Bruce G. Colpitts

Harmonic radar transceiver design: miniature tags for insect tracking

The design and operation along with verifying measurements of a harmonic radar transceiver, or tag, developed for insect tracking are presented. A short length of wire formed the antenna while a beam lead Schottky diode across a resonant loop formed the frequency doubler circuit yielding a total tag mass of less than 3 mg. Simulators using the method-of-moments for the antenna, finite-integral time-domain for the loop, and harmonic balance for the nonlinear diode element were used to predict and optimize the transceiver performance. This performance is compared to the ideal case and to measurements performed using a pulsed magnetron source within an anechoic chamber. A method for analysis of the tag is presented and used to optimize the design by creating the largest possible return signal at the second harmonic frequency for a particular incident power density. These methods were verified through measurement of tags both in isolation and mounted on insects. For excitation at 9.41 GHz the optimum tag in isolation had an antenna length of 12 mm with a loop diameter of 1 mm which yielded a harmonic cross-section of 40 mm/sup 2/. For tags mounted on Colorado potato beetles, optimum performance was achieved with an 8 mm dipole fed 2 mm from the beetle attached end. A theory is developed that describes harmonic radar in a fashion similar to the conventional radar range equation but with harmonic cross-section replacing the conventional radar cross-section. This method provides a straightforward description of harmonic radar system performance as well as provides a means to describe harmonic radar tag performance.

Dark-Pulse Brillouin Optical Time-Domain Sensor With 20-mm Spatial Resolution

Brillouin scattering-based distributed fiber-optic sensing is a powerful measurement tool that uses the inelastic scattering of incident light by an acoustic wave (phonon) to determine strain and/or temperature conditions of the fiber. Since the original Brillouin-time-domain-analysis (BOTDA) technique was proposed, several other analysis methods have been introduced to improve sensing performance in four key areas: spatial resolution; measurement accuracy; total sensing length; and measurement-acquisition time. The four factors are generally interrelated and improvements to one factor often come at the cost of one or more of the others. For example, one system might sacrifice spatial resolution for total sensing length, while another might sacrifice accuracy to gain acquisition speed. We present a BOTDA system based on dark-pulse scattering that provides improved resolution, accuracy, and acquisition time over conventional BOTDA systems, without the severe limitations on sensing length often imposed by other high-resolution techniques. Theoretical validation of the method is given, and experimental results are presented that demonstrate 20-mm resolution strain measurements with an accuracy of plusmn20 muepsiv, which is the highest spatial resolution yet reported for a BOTDA system

Distributed sensor based on dark-pulse Brillouin scattering

A novel dark-pulse-based technique has been used for the first time in a Brillouin scattering-based distributed fiber sensor. Experimentally obtained Brillouin spectra demonstrate that the dark-pulse configuration is as capable of strain and temperature measurement as conventional pulse-based systems but at much higher spatial resolution. A spatial resolution of 50 mm is reported with a strain measurement accuracy of 6 /spl mu//spl epsiv/ on a 100-m sensing fiber.

Arbitrary magnetic field gradient waveform correction using an impulse response based pre-equalization technique

The time-varying magnetic fields used in magnetic resonance applications result in the induction of eddy currents on conductive structures in the vicinity of both the sample under investigation and the gradient coils. These eddy currents typically result in undesired degradations of image quality for MRI applications. Their ubiquitous nature has resulted in the development of various approaches to characterize and minimize their impact on image quality. This paper outlines a method that utilizes the magnetic field gradient waveform monitor method to directly measure the temporal evolution of the magnetic field gradient from a step-like input function and extracts the system impulse response. With the basic assumption that the gradient system is sufficiently linear and time invariant to permit system theory analysis, the impulse response is used to determine a pre-equalized (optimized) input waveform that provides a desired gradient response at the output of the system. An algorithm has been developed that calculates a pre-equalized waveform that may be accurately reproduced by the amplifier (is physically realizable) and accounts for system limitations including system bandwidth, amplifier slew rate capabilities, and noise inherent in the initial measurement. Significant improvements in magnetic field gradient waveform fidelity after pre-equalization have been realized and are summarized.

Electronic tags for the tracking of insects in flight: effect of weight on flight performance of adult Colorado potato beetles

The wing loading of the Colorado potato beetle, Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae), was found to decrease slightly with increasing size over a wide range of individual sizes and independent of sex. This makes it possible to use tags of the same weight for beetles of all sizes and suggests that if the addition of light electronic tags has any effect on the beetles flight it will be similar across beetle size. The wing loading of individual potato beetles ranged from an average minimum 10.9 N m−2 to an average maximum of 15.6 N m−2 as their weights fluctuated over time following water and food uptake or dispersal. However, tests carried out in flight chambers indicate that beetles become incapable of upward flight as they go beyond an average wing loading of 11.8 N m−2, that is 101 μN (10.3 mg) beyond their minimum weight. It is estimated from our results that electronic tags should weigh no more than 23–33% of the potato beetles acceptable extra loading for the technique to have no or minimal impact on the number and quality of upward flights taken.

Distributed Fiber-Optic Sensor for Dynamic Strain Measurement

A novel Brillouin-based distributed sensing technique is presented that can obtain the strain profile of the entire sensing fiber with a single optical pulse. Experimentally obtained spectra show that this comb excited pump signal is capable of strain and temperature measurement in a dynamic system. The results show the expected sinusoidal strain profile for a 12 s periodic strain cycle applied to the test fiber. The strain distribution for the entire fiber was obtained in 256 mus , thereby demonstrating for the first time, the potential to measure dynamic strains up to 3.9 kHz with 12 m resolution. Increased acquisition speed comes at the expense of achievable spatial resolution.

Reduction in the Number of Averages Required in BOTDA Sensors Using Wavelet Denoising Techniques

This paper reports on a new mechanism to decