Showing papers on "Center frequency published in 1995"

Frequency discrimination as a function of frequency, measured in several ways

Abstract: Frequency discrimination was measured for a wide range of center frequencies (0.25–8 kHz) using three different tasks. In the first (difference limens for frequency, DLFs) subjects were required to indicate which of two successive tone pulses was higher in frequency. In the second (difference limens for change, DLCs), two successive pairs of tone pulses were presented; one pair had the same frequency and the other pair differed in frequency. Subjects were required to indicate which pair differed in frequency. In the third (frequency‐modulation difference limens, FMDLs), subjects were required to indicate which of two successive tone pulses was frequency modulated. Modulation rates were 2, 5, or 10 Hz. For frequencies up to 2 kHz, DLFs and DLCs were small (less than 0.6% of the center frequency) and were similar to one another. For frequencies of 4 kHz and above, both DLFs and DLCs increased markedly, but the increase was greater for DLFs. Thus the worsening of performance at high frequencies is greater when subjects are required to indicate the direction of a frequency change than when they just have to detect any change. FMDLs, when expressed relative to the carrier frequency, varied much less with frequency than DLFs or DLCs. At 2 kHz and below, FMDLs were larger than DLFs or DLCs. Above 4 kHz, FMDLs were smaller than DLFs or DLCs. At 2 kHz and below, FMDLs usually worsened with increasing modulation frequency. Above 4 kHz, FMDLs improved with increasing modulation frequency. The pattern of results suggests that two mechanisms play a role in frequency discrimination, one based on changes in the excitation pattern (a ‘‘place’’ mechanism) and one based on phase locking in the auditory nerve (a ‘‘temporal’’ mechanism). The temporal mechanism only operates below about 4 kHz and, within this range, it determines DLFs and DLCs. The temporal mechanism is probably sluggish, and it affects FMDLs only for very low modulation rates. The place mechanism dominates for high frequencies, and for lower carrier frequencies when stimuli are frequency modulated at high rates.

Phase coded, micro-power impulse radar motion sensor

Abstract: A motion sensing, micro-power impulse radar MIR impresses on the transmitted signal, or the received pulse timing signal, one or more frequencies lower than the pulse repetition frequency, that become intermediate frequencies in a "IF homodyne" receiver. Thus, many advantages of classical RF receivers can be thereby be realized with ultra-wide band radar. The sensor includes a transmitter which transmits a sequence of electromagnetic pulses in response to a transmit timing signal at a nominal pulse repetition frequency. A receiver samples echoes of the sequence of electromagnetic pulses from objects within the field with controlled timing, in response to a receive timing signal, and generates a sample signal in response to the samples. A timing circuit supplies the transmit timing signal to the transmitter and supplies the receive timing signal to the receiver. The relative timing of the transmit timing signal and the receive timing signal is modulated between a first relative delay and a second relative delay at an intermediate frequency, causing the receiver to sample the echoes such that the time between transmissions of pulses in the sequence and samples by the receiver is modulated at the intermediate frequency. Modulation may be executed by modulating the pulse repetition frequency which drives the transmitter, by modulating the delay circuitry which controls the relative timing of the sample strobe, or by modulating amplitude of the transmitted pulses. The electromagnetic pulses will have a nominal center frequency related to pulse width, and the first relative delay and the second relative delay between which the timing signals are modulated, differ by less than the nominal pulse width, and preferably by about one-quarter wavelength at the nominal center frequency of the transmitted pulses.

