Showing papers on "Center frequency published in 1997"

Waveform adaptive ultra-wideband transmitter

Abstract: A waveform adaptive transmitter that conditions and/or modulates the phase, frequency, bandwidth, amplitude and/or attenuation of ultra-wideband (UWB) pulses. The transmitter confines or band-limits UWB signals within spectral limits for use in communication, positioning, and/or radar applications. One embodiment comprises a low-level UWB source (e.g., an impulse generator or time-gated oscillator (fixed or voltage-controlled)), a waveform adapter (e.g., digital or analog filter, pulse shaper, and/or voltage variable attenuator), a power amplifier, and an antenna to radiate a band-limited and/or modulated UWB or wideband signals. In a special case where the oscillator has zero frequency and outputs a DC bias, a low-level impulse generator impulse-excites a bandpass filter to produce an UWB signal having an adjustable center frequency and desired bandwidth based on a characteristic of the filter. In another embodiment, a low-level impulse signal is approximated by a time-gated continuous-wave oscillator to produce an extremely wide bandwidth pulse with deterministic center frequency and bandwidth characteristics. The UWB signal may be modulated to carry multi-megabit per second digital data, or may be used in object detection or for ranging applications. Activation of the power amplifier may be time-gated in cadence with the UWB source thereby to reduce inter-pulse power consumption. The UWB transmitter is capable of extremely high pulse repetition frequencies (PRFs) and data rates in the hundreds of megabits per second or more, frequency agility on a pulse-to-pulse basis allowing frequency hopping if desired, and extensibility from below HF to millimeter wave frequencies.

Receiver with sigma-delta analog-to-digital converter for sampling a received signal

Abstract: A receiver comprising a sigma-delta analog-to-digital converter (ΣΔ ADC) can be utilized in one of four configurations, as a subsampling bandpass receiver, a subsampling baseband receiver, a Nyquist sampling bandpass receiver, or a Nyquist sampling baseband receiver. For subsampling ΣΔ receivers, the sampling frequency is less than twice the center frequency of the input signal into the ΣΔ ADC. For Nyquist sampling ΣΔ receivers, the sampling frequency is at least twice the highest frequency of the input signal into the ΣΔ ADC. For baseband ΣΔ receivers, the center frequency of the output signal from the ΣΔ ADC is approximately zero or DC. For bandpass ΣΔ receivers, the center frequency of the output signal from the ΣΔ ADC is greater than zero. The sampling frequency can be selected based on the bandwidth of the input signal to simplify the design of the digital circuits used to process the output samples from the ΣΔ ADC. Furthermore, the center frequency of the input signal can be selected based on the sampling frequency and the bandwidth of the input signal. The ΣΔ ADC within the receiver provides many benefits.

Universal pressure and temperature SAW sensor for wireless applications

Abstract: This work presents a universal SAW sensor device for measurement of either pressure or temperature or both simultaneously. An arrangement of three surface acoustic wave resonators (SAWRs) was used. The experimental sensor described in this paper employs a 35/spl deg/ Y cut quartz diaphragm and utilizes center frequencies ranging from 433 to 436 MHz. The sensor relies on differences in the dependence of each SAWR center frequency on pressure and temperature. A difference frequency measurement was used. The difference frequency outputs were used to determine pressure and temperature.

A highly linear CMOS G/sub m/-C bandpass filter with on-chip frequency tuning

Abstract: A fourteenth-order CMOS transconductance-C (G/sub m/-C) bandpass filter with on-chip automatic frequency tuning is described. By using highly linear G/sub m/-C integrators, the filter achieves 75 dB dynamic range over 700 kHz noise bandwidth. The measured intermodulation distortion (IM3) @ 600 kHz for a 4 V/sub pp/ input signal is only -61 dB. On-chip automatic frequency tuning provides more than 300% center frequency range (i.e., 165-505 kHz) of the filter with /spl plusmn/1% frequency accuracy. The 0.7-/spl mu/m CMOS filter measures 4.8 mm/sup 2/ and consumes 70 mW from a single 5 V power supply.

Fast frequency sweep technique for the efficient analysis of dielectric waveguides

Abstract: This paper describes a new approach to spectral response computations of an arbitrary two-dimensional (2-D) waveguide. This technique is based on the tangential-vector finite-element method (TVFEM) in conjunction with the asymptotic waveform evaluation (AWE) technique. The former is used to obtain modes characteristics for a central frequency, whereas the latter employs an efficient algorithm to compute frequency moments for each mode. These moments are then matched via Pade approximation to a reduced-order rational polynomial, which can be used to interpolate each mode over a frequency band with a high degree of accuracy. Furthermore, the moments computations and subsequent interpolation for a given set of frequency points can be done much more rapidly than just simple simulations for each frequency point.

