Showing papers on "Band-stop filter published in 2001"

High-input impedance voltage-mode universal biquadratic filter using three plus-type CCIIs

Abstract: A new three-input single-output voltage-mode universal biquadratic filter with high-input impedance using only three plus-type second-generation current conveyors (CCIIs) is presented. The proposed configuration uses only two capacitors and two resistors and can realize all the standard filter functions, that is, high-pass, bandpass, low-pass, notch, and allpass filters without changing the passive elements. The proposed circuit has no requirements for component matching conditions. The use of only plus-type CCIIs simplifies the configuration.

Interference detection, identification, extraction and reporting

Abstract: An adaptive notch filter (ANF) module selectively filters a received wideband communication signal to eliminate narrowband interference that lies within the frequency spectrum of the wideband communication signal. To determine the presence of narrowband interference, the ANF module scans various known narrowband channels that lie within the frequency spectrum of the wideband communication signal and determines signal strengths for each of the narrowband channels. The signal strengths from the narrowband channels are compared to a threshold that is derived from the narrowband signal strengths. Narrowband channels having signal strengths that are greater than the threshold are determined to have interference.

Method and apparatus for interleaved optical single sideband modulation

Abstract: An optical carrier notch filter includes an optical coupler with at least first, second and third ports. The first port is configured to receive an output that includes an optical carrier and interleaved optical single sideband signals. An optical bandpass filter is coupled to a port of the optical coupler. The optical bandpass filter separates the output into a transmitted signal that contains the optical carrier, and a reflected signal that includes the interleaved optical single sideband signals. The reflected signal is reflected from the optical bandpass filter to the third port of the optical coupler.

Devices based on surface plasmon interference filters

Abstract: Devices based on surface plasmon filters having at least one metal-dielectric interface to support surface plasmon waves. A multi-layer-coupled surface plasmon notch filter is provided to have more than two symmetric metal-dielectric interfaces coupled with one another to produce a transmission spectral window with desired spectral profile and bandwidth. Such notch filters can form various color filtering devices for color flat panel displays.

Stable IIR notch filter design with optimal pole placement

Abstract: This paper presents a two-stage approach for designing an infinite impulse response (IIR) notch filter. First, the numerator of the transfer function of the IIR notch filter is obtained by placing the zeros at the prescribed notch frequencies. Then, the denominator polynomial is determined by using an iterative scheme in which the optimal pole placements are found by solving a standard quadratic programming problem. For stability, the pole radius in the single notch filter design is specified by the designer, and in the multiple notch filter design, the pole radius is constrained by using the implications of Rouche's theorem. Examples are included to illustrate the effectiveness of the proposed techniques.

Steady-state analysis of a plain gradient algorithm for a second-order adaptive IIR notch filter with constrained poles and zeros

Abstract: Gradient-type algorithms for adaptive infinite-impulse response (IIR) notch filters are very attractive in terms of performance and computation cost. However, it is generally quite difficult to assess their performances analytically. There are several trials to analyze adaptive algorithms, such as the sign and the plain gradient algorithms for some types of adaptive IIR notch filters, but the analysis techniques used cannot be directly applied to different types of adaptive IIR notch filters. This brief presents closed form expressions for steady-state estimation bias and mean square error (MSE) of a well known plain gradient (LMS-like) second-order adaptive IIR notch filter with constrained poles and zeros. First, theoretical expressions for output signals of the notch filter and its corresponding gradient filter at their steady states are developed based on the Taylor series expansions of transfer functions of these two filters in the vicinity of the sinusoidal signal frequency difference equations for convergences in the mean and mean square are then established by using these output signals, from which the steady-state bias and MSE of the algorithm are derived. Stability bound of the algorithm is also investigated based on the difference equations. Extensive simulations are provided to support the analytical findings.

Efficient FIR filter for high-speed communication

Abstract: A method for reducing a propagation delay of a digital filter. The digital filter has an input path and an output path and includes a set of delay elements and a number of taps. The taps couples the input path to the output path. Each of the taps includes a coefficient, a multiplier and an adder. Each of the delay elements is disposed between two adjacent taps. The delay elements are placed in both the input path and the output path of the digital filter, such that the digital filter has fewer delay elements in the input path than a direct-form digital filter having the same number of taps in a direct-form structure and has fewer delay elements in the output path than a transposed-form digital filter having the same number of taps in a transposed-form structure, and such that the digital filter has same transfer function as the direct-form digital filter and the transposed-form digital filter.