High-power HTS microstrip filters for wireless communication

Abstract: The performance of narrowband microstrip filters with low insertion loss and high power-handling capabilities made from YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) high-temperature superconducting (HTS) thin films is presented. Results are shown for two different designs that were chosen to optimize the power-handling capability. Both filters have a 2-GHz center frequency and 5 poles that incorporate coupled resonators with 10-/spl Omega/ internal impedances. They were made on 5-cm-diameter LaAlO/sub 3/ substrates. Both designs use parallel-coupled feed lines to avoid current crowding, The first design includes backward- and forward-coupled filters, has 1% bandwidth, and has handled over 25 watts of input power at 10 K with less than 0.25 dB compression. The second design has 1.2% bandwidth and uses only forward-coupled resonators. The dissipation loss is less than 0.2 dB at 45 K and it has a third-order intercept of 62 dBm. Another similar filter handled 36 watts of power at 45 K with less than 0.15 dB compression across the passband. We have developed a technique to visualize the power dissipation of the filter by observing the bubbles created by the filter when submerged in liquid helium, showing areas with local defects or where the current distribution is at its peak value. We also discuss several planar high-power filter issues, including material selection and fabrication, device configuration trade-offs, filter structure optimization, and design approaches to maximize power-handling capacity.

Sampled amplitude read channel employing a user data frequency synthesizer and a servo data frequency synthesizer

Abstract: A sampled amplitude read channel incorporated within a magnetic disk storage system for reading data recorded in concentric tracks on a magnetic medium, where the data comprises user data sectors recorded at varying data rates across a plurality of predefined zones and embedded servo data sectors recorded at the same data rate across the zones. The sampled amplitude read channel comprises a timing recovery component for synchronous sampling of a read signal from a magnetic read head positioned over the magnetic medium, a gain control component for adjusting the amplitude of the read signal, and a DC offset component for cancelling a DC offset in the read signal. These components are dynamically configured to operate according to whether the read channel is processing user data or embedded servo data. A user data frequency synthesizer and a servo data frequency synthesizer lock the timing recovery component to a reference frequency and provide a coarse center frequency control signal corresponding to the user data or servo data mode. The read channel further employs pipelined reads to reduce the physical gap between sectors on the medium. In addition, an improved sync mark detector and an improved asynchronous servo address mark detector increase the accuracy and reliability of the read channel.

Stacked crystal filter device and method of making

Abstract: A piezoelectric device includes a stacked crystal filter, a supporting substrate and an acoustic reflector between the filter and substrate for suppressing undesired resonant frequencies. The acoustic reflector includes a set of quarter wavelength layers of alternating higher and lower impedances. The reflector provides a large reflection coefficient for a bandwidth that encompasses only desired frequencies generated by the stacked crystal filter. By changing the nature of the material and the number of layers, the bandwidth and center frequency of the acoustic reflector may be varied to encompass a desired range of frequencies. To reduce the magnitude of side lobes at undesired frequencies, the impedance differences between layers of the reflector are reduced with depth into the reflector. The side lobes may also be reduced by serially connecting two stacked crystal filters having acoustic reflectors of the same material but with a different number of quarter wavelength layers.

A simple relationship between frequency and range averages for broadband sonar

Abstract: All sonars have a bandwidth, and often the assumption of single frequency propagation will mislead the sonar designer. It is shown here theoretically, and by demonstration using SAFARI, that a frequency average can often be approximated closely by a variable width running range average in which the width or window size is proportional to range. In fact if the bandwidth is expressed as a fraction α of the center frequency one should use a range averaging width of α times the range. The limitations of such a scheme are also addressed.