A micromachine-based RF low-noise voltage-controlled oscillator

Abstract: A voltage-controlled oscillator (VCO) employs an aluminum micromachined variable capacitor for frequency tuning. Unlike conventional varactor diodes, the capacitor is fabricated on a silicon substrate and thus amenable to monolithic integration with a standard IC process. Experimental capacitors achieve a 16% tuning range with a nominal capacitance value of 2 pF and a quality factor above 60 at 1 GHz. A prototype VCO exhibits -107 dBc/Hz phase-noise at 100 kHz offset frequency from the carrier. The center frequency of 714 MHz and 14 MHz tuning range are limited by the test setup.

Automatic frequency control apparatus for stabilization of voltage-controlled oscillator

Abstract: The output of a voltage-controlled oscillator (VCO), after being passed through a resonator having a single-peak characteristic, is detected by a detector. The output of the detector is sampled and held at a time instant that the control input voltage to the VCO reaches a maximum and also at a time instant that it reaches a minimum, and the difference between them is fed back to the control input of the VCO. In this way, the center frequency of the VCO is controlled so that it becomes equal to the center frequency of the resonator. In the case of a VCO as an FM modulator in an FM-CW radar, the sample-and-hold timing is derived from a clock signal based on which a triangle wave is generated. In the case of a VCO as an FSK modulator, the sample-and-hold timing is obtained by detecting a 0 and a 1 in input data.

Voice analysis-synthesis method using noise having diffusion which varies with frequency band to modify predicted phases of transmitted pitch data blocks

Abstract: A high efficiency encoding method for encoding data on frequency axis obtained by dividing an input audio signal on block-by-block basis and converting the signal onto the frequency axis, wherein V bands are searched for a band BVH with the highest center frequency if it is decided that there are one or more shift points of voiced (V)/unvoiced (UV) decision data of all bands on the frequency axis, and wherein the number of V bands NV up to the band BVH is found, so as to decide whether proportion of the V bands is equal to or higher than a predetermined threshold Nth, thereby deciding one V/UV boundary point. Thus, it is possible to replace the V/UV decision data for each band by information on one demarcation in all bands, thereby to reduce data volume and to reduce bit rate. Also, by using two-stage hierarchical vector quantization in quantizing the data on the frequency axis, operation volume for codebook search and memory capacity of the codebook are reduced.

The nonstationary strain filter in elastography: Part I. Frequency dependent attenuation

Abstract: The accuracy and precision of the strain estimates in elastography depend on a myriad number of factors. A clear understanding of the various factors (noise sources) that plague strain estimation is essential to obtain quality elastograms. The nonstationary variation in the performance of the strain filter due to frequency-dependent attenuation and lateral and elevational signal decorrelation are analyzed in this and the companion paper for the cross-correlation-based strain estimator. In this paper, we focus on the role of frequency-dependent attenuation in the performance of the strain estimator. The reduction in the signal-to-noise ratio (SNRs) in the RF signal, and the center frequency and bandwidth downshift with frequency-dependent attenuation are incorporated into the strain filter formulation. Both linear and nonlinear frequency dependence of attenuation are theoretically analyzed. Monte-Carlo simulations are used to corroborate the theoretically predicted results. Experimental results illustrate the deterioration in the precision of the strain estimates with depth in a uniformly elastic phantom. Theoretical, simulation and experimental results indicate the importance of high SNRs values in the RF signals, because the strain estimation sensitivity, elastographic SNRe and dynamic range deteriorate rapidly with a decrease in the SNRs. In addition, a shift in the strain filter toward higher strains is observed at large depths in tissue due to the center frequency downshift.

Method for lowpass filter calibration in a satellite receiver

Abstract: An improved satellite receiver front end architecture having a tuner chip and a demodulator/decoder chip. The tuner chip includes a lowpass filter having a configurable cutoff frequency, and the tuner chip uses a frequency signal to provide accurate adjustment of the cutoff frequency. A clock signal having a clock frequency is converted into a control voltage which determines the cutoff frequency of the lowpass filter. Consequently, the cutoff frequency may be increased by increasing the clock frequency, or decreased by decreasing the clock frequency. This configuration provides for improved cutoff frequency control in the presence of signal interference.

