can tune from 4 to 40 GHz has become evident. This filter also needs a passband much wider than that of any of the previously available yttrium-iron-garnet (YIG) filters. To meet these needs, a special non-YIG ferrite filter was designed and built. A discussion is presented of the key problems and solutions in the development of this new microwave filter. It is shown that a 4-40-GHz bandpass filter can be realized if the tuning magnet uses conical poles and the filter's coupling structure can be made small enough to prevent self-resonance from occurring below 40 GHz. Additionally, the passband bandwidth can be made greater than 100 MHz by using lithium-aluminum-ferrite (LAF) resonators instead of YIG. >

Finite-gain-insensitive circulating-delay type pseudo-N-path filter

Abstract: A novel circulating-delay switched-capacitor (SC) pseudo-N-path (PNP) filter is described which has low sensitivity to finite amplifier gain. The circuit exploits the narrowband characteristic of the PNP filter to predict its output voltage a number of cycles in advance and hence give accurate error compensation.

Tchebyscheff time-delay dielectric band-pass filter using Q control method of normal modes

Abstract: A Chebyshev time-delay three-pole bandpass filter has been developed in the 900-MHz band by using the Q control method of normal modes. The group delay ripple of the filter is 50 ns within the 300-kHz bandwidth. The size of the equipment is 482.0-mm wide*300.0-mm deep*132.6-mm high, and it can be put into a 19-in standard rack. This filter is expected to be applicable for channel filters used in digital cellular base stations. >