Detection of decrements and increments in sinusoids at high overall levels

Abstract: Thresholds for the detection of decrements in level of sinusoidal signals were measured as a function of duration (2, 4, 6, 10, and 14 ms), level (70, 80, and 90 dB SPL) and frequency (250, 500, 1000, 2000, and 4000 Hz). Seven normally hearing listeners were tested at each frequency (with different subjects for each frequency). Thresholds for detecting a 10‐ms increment in level were also measured. The sinusoids were presented in a background noise low‐pass filtered at 5 kHz, which was intended to mask spectral splatter associated with the decrement or increment. Performance improved with increasing frequency for all decrement and increment durations. Performance also tended to improve with increasing level at 2000 and 4000 Hz. The results were analyzed using a four‐stage model consisting of an auditory filter centered on the signal frequency, a compressive nonlinearity, a sliding temporal integrator and a decision mechanism. The analysis indicated that the improved performance with increasing frequency and increasing level could be attributed partly to off‐frequency listening; for the two highest center frequencies, subjects probably made use of the output of an auditory filter centered above the signal frequency, where changes in excitation level associated with an increment or decrement were magnified. The measurements at 4000 Hz were repeated using a broadband background noise (15‐kHz bandwidth), which would prevent the use of information from auditory filters centered far above the signal frequency. Performance was poorer than when low‐pass noise was used, but still improved somewhat with increasing level. The slight improvement in performance with increasing level can be accounted for by a reduced compressive linearity at high levels. A good fit to the data could be obtained by assuming that the equivalent rectangular duration (ERD) of the temporal integrator was invariant with level, but that the compressive nonlinearity varied with level in a similar way to basilar‐membrane input–output functions. The nonlinearity appears to be somewhat less compressive at 250 Hz than at higher center frequencies. The ERD is about 7 ms regardless of center frequency.

Nuclear magnetic resonance receiver, method and system

Abstract: An NMR receiver includes an analog to digital converter for sampling a nuclear magnetic resonance probe output signal at a frequency that is less than the probe signal center frequency and is at least twice the probe signal bandwidth.

Phase aberration correction on two-dimensional conformal arrays

Abstract: Two-dimensional conformal arrays are proposed to enhance low contrast lesion detection deep in the body. The arrays conform to the body maintaining good contact over a large area. To provide full three-dimensional focusing for two-dimensional imaging, such arrays are densely sampled in the scan direction (x) and coarsely sampled in the nonscan direction (y), i.e., the arrays are anisotropic. To illustrate reduction in slice thickness with increased array length in y, a two-dimensional array is synthesized using a one-dimensional, 128 element array with a 3.5 MHz center frequency. A mask is attached confining transmission and reception of acoustic waves to 2 mm in y. Using a mechanical scan system, the one-dimensional array is moved along y covering a 28.16 mm/spl times/20.0 mm aperture. Accordingly, the synthetic array has 128 elements in x and 10 elements in y. To correct for geometric irregularities due to array movement, a gelatin based phantom containing three-dimensional point targets is used for phase aberration correction. Results show that elevational beam quality is degraded if small geometric errors are not removed. Emulated conformality at the body surface and phase aberrations induced by spatial inhomogeneities in tissue are further imposed and shown to produce severe beam-forming artifacts. Two-dimensional phase aberration correction is applied and results indicate that the method is adequate to compensate for large phase excursions across the entire array. To fully realize the potential of large, two-dimensional, conformal arrays, proper two-dimensional phase aberration correction methods are necessary. >

Sampling system for radio frequency receiver

Abstract: A radio frequency (rf) receiver adapted to receive a number of different digitally modulated rf input signals such as quadrature amplitude modulated (QAM) and vestigial side band (VSB) rf input signals includes circuitry for down converting the rf input signals to an intermediate frequency (IF) range having a center frequency fc2 and a bandwidth of Bhz and converter circuitry for sampling the IF signals and then producing corresponding baseband signals. In a preferred embodiment, the intermediate frequency signals are applied to a sample and hold circuit which is sampled at a frequency fs and whose output is coupled via a low pass filter to an analog-to-digital converter whose output is then applied to a Hilbert filter for demodulating the sampled signals and producing baseband signals. In-phase (I) and quadrature (Q) signals are produced whose phase and amplitude are not a function of different components and their tolerance of different conduction paths, as in the prior art.