A Q-enhanced active-RLC bandpass filter

Abstract: A fully differential high-Q bandpass filter that uses lossy integrated inductors is presented. The circuit is implemented in a 0.8 /spl mu/m BiCMOS technology and realizes a center frequency of 750 MHz with a Q-factor that is tunable from 10 to 490 while dissipating 80-100 mW from a single 5 V supply. Since the objective of the prototype was to explore the proposed Q-enhancement technique, the dynamic range is limited to 25 dB for Q=20.

Filter having tunable center frequency and/or tunable bandwidth

Abstract: In an RF filter having an input terminal, an output terminal, a plurality of resonator elements, and a plurality of coupling reactances for coupling one resonator element to another resonator element, for coupling the input terminal to a resonator element, and for coupling the output terminal to a resonator element, techniques are disclosed for providing an RF filter having an electronically tunable center frequency and an electronically tunable bandwidth. According to a specific embodiment disclosed herein, a first shunt reactance is provided from the input terminal to ground, and a second shunt reactance is provided from the output terminal to ground. The values of the coupling reactances remain constant, while the values of the resonator elements, the first shunt reactance, and the second shunt reactance are tuned to provide a specified filter bandwidth and/or a specified center frequency. According to a further embodiment, such a tunable filter is fabricated on a physical substrate having a substantially constant dielectric constant per unit volume, and the coupling reactances include capacitors fabricated using the physical substrate.

Bandpass delta-sigma modulator with 800 MHz center frequency

Abstract: A fourth-order bandpass /spl Delta/-/spl Sigma/ modulator with center frequency of 800 MHz designed and fabricated in AlGaAs/GaAs heterojunction bipolar transistor (HBT) technology is reported. The modulator can be clocked at a continuum of frequencies from 2-4 GHz. Its performance was characterized at one convenient clock frequency, 3.2 GHz, since clocking the modulator at 4 times the center frequency allows for trivial extraction of in-phase and quadrature components of the bandpass signal in the digital domain. The 1-bit modulator output achieves a signal-to-noise ratio of 66 dB over a 100 kHz bandwidth and 41 dB over a 25 MHz bandwidth (which covers the entire cellular band).

The Detection Thresholds of Resonances at Low Frequencies

Abstract: New experimental data on the detection thresholds of low-frequency resonances and antiresonances are presented. Using an adaptive procedure known as the up-down transformed response (UDTR) rule, the 70.7% detection thresholds were measured for a single added resonance (peak and notch) for different Q values and center frequencies. The signals included pink noise and pulses auditioned through earphones. The results show that detection thresholds are affected in complicated ways by Q, center frequency, and signal type. This makes their detection difficult to predict using current hearing models and frequency response measurements.

Simple channel model for 60 GHz indoor wireless LAN design based on complex wideband measurements

Abstract: Based on practical complex wideband measurements, a simple model for the 60 GHz indoor wireless radio channel is presented in this paper. The multipath channel is modeled by a conventional time invariant FIR filter structure. Two sets of filter coefficients are provided for typical indoor wireless LAN application scenarios with an RF bandwidth of 200 MHz and 62 GHz center frequency. The scenarios represent the line of sight (LOS) and non line of sight (NLOS) cases where omni-directional antennas are used for both transmit and receive sides. The present channel model is used for system design simulations in the European wireless ATM project MEDIAN and can easily be adopted for similar simulation purposes. Thus, a new pragmatic system design tool for the 60 GHz indoor radio environment is given with this paper.

A fully integrated low-noise 1-GHz frequency synthesizer design for mobile communication application

Abstract: This paper describes a fully monolithic phase-locked loop (PLL) frequency synthesizer circuit implemented in a standard 0.8-/spl mu/m CMOS technology. To be immune to noise, all the circuits in the synthesizer use differential schemes with the digital parts designed by static logic. The experimental voltage controlled oscillator (VCO) has a center frequency of 800 MHz and a tuning range of /spl plusmn/25%. The measured frequency synthesizer performance has a frequency range from 700 MHz to 1 GHz with -80 dBc/Hz phase noise at a 100 kHz carrier offset. With an active area of 0.34 mm/sup 2/, the test chip consumes 125 mW at maximum frequency from a 5 V supply. The only external components are the supply decoupling capacitors and a passive filter.