Swept performance monitor for measuring and correcting RF power amplifier distortion

Abstract: RF power amplifier distortion is measured in the presence of multi-frequency input signals, by sweeping a local oscillator to tune RF input and output receivers. When the power detected by the input receiver exceeds a carrier energy threshold, the operation of a predistortion processor is blanked. This creates an adaptive notch filter, which allows for the direct measurement of low level distortion power in the presence of high power carriers. The unnecessary complexity of using controllably interrupted high isolation switches in the signal flow path of the output receiver in certain applications may be effectively obviated by buffer amplifier—passband filter stages. These buffer-filter stages provide additional gain to offset the fact that the signal level extracted from the output amplifier is very low, and prevent producing IMDs in the swept receiver's mixer. They are preferably implemented of the same bandpass filter in the swept input receiver.

Passband filter having an asymmetrical filter response

Abstract: A filter, such as a transmit filter of a duplexer, includes an array of acoustic resonators that cooperate to establish an asymmetrically shaped filter response over a target frequency passband. The acoustic resonators are preferably film bulk acoustic resonators (FBARs). The filter response defines an insertion loss profile in which a minimum insertion loss within the target passband is located at or near a first end of the frequency passband, while the maximum insertion loss is located at or near the opposite end of the frequency passband. In the transmit filter embodiment, the minimum insertion loss is at or near the high frequency end of the filter response, which is tailored by selectively locating poles and zeros of the array of FBARs.

Electrically tunable notch filters

Abstract: This invention provides a notch filter including a main transmission line, a coupling mechanism, and at least one electrically tunable resonator coupled to the transmission line through the coupling mechanism. A tunable dielectric varactor or a microelectromechanical variable capacitor is provided in each of the resonators. Wireless telephone handsets that include the filter are also included.

Broad-band tunable microwave transversal notch filter based on tunable uniform fiber Bragg gratings as slicing filters

Abstract: We demonstrate a multigigahertz tunable transversal notch filter based on two uniform fiber Bragg gratings as slicing elements of a broad-band optical source. The tunability of the filter is achieved by wavelength tuning of the fiber gratings to adjust the basic time delay provided by a dispersive element.

Tunable optical filter

Abstract: A tunable optical notch filter employing a Fabry-Perot etalon has a first partially reflective mirror and a second mirror with variable effective reflectivity Both the gap of the etalon and the effective reflectivity of the second mirror can be controlled, eg by TAB actuators, enabling a control of the central wavelength and the depth (loss) of the notch of the spectral response of the filter

Removing power line noise from recorded EMG

Abstract: Three methods for offline removal of power line interference (hum) from electromyograms (EMGs) were compared using both simulated and recorded EMG signals. The first method was a simple recursive digital notch filter. In the second method (Regression-Subtraction), the amplitude and phase of the interference were estimated by regressing sine and cosine functions onto a 'quiet period' before the start of the muscular contraction. A sinusoid with this frequency, magnitude and phase was then subtracted from the entire length of the signal. In the third method (Spectrum Interpolation), it was assumed that the magnitude of the original component of the signal at the frequency of the interference can be approximated by interpolating between the adjacent frequency bins in the power spectrum. While Regression-Subtraction was found to give the highest SNR for the output signal under ideal conditions, Spectrum Interpolation was found to be comparable if the phase of the interference was not constant and superior if the interference contained strong harmonic components.

Methods and apparatus for tuning pre-selection filters in radio receivers

Abstract: A tuning arrangement in a radio receiver includes a front-end circuit having a tunable band-pass filter that is capable of tunably selecting channels within at least one frequency band of an RF signal. A noise source coupled to an input of the tunable band-pass filter introduces a wide-band noise signal into the front-end circuit. A signal detector coupled to an output of the front-end circuit measures at least one signal power associated with a filtered noise signal derived from the wide-band noise signal. A tuning controller, coupled to the tunable band-pass filter by a tuning control signal, adjusts the tuning control signal in response to the at least one measured signal power for tuning the tunable band-pass filter to a desired filter response.

Harmonic filtering of high-power 12-pulse rectifier loads with a selective hybrid filter system

Abstract: Current distortion of 12-pulse rectifier loads is significantly lower compared to six-pulse rectifier loads. However, in passive filtering of the lowest and dominant characteristic 11th and 13th harmonics, the use of 5th and 7th filters is often required in order to prevent possible parallel and series resonance between the passive filter and source impedance which can be excited by source background distortion or by load current residual noncharacteristic harmonics at the 5th and 7th harmonic frequencies. In hybrid filter systems, an active filter (AF) can be added in series with the passive filter in order to isolate the source and load. In most proposed hybrid filter systems, AF control is based on the detection of total current distortion and high-frequency inverters. With a selective AF control system and voltage-controlled inverter, the AF can be controlled to isolate the load at the critical frequencies only while at all other frequencies the passive filter function is preserved so that lower switching frequency and AF rating is required. In this paper, the authors present a selective AF filter control system and simple hybrid filter topology suitable for the compensation of high-power 12-pulse rectifier loads. Harmonic current controllers based on the second-order infinite-impulse response digital resonant filters are used, as they can be considered as simple digital algorithms for more complex double cascaded synchronous-reference-frame-based proportional plus integral controllers. They are centered to the targeted harmonic frequencies by using an adaptive fundamental frequency tracking filter. This approach gives good results, even if the reference waveform (in our case, a load voltage) is highly distorted or unbalanced and no separate phaselocked loop is required. Test results for a laboratory model of this system and stability analysis are presented and the importance of delay-time compensation is discussed.

Devices based on an array of light-filtering channels with surface plasmon interference filters

Abstract: Devices based on surface plasmon filters having at least one metal-dielectric interface to support surface plasmon waves. A multi-layer-coupled surface plasmon notch filter is provided to have more than two symmetric metal-dielectric interfaces coupled with one another to produce a transmission spectral window with desired spectral profile and bandwidth. Such notch filters can form various color filtering devices for color flat panel displays.

A robust hysteresis current-controlled PWM inverter for linear PMSM driven magnetic suspended positioning system

Abstract: Hysteresis current-controlled pulsewidth modulation (PWM) is very robust but it possesses nonconstant switching frequency, and it is difficult to use for high-performance position servo applications. This paper presents a robust hysteresis current-controlled PWM scheme for a magnetic suspended positioning system driven by an inverter-fed linear permanent-magnet synchronous motor having improved performance in these two areas. In the proposed control scheme, the conventional hysteresis PWM mechanism is augmented by a robust harmonic spectrum-shaping controller. The error signal, which represents the switching frequency deviated from the set one, is detected using a notching filter inverse model. Then, the current command is adjusted by a robust compensation signal. The hysteresis band can be equivalently varied to let the dominant harmonic frequency of inverter output be constant, wherein the frequency can easily be changed by tuning the center frequency of the notch filter. The gating signal of switches is not needed to be sensed for making the proposed control. The constant-frequency control performance yielded by the proposed controller is rather insensitive to the system disturbances and the neutral voltage variation due to isolated Y connection. Through applying the proposed PWM inverter, satisfactory position control requirements can be achieved by properly setting the dominant harmonic frequency according to the electromechanical model of the positioning system.

Global positioning system receiver capable of functioning in the presence of interference

Abstract: Systems and methods are described for a GPS receiver capable of functioning in the presence of interference. A method includes detecting an interfering signal including: tuning a band pass filter over a frequency range; and at each of a plurality of incremental frequencies: computing a set of band pass filter coefficients; sending the set of band pass filter coefficients to a digital filter; repeatedly transforming an analog-to-digital converter output having a quantization level in excess of 2 bits into a band pass filter output with the digital filter to obtain a plurality of samples; computing an average of the plurality of samples; and comparing the average to a threshold to detect peaks that exceed a threshold. An apparatus, comprising: an analog radio frequency circuit; an analog-to-digital converter coupled to the analog radio frequency circuit, the analog-to-digital converter providing a quantization level in excess of 2 bits; a digital filter coupled to the analog-to-digital converter; and a digital circuit coupled to the digital filter.