Filter using enhanced quality factor resonator and method

Abstract: A filter (500) including a first resonator (505) coupled in shunt with a first port (507), a first bridging network (510) coupled between the first port (507) and a first node (509), a second resonator (515) coupled in shunt with the first node (509) and a second port (547) coupled to the first node (509). The first (505) and second (515) resonators each include acoustic resonators (15) and the first bridging network (510) has an electrical length Θ at a frequency ω in a system having a characteristic admittance Y in accordance with ωCo =YcotΘ where Co is a shunt capacitance including the clamping capacitance of the first resonator (505) and ω is a center frequency of the filter (500).

Spread spectrum correlation using saw device

Abstract: A technique is provided using a SAW device for correlating to a plurality of different chip code sequences, such as M different chip code sequences, using fewer than M individual SAW correlators. A received signal is provided to a SAW correlator capable of recognizing a plurality of different chip code sequences. The SAW correlator is pre-programmed so as to recognize each different chip code sequence at a different frequency. The received signal is given a programmably selected frequency offset from the center frequency of the received signal (or a downconverted version thereof) and applied to the SAW correlator. The SAW correlator correlates for the particular chip code sequence at the offset frequency. The output of the SAW correlator is given an opposite frequency offset and filtered with a bandpass filter to generate a correlator output at the designated center frequency. In one embodiment, a comb generator signal is multiplied with the received signal to provide a plurality of signals at offset frequencies. The plurality of offset signals are provided to the SAW correlator which simultaneously correlates for all possible chip code sequences. The output of the SAW is connected to a plurality of bandpass filters, each attuned to one of the offset frequencies generated by the comb generator.

Frequency discrimination of bandlimited harmonic complexes related to vowel formants

Abstract: Just‐noticeable differences (jnd’s) in the center frequency of bandlimited harmonic complexes were measured for normal hearing subjects. A triangular and a trapezoidal spectral envelope were used. The center frequency ranged from 2000 to 2100 Hz for the triangular envelope and from 2000 to 2200 Hz for the trapezoidal envelope. For both envelope shapes the values 100, 200, and 400 dB/oct were used for the slope. The fundamental frequency was 100 Hz for all complexes. A three‐interval, three‐alternative forced‐choice task was used. All measurements were performed with and without roving of the signal level. Without roving, the jnd’s were in most cases about a factor of 1.5 smaller than with roving. The bandwidth and the center frequency were factors having the major influence on the jnd’s. For the triangular envelope, the jnd’s were smallest for center frequencies that were halfway between two signal components. For the trapezoidal envelope, the jnd’s were smallest for center frequencies that coincided with a signal component. For the triangular envelope, the smallest jnd’s for each slope were about half the value of those for the trapezoidal envelope. In the case of the smallest jnd’s for each spectral envelope shape, the jnd’s were smaller when the spectral slope was steeper. Part of the data for the triangular envelope can be explained by modulation depth discrimination data. The data for the trapezoidal envelope can be interpreted in terms of a spectral profile comparison. The remaining part might be interpreted either in terms of a temporal mechanism, or of the mentioned spectral profile comparison. It is argued that, for this spectral comparison, the ear probably combines information over a limited frequency range by comparing the spectral profiles of the two presented stimuli, rather than comparing individual component levels.

Soliton transmission control by Butterworth filters.

Abstract: Butterworth filters of second or higher order periodically inserted along a soliton transmission line, although they are flat at bandpass center, give to first order a nonzero restoring force for frequency and amplitude displacements. The amplified spontaneous emission generated at the filter center frequency is significantly less than that generated with fixed etalons for the same damping strength.

Method and apparatus for high frequency time domain reflectometry

Abstract: An apparatus and method is disclosed for the location of faults occurring within transmission systems. The disclosed apparatus and method allows a user to adjust the center frequency of a pulse output by a standard narrow bandwidth time domain reflectometer, without altering the bandwidth of the output pulse. As a result, the detection of faults occurring at a wide range of frequencies is enabled--while the cost associated with widening the bandwidth of the transmitted pulse is not incurred. Moreover, due to the narrower bandwidth, efficient fault detection is achieved as well for high frequency systems as for low frequency systems.