Three‐dimensional spectral‐spatial excitation

Abstract: A method of three-dimensional spectral-spatial excitation is presented which is selective simultaneously in two spatial dimensions and in the spectral or chemical shift dimension. This method can be used to create spectral passbands whose center frequency varies as a function of spatial location within an imaging plane. This variation of passband center frequency may be specified by an acquired main field (B0) map; the resulting excitations compensate for inhomogeneity of the B0 field. In vivo images are presented in which three-dimensional spectral-spatial excitation allows selective water-only imaging in the presence of large B0 inhomogeneity where conventional spectrally selective imaging falls. Phantom studies give a detailed profile of the performance of three-dimensional spectral-spatial pulses suitable for water-only or fat-saturation imaging. These pulses may also be useful for fat and water suppression in spectroscopic imaging. Performance constraints imposed by limited gradient slew rates are analyzed and quantified.

A novel monolithic piezoelectric sensor

Abstract: A novel monolithic piezoelectric sensor (MPS) is presented for the detection of physical, chemical and biochemical measurands. This new sensor overcomes specific deficiencies associated with the quartz crystal microbalance (QCM) while still employing a well-characterized, temperature-stable thickness-shear mode (TSM). The sensor is applicable to both gaseous and liquid phase measurements; however, the principal benefit of the MPS is in liquid phase measurements. In these applications, it offers the ability to operate simple, yet stable, oscillator circuits in relatively viscous media. The proposed sensor structure is based on a two-pole coupled resonator, in which mechanical coupling between the electrical input and output determines the electrical properties. This structure offers approximately 180/spl deg/ of phase shift over its 3dB bandwidth with nominally 180/spl deg/ of insertion phase at the symmetric resonant frequency and approximately 0/spl deg/ of insertion phase at the antisymmetric resonant frequency. Simple oscillator circuits may be implemented which measure the symmetric frequency (f/sub s/), the antisymmetric frequency (f/sub a/) or the nominal center frequency (f/sub s/). Suitable switching allows combinations of these frequencies to be sequentially measured, expanding the capabilities of the MPS versus the QCM. This novel MPS sensor structure should accelerate the commercialization of piezoelectric sensor technology, particularly in such areas as chemical, biochemical and environmental testing.

Detection and discrimination of frequency glides as a function of direction, duration, frequency span, and center frequency

Abstract: The study investigated the ability to detect and discriminate frequency glides under a variety of experimental conditions. The subjects distinguished between a comparison signal that either was level in frequency or was swept across a fixed frequency span, and a target signal that changed more in frequency than the comparison signal. Tone durations were 50 and 400 ms. Nominal center frequencies were 0.5, 2, and 6 kHz; actual center frequencies were varied randomly, or roved, over a range equal to 0.1 times the nominal center frequency. Up- and down-glides were used. The transition span of the comparison signal was either 0, 0.5, 1, or 2 times the equivalent rectangular bandwidth of the auditory filter at the nominal center frequency. Discrimination thresholds were obtained for all combinations of center frequency, direction, and span. Overall, thresholds expressed as ΔHz/ERB varied little as a function of center frequency. Glide duration had no effect on discrimination. The 50-ms down-glides were more dif...

A MOSFET-only continuous-time bandpass filter

Abstract: A MOSFET-only filter is described which employs an amplifier-regulated cascode structure to realize both conductance and transconductance, while the required capacitors are realized by MOSFET gate capacitance. Distortion due to mobility degradation and MOSFET capacitor nonlinearity is analyzed, as is the effect of finite gain-bandwidth product of the regulating amplifiers on the filter's linearity and high frequency performance. A prototype second-order bandpass filter has a center frequency of 560 kHz, a third-order intermodulation of -41 dB for 0 dBm input and a dynamic range of 60 dB. It consumes 2.5 mW from a 5 V supply and occupies 0.18 mm/sup 2/ in a 1-/spl mu/m digital CMOS technology.

System and method for characterizing undesirable noise of a signal path within a selected frequency band

Abstract: A test (600) system for characterizing the nature and the severity of the impairments affecting a radio frequency signal path (148) which may be an upstream or a downstream channel in a cable system. Testing is done by monitoring the output of an unused signal path (614) with a filter (608) and a totaling counter (612). The filter passes impairment energy from the signal path to the counter in a frequency band of interest, thereby increasing the count value on the counter. The bandpass filter limits the ability of impairments or signals from other frequency bands to increase the counter's count value. The counter's input threshold voltage level (404) is set to trigger on impairments that are sufficiently strong to cause data errors. The count value may be used to determine the time duration of an impairment by dividing the count accumulated in one second by the center frequency of the filter. An optional digital time trace acquisition unit (710), connected to the broadband signal path output (614), may be employed to capture time traces (734) to help identify the source and the nature of the impairment. The time traces can be stored, retrieved, and analyzed by conversion into the frequency domain, by digital filtering, by plotting as a histogram, and by plotting as a waterfall display.