Carrier blanking mechanism for sweeping detector used to measure and correct RF power amplifier distortion

Abstract: RF power amplifier distortion can be accurately measured in the presence of multi-frequency input signals, by using a swept local oscillator (45) to tune RF input (20) and output (30) receivers. The power detected by the input receiver (20) is compared with a threshold associated with the carrier. Whenever the power detected by the input receiver (20) exceeds the threshold indicating that the input receiver is tuned on carrier energy and the signal path through the output receiver is blanked. The sweeping action combined with selective blanking of the output receiver (30) creates an adaptive notch filter, which allows for the direct measurement of low level distortion power in the presence of high power carriers. This distortion power is digitized and can be processed to control pre-distortion correction circuitry (14) or gain/phase adjustment circuitry (14) of a feed-forward error correction loop.

Photonics-based interference mitigation filters

Abstract: New photonic filter structures for interference mitigation of microwave signals are presented. These fiber filters have a parallel topology, and comprise a grating based photonic bandpass filter, or a dual offset cavity structure based on a new noncommensurate delay-line approach. The new topologies overcome the problem of synthesizing both a narrow stopband and a very-wide and flat passband, to simultaneously excise interference with minimal impact on the wanted signal over a wide microwave range. Results demonstrate stopbands of around 1% of center frequency, wide-band flat transmission, and a shape factor of 10.5 that is the lowest reported for a photonic notch filter, in excellent agreement with predictions.

Varactor-tuned active notch filter with low passband noise and signal distortion

Abstract: The frequency-tunable band-reject filter described in this paper is designed to suppress signal interference in receivers of frequency-agile systems, while offering maximum transparency at passband frequencies. The presented solution is based on a new channelized-active-filter architecture in which the portion of the circuit that determines passband behavior is free of semiconductor devices. This permits passband residual noise and intermodulation distortion to be reduced to that of a simple nonresonant passive network. The concept is demonstrated with a varactor-tuned hybrid-integrated filter whose 40-dB-deep rejection notch of constant 70-MHz width can be tuned over a 9.5-10.5-GHz frequency span. Included are measurements of pertinent small-signal transfer characteristics and noise properties, as well as detailed assessments of nonlinear effects, such as third-order intermodulation distortion and compression of notch depth as a function of drive level.

Narrowband interference canceller for wideband communication systems

Abstract: A method and apparatus for adaptively removing interference from a signal. In one embodiment, the invention comprises an infinite impulse response (IIR) notch filter configured to receive a wideband signal and provide a filtered output signal, and a controller coupled to the notch filter to adaptively control the null frequency of the notch filter thereby removing narrowband interference from the received wideband signal. The controller may employ a gradient-based algorithm to detect the highest power frequency band in the output signal and modify the null frequency of the notch filter to minimize the power of the output signal.

Tunable variable bandpass optical filter

Abstract: A variable bandwidth tunable optical filters comprised of two transparent optical substrates, upon each of which is deposited a linearly variable multilayer interference filter coating which varies in total thickness from end to end in a wedged fashion. This wedge imparts a corresponding variation of the center wavelength of the filter bandpass with respect to the linear position of the coated filter substrates. The variable bandpass tunable filter can be adjusted to a specific center wavelength by moving the filter pair together linearly through the incident beam. The bandwidth of the filter pair is maximized when the two filters are spectrally aligned, and is determined by the bandwidth of the individual filters. The variable bandwidth of the tunable filter can be adjusted by introducing a relative linear offset between the linearly variable filter pair.

A novel tunable microwave optical notch filter

Abstract: A new topology for a tunable microwave photonic notch filter, which achieves a wide and continuous tuning range of FSR/2, is presented. The novel principle of tuning is based on changing optical variable attenuators only; consequently, a fixed wavelength laser can be used as the optical source. Experimental filter tuning results demonstrate a wide fractional tuning range of 50%, continuous tuning capability, and a notch filter shape that does not change as it is tuned, in very good agreement with predictions.

Spontaneous Brillouin-based distributed temperature sensor utilizing a fiber Bragg grating notch filter for the separation of the Brillouin signal

Abstract: We report on a Brillouin optical time domain reflectometer-based distributed temperature sensor utilizing a fiber Bragg grating notch filter to suppress the Rayleigh backscatter in order to separate the Brillouin signal. The Brillouin light path is thus subject to minimum attenuation and is frequency independent. A 2-m spatial resolution is achieved over a range of 25 km with the temperature resolution rising from 1/spl deg/C at the near end to 7/spl deg/C at the far end in a measurement time of 10 min. This is reduced to <1/spl deg/C over /spl sim/20 km if the measurement time is increased to 180 min.