Bridge-output-to-frequency converter for smart thermal air-flow sensors

Abstract: A fully integrated bridge-output-to-frequency converter has been realized as signal-conditioning circuitry for a one-chip smart thermal mass-flow sensor. The frequency converter reads out four separate resistor bridges of the air-flow sensor in order to realize high sensitivity and directional flow measurement over the full angle range of 360/spl deg/. The converter selects the sensor bridges individually by means of a microprocessor-controlled analog multiplexer. The flow velocity and direction are also calculated by the microprocessor. The relative measurement cancels all first-order errors. The center frequency is 10 kHz and the sensitivity 1 Hz/(/spl mu/V/V). A bridge-offset of 10 mV/V is allowed, and the linearity error is 0.1%. >

Digital demodulation apparatus

Abstract: A demodulation apparatus of digital detection processing type of the invention offers versatility as consumer equipment in mobile communications, ATV, satellite broadcasting, CATV, and the like A modulated wave output is obtained by multiplying an input digitally modulated wave signal by a local oscillating signal from a local oscillator The obtained modulated wave output has a center frequency which is substantially equal to the symbol frequency The modulated wave output is A/D converted at a rate which is four times as high as the symbol frequency, so as to be output as interleaved I and Q digital data The I and Q data is split, and the split I and Q data are multiplied by coefficients of "+1" and "-1", respectively The multiplied two output signals are selectively output Thus, the data multiplied by the coefficients of "+1" and "-1" are alternately output for the I and Q signals, so as to perform the digital detection The processed I and Q data are subjected to digital channel filter processing for spectrum shaping/An interpolation signal for one of the output signals of digital channel filters is generated and output An amplitude level value of the interpolation signal is controlled, and then the timing of the interpolation signal is matched with the timing of the other one of the output signals of the digital channel filters The phase error detection and the waveform shaping are performed for the I and Q data having the matched timings, and the data identification is performed

Analog Integrated Polyphase Filters

Abstract: When it comes to integratability, the zero-IF receiver is an alternative for the heterodyne or IF receiver. In recent years it has been introduced in several applications, but its performance can not be compared to that of the IF receiver yet. In this paper the principle of the low-IF receiver is introduced. The low-IF receiver has a topology which is closely related to the zero-IF receiver. Like the zero-IF receiver, the implementation of a low-IF receiver can be done with a high degree of integration. Its performance can however be better. The low-IF receiver is not sensitive to DC-offsets or LO to RF crosstalk. The principle of the low-IF receiver is based on the replacement of the LF lowpass filter of a zero-IF receiver with an analog integrated active polyphase filter. An active polyphase filter can perform a lowpass filter operation shifted on to a center frequency. In this paper the realization of a 5th order lowpass Butterworth filter centered around a low IF of 250 kHz is presented. This polyphase filter is designed for a low-IF receiver for high quality applications. The filter is implemented with the active-RC technique in a standard 1.2 μm CMOS process.

Pressure pulse distortion by hydrophones due to diminished low frequency response

Abstract: In characterizing the bandwidth of measurement devices used in ultrasound exposimetry, attention has been focused on the high frequency response. However, current diagnostic ultrasound measurement standards have low frequency specifications for hydrophones and associated amplifiers, and the response below 1 MHz can be especially significant when measuring lithotripsy pulses. To model the effects of diminished low frequency response, simulated diagnostic and lithotripsy pulses were filtered with a single-pole high-pass filter for a range of -3 dB cutoff frequencies (denoted f/sub a/). For lithotripsy pulses, it was found that the pulse quantities peak rarefactional pressure (p/sub r/) and pulse width (t/sub w/) were most sensitive to f/sub a/, and to keep errors in p/sub r/, and t/sub w/ below 10%, f/sub a/ should be in the 10-60 kHz range for the pulses examined. For the diagnostic case, p/sub r/ was the quantity most significantly affected, and for an f/sub a/ value approximately one-half the center frequency, p/sub r/ was decreased by more than 30% for a pulse modeled to show the effects of finite amplitude distortion typical of diagnostic pulses measured in water. Given this latter result, current hydrophone and amplifier low frequency specifications may need to be reconsidered.

A 1.5 V CMOS balanced differential switched-capacitor filter with internal clock boosters

Abstract: A 1.5 V SC filter employing a balanced differential structure and a internal clock booster is proposed. The design technique is demonstrated by a fourth-order bandpass biquad filter fabricated with a standard 0.8 /spl mu/m CMOS technology. This prototype fourth-order filter which has a center frequency of 8 kHz and a clock frequency of 400 kHz dissipates about 330 /spl mu/W with a 1.5 V power supply. Including the clock generator and boosters, it occupies 600/spl times/1500 /spl mu/m/sup 2/. The measurement result shows this filter has the IMD of 0.1 percent for 1.2 V/sub IP/ differential signal.

High frequency fundamental resonators and filters fabricated by batch process using chemical etching

Abstract: The small sized 1st intermediate frequency (IF) filters at center frequency range of 70 MHz to 150 MHz and passband widths of /spl plusmn/5 to /spl plusmn/100 kHz with sharp selectivity are required in mobile communication systems such as mobile and portable cellular phone. Our solution to employ fundamental mode monolithic crystal filter (MCF) assembled in surface mountable package. We describe here in detail, the design approach including batch process etching technology. Through photolithography, 56 patterns are chemically etched on one wafer (25 mm/spl times/20 mm). Then, a similar etching process automatically adjusts the wafer thickness in accordance with frequency. For the MCF, the frequency of split electrodes and the degree of coupling between them are automatically adjusted by an accurately positioned mask evaporation process controlled by a computer. Further, we describe suppression of the spurious response, technology for realizing wide bandwidth and high stopband attenuation characteristics. By our above developed technology, we achieved 90 MHz miniaturized IF filter with 1/15 volume reduction of conventional 3rd overtone mode MCF, still possessing the same characteristic of conventional one. Also, we achieved 130 MHz of middle band with suppression of spurious response in wide stopband frequency range, and 71 MHz linear phase wide /spl plusmn/88 kHz band with the group delay distortion 0.9 /spl mu/s over f/sub 0//spl plusmn/80 kHz.

Tunable tone control circuit and a device and method for tuning the RC constants

Abstract: An equalizer, for providing a tunable pole/zero compensated output signal, includes a integrated tone control circuit with a MOSFET-C configuration such that the spacing between the pole and the zero and the center frequency of the pole/zero pair can be tuned by varying the resistance of MOSFET resistors using variable voltage sources applied to the gates of the MOSFETs

Automatic frequency offset compensation apparatus

Abstract: An automatic frequency compensation apparatus comprises: a frequency offset prediction portion for predicting a frequency offset of an oscillation signal used for receiving a TDM digital modulation signal from the center frequency of the TDM digital modulation signal using a quadrature baseband signal derived from the TDM digital modulation signal, a vector correlation portion detecting a correlation between the quadrature baseband signal and a predetermined signal, the frequency offset prediction portion predicting when the correlation exceeds a reference value, and a frequency offset compensation portion effects a frequency offset compensation to the quadrature baseband. A convergent coefficient may be decreased with the number of events that the correlation exceeds the reference value. An equalizer may be further provided after the frequency offset compensation portion and the prediction is made using the output of the equalizer. The prediction is effected during the training period or during the training period when the correlation exceeds the reference value. The correlation may be detected using the output of the equalizer and a signal obtained by Judging or digitizing the output of the equalizer.

Soliton transmission system having plural sliding frequency guiding filter groups

Abstract: Timing jitter problems are effectively eliminated in a soliton transmission system realized by deploying a series of optical filters in groups whose group average center frequency intentionally differs from the group average center frequency of other optical filter groups. The center frequency of the series of optical filter groups is translated along the desired length of the system in a predetermined manner such as frequency increasing, frequency decreasing, and combinations of both. This creates a transmission environment which is substantially opaque to noise while remaining perfectly transparent to solitons. By arranging the optical filters in groups, it is possible to simplify the system design by reducing the number of filters having different nominal center frequencies.

Transient and optoelectronic feedback-sustained pulsation of laser diodes at 1300 nm

Abstract: We report the observation of self-sustained pulsation and transient self-pulsation in laser diodes at 1300 nm and the effects of optoelectronic feedback on the pulsations. Transient self-pulsation has a lifetime of a few minutes with frequencies up to 7 GHz. The linewidth of self-pulsation is on the order of 0.5 GHz. With optoelectronic feedback, the transient self-pulsation can be sustained and the linewidth significantly reduced to about 20 kHz. The center frequency of feedback-sustained pulsation is dependent on the passband of the bandpass filter in the feedback loop. The feedback sustained pulsation can be frequency modulated for applications in subcarrier multiplexed optical networks such as cable TV distribution and antenna remoting. >

Method for instantaneous frequency measurement

Abstract: This invention discloses a method of instantaneous frequency measurement. The frequency band of interest is divided into a number of virtual sub-bands. Each sub-band is folded into a common baseband by a band folding down converter. Frequency measurement is achieved in a two part process, part 1 determines the center frequency of the virtual sub-band, while part 2 determines the baseband frequency, the frequency within the virtual sub-band. Both components of the RF frequency are then combined to form the frequency measurement. The frequency determinations for both sub-band frequency and baseband frequency are made by digital processing of the baseband phase measurements. The baseband phase is measured by a multi bit phase sampler, an integral part of the invention.

High-temperature superconducting microstrip filters with high power-handling capability

Abstract: The performance of narrowband microstrip filters with low insertion loss and high power-handling capabilities made from YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) high-temperature superconducting (HTS) films is presented. Results are shown for two different novel designs that were chosen to optimize the power-handling capabilities. Both have a 2-GHz center frequency and 5-poles that incorporate coupled resonators with 10-/spl Omega/ internal impedances on 50-mm-diameter LaAlO/sub 3/ substrates. Both designs use parallel-coupled feed lines to avoid current crowding. The first design includes backward- and forward-coupled filters, has 1% bandwidth, and has handled over 25 watts of input power at 10 K with less than 0.25 dB compression. The second design has 1.2% bandwidth and use only forward-coupled resonators. The minimum insertion loss is less than 0.2 dB at 45 K, it has a third-order intercept of 62 dBm. Another similar filter handled 36 watts of power at 45 K with less than 0.15 dB compression across the passband. We have developed a technique to visualize the power dissipation of the filter by observing the bubbles created by the filter when submerged in liquid helium, showing areas with local defects or where the current distribution is at its peak value. >

Technique for selective fat saturation in MR imaging

Abstract: A system is provided for MR imaging wherein a volume within an imaging subject positioned with respect to the system contains both water molecules and fat tissue proximate to one another. The water molecules and fat tissue are selectively saturated by first determining whether application of a first RF excitation pulse of a first RF frequency to the volume, in association with a gradient magnetic field having a selected polarity with respect to an axis, would saturate fat tissue in a band lying closer to or farther from a selected location than a band of saturated water molecules. The first frequency comprises the sum of a center frequency and a first offset frequency having a magnitude and polarity determined by the gradient magnetic field. In the event the saturated fat band associated with the first frequency would lie farther from the specified location, a second RF pulse of a second RF frequency is instead applied to the volume to saturate fat tissue within a band lying closer to the specified location, the second RF frequency comprising the sum of the center frequency and a second offset frequency having a magnitude equal to the magnitude of the first offset frequency and a polarity opposite to the polarity thereof. The polarity of the selection gradient of the second RF pulse is similarly negated compared to the polarity of the selection gradient of the first pulse.