A bandpass /spl Sigma//spl Delta/ modulator with 92 dB SNR and center frequency continuously programmable from 0 to 70 MHz

Abstract: Use of a bandpass /spl Sigma//spl Delta/ modulator permits direct conversion of an analog signal to digital form at IF frequencies. This allows the ADC to be moved closer to the receiver front end. Moving the digital interface closer to the antenna reduces receiver analog circuit complexity, eliminates DC-offset cancellation, inphase/quadrature (I/Q) gain calibration, dual I/Q mixers and improves system robustness as mixing is in the digital domain. This second-order bandpass /spl Sigma//spl Delta/ modulator is targeted for an airborne radar system but is also expected to find use in a variety of communications applications. Measurements yield signal to noise+distortion ratio (SNR) from 92 dB (15 b) in narrowband ( 366 kHz) to 44 dB (7 b) in broadband (62.6 MHz) about a center frequency of 55.5 MHz. Modulator sampling rate is 4 GHz and it is implemented in AlInAs-InGaAs HBT technology. The performance represents an improvement of approximately a factor of 10 in bandwidth, resolution and center frequency over other reported bandpass modulators.

Intermediate frequency selecting device for use in dual band cellular telephone and method thereof

Abstract: A dual band cellular telephone includes an antenna, a power amplifier, RF transmission and reception chains, first and second local oscillators, and IF transmission and reception chains. In order to select an intermediate frequency, processing circuitry within the telephone determines center frequencies of transmission radio frequency sweep ranges of first and second bands, calculates a difference between the center frequencies of the first and second transmission bands, and multiplies the calculated difference value by a scaling coefficient to calculate a center local oscillating frequency associated with one of the RF transmission bands. It then subtracts the transmission center frequency of one of the RF transmission bands from the local oscillator center frequency to determine the transmission intermediate frequency. Preferably, the cellular telephone has a single local oscillator for transmit and a single local oscillator for receive. The IF selection method enables the same IF frequency separation between transmit and receive when operating at any frequency in the dual band system.

Frequency tuning of the dolphin’s hearing as revealed by auditory brain-stem response with notch-noise masking

Abstract: Notch-noise masking was used to measure frequency tuning in a dolphin (Tursiops truncatus) in a simultaneous-masking paradigm in conjunction with auditory brain-stem evoked potential recording. Measurements were made at probe frequencies of 64, 76, 90, and 108 kHz. The data were analyzed by fitting the rounded-exponent model of the auditory filters to the experimental data. The fitting parameter values corresponded to the filter tuning as follows: QER (center frequency divided by equivalent rectangular bandwidths) of 35 to 36.5 and Q10 dB of 18 to 19 at all tested frequencies.

Space diversity receiving apparatus

Abstract: A space diversity receiver apparatus receives signals by two spatially separated antennas, controls the phase of the signal received by one of the antennas by a phase control circuit, combines the phase-controlled signal and the signal received by the other antenna by a combiner and outputs the combined signal. A digital detector digitally detects the center frequency level and the levels on high- and low-frequency sides of the center frequency of the combined signal. Phase is controlled in such a manner that the center frequency level will coincide with a set level and a deviation between the levels on the high- and low-frequency sides of the center frequency will become zero.

Low insertion phase variation dielectric material

Abstract: This invention is directed to an apparatus for transmitting an electromagnetic wave. The apparatus includes a dielectric material with a permittivity that decreases with increasing frequency over an operating frequency bandwidth that includes the frequency of the electromagnetic wave. With this behavior, the electrical or phase length of the dielectric material remains relatively constant and independent of frequency over the operating band, a feature that is highly advantageous for many important applications including circuit boards, radomes, and antennas. In addition, the dielectric material can be engineered to exhibit this behavior at a desired frequency, including frequencies in the millimeter, microwave, VHF and UHF frequency ranges. In a preferred embodiment, the dielectric material is an artificial dielectric manufactured by mixing conductive particulate into a host dielectric material. The conductive particulate can be microspheres or microfibers particularly with poor or medium conductivities, that have determined shapes and that are mixed in a volumetric proportion with the host dielectric material, to achieve the desired permittivity versus frequency behavior. With the microfibers, the maximum loss tangent occurs off the center frequency of the operating frequency bandwidth so that constant phase behavior can be obtained with relatively little wave transmission loss. The apparatus can also include a conductive material disposed in contact with the dielectric material, for transmitting the electromagnetic wave. The invention also includes related methods.

Decomposition of vibroseis data by the multiple filter technique

Abstract: A harmonically distorted vibroseis signal is decomposed using the “multiple filter technique” in a similar manner as the wavelet transform The 1-D vibroseis signal is transformed into a 2-D function of time and frequency The procedure consists of applying a family of narrow Gaussian filters whose center frequencies are close to the instantaneous frequencies of the harmonically distorted vibroseis signal As a result of this decomposition, an envelope trace is obtained for every center frequency For a linear fundamental (pilot) sweep, the fundamental sweep and its harmonic distortions have different arrival times in the decomposed envelope trace An accurate analysis of the harmonic distortion can be carried out using the amplitudes along the slopes of the instantaneous frequency defined for the linear fundamental sweep and its harmonic distortions Plotting the contoured amplitudes of each envelope trace on a frequency-versus-time scale yields the amplitude distribution between the fundamental sweep and its harmonic distortions They represent the frequency content of the uncorrelated vibroseis signal and can be used to examine the interaction and leakage of the signal and its harmonics Similarly, the correlation noise, the so-called “ghost sweep” in the correlated vibroseis data, can be decomposed with this technique The method is applied to synthetic, harmonically distorted, vibroseis signals and to a real vibroseis vertical seismic profile (VSP) data set

A completely integrated single-chip PLL with a 34 GHz VCO using 0.2 /spl mu/m E-/D-HEMT-technology

Abstract: A completely integrated single-chip phase locked loop based on a 0.2 /spl mu/m gate length enhancement/depletion AlGaAs-GaAs-AlGaAs-HEMT technology has been designed and characterized. The chip contains a VCO with 34 GHz center frequency, a dynamic frequency divider by two, a static divider by eight, a phase detector, and a loop filter. The chip size is 2.0/spl times/1.5 mm/sup 2/. The power consumption is 1.2 W at a supply voltage of -5.0 V. The locking range is approximately /spl plusmn/700 MHz. The phase noise of locked PLL is -83 dBc/Hz at 100 kHz and -102 dBc/Hz at 1 MHz offset from the carrier frequency, respectively.

A Fourth Order Bandpass Delta-Sigma Modulator with DigitallyProgrammable Passband Frequency

Abstract: The design and implementation of a fourth order switched-capacitor bandpass delta-sigma modulator with digitally programmable passband is described. The quantization noise null can be programmed from 0.4π (0.2f_s) to 0.6π(0.3f_s) in steps of 0.01π (f_s/200) by changing digital switch settings. This design enables the A/D conversion of a bandpass signal with digital tuning of the center frequency for application in systems such as a transceiver IF stage. The modulator IC measures 4.8mm^2 in a 2µ m CMOS process and achieves an SNR of 47 and 59 dB over a 0.01π bandwidth at sampling rates of 2.358 MHz and 1.25 MHz, respectively.

Frequency dependence of broadband propagation in coastal regions

Abstract: To study the frequency-dependent spatial and temporal variabilities of sound propagation in coastal regions two experiments were conducted by transmitting sound impulses in the form of M sequences centered from 0.6 to 22 kHz. The site of the first experiment was the Atlantic Generating Station (AGS) where the source–receiver range was 214 m. The site of the second experiment was Delaware Bay where the range was approximately 760 m. The mean water depth was 14–15 m in both locations. Oceanographic data and acoustic data were collected simultaneously in both experiments. It was found that the temporal coherence of the propagated broadband signal changes significantly with pulse center frequency, as well as varying with geographic location and time. Trends of increasing signal decorrelation (between consecutive pulses) with increasing center frequency, and increasing signal decorrelation for paths with increasing number of interface interactions, were observed. It was found that for lower center frequencies there is hardly any decorrelation in signal over several hours, while for signals with center frequencies only a few kHz higher there was substantial decorrelation over times as short as 10 to 20 min. No significant spatial decorrelation of the signal was observed over the hydrophone spacing of a few meters for these experiments.