Sliding-window transform with integrated windowing

Abstract: A system for a sliding-window transform with integrated windowing is described. The system provides a Direct Fourier Transform kernel with an integrated windowing filter having a desired number of stages. In one embodiment, the windowing filter is a lowpass filter. In one embodiment, the lowpass filter has a rectangular filter transfer characteristic. The DFT includes a complex multiplier. A first portion of the windowing filter is provided before the complex multiplier and can be implemented using real arithmetic. A second portion of the windowing filter is provided after the complex multiplier and is implemented using complex arithmetic. In one embodiment, the filter weights of the second portion of the windowing filter are unity and thus no multiplier is needed for the filter weights in the second portion of the windowing filter.

Acquisition matched filter for W-CDMA systems providing frequency offset robustness

Abstract: A system and method for acquiring a transmitted spread-spectrum signal uses a matched filter configuration that is preferably employed as a Stage 2 filter in the second phase of an initial signal acquisition procedure performed in the receiver. In operation, a Stage 1 filter matches a sequence which is repeated a number of times according to a second sequence within a secondary synchronization sub-channel of the spread-spectrum signal. The Stage 2 filter of the invention then filters the output of the Stage 1 filter in order to recover a secondary synchronization code. The Stage 2 filters the output of the Stage 1 filter using three matched filters of its own. The first and second filters sample different taps of a sequence output from the Stage 1 filter. The third filter samples a predetermined number of taps overlapping taps for the first and second filters in a way that resolves ambiguities with respect to the identification of the sequence. The ambiguity resolution performed by the third filter improves the frequency offset robustness and thus the overall performance of the receiver.

System and method for peak power reduction in spread spectrum communications systems

Abstract: A system and method for signal peak reduction in a spread spectrum communication system of the type including a filter for limiting signal bandwidth of symbols transmitted from the system. A signal peak reduction unit (122) is provided before the filter (126) the includes a filter predictor (146) that predicts the effect of that filter on input symbols by using filter coefficient values corresponding to the filter impulse response function. Input symbols that are predicted to cause the output signal to exceed a predetermined peak limit value are adjusted. Several examples of suitable algorithms for calculating the necessary peak reduction to be applied to the input symbols are disclosed. The peak reduction unit provides adjusted symbols to the filter for processing and communication system output.

On the design of adaptive notch filters

Abstract: In instrumentation and other applications, the on-line estimation of the frequency and amplitude of a noise-corrupted sinewave is of great practical interest. Recently an adaptive notch filter (ANF) with global convergence properties has been developed, and is a candidate approach to our problem. This paper analyses the transient and noise properties of this ANF and equips the method with design equations. Using frequency ranges greater than (up to 2 decades) and signal/noise ratios less than (down to −16 dB) those commonly found in the ANF literature, it is verified by extensive simulations that the new frequency estimator has excellent tracking and noise-rejection properties, provided that the signal/noise ratio is not too small. A comparison is made of its behaviour with that of a phase-locked loop, a method commonly used in practice. Copyright © 2001 John Wiley & Sons, Ltd.

Efficient digital ITU-compliant zero-buffering DTMF detection using the non-uniform discrete fourier transform

Abstract: Computationally efficient DTMF detection methods and apparatus are presented that meet all of the ITU recommendations using the modified non-uniform DFT. The system of the present invention employs a high band filter block and two low band filter blocks to detect power at the 8 DTMF tones. The frame length of the high band filter blocks is one half the length of the low band filter blocks. The frame lengths are chosen to meet the ITU frequency selectivity requirements for all DTMF frequencies. The frames of the two low band filter blocks are staggered to produce outputs alternately, and are aligned with respect to the frame of the high band filter block to produce low band filter block outputs that coincided with the high band filter block outputs without the need for signal buffering. A system of power level tests are employed in conjunction with a system of timing tests to ensure that all ITU timing and frequency constraints are met. The present invention requires no buffering of input samples, and can perform DTMF decoding of 24 telephone channels of a T1 time-division multiplexed communication line, using a single fixed-point commercially available digital signal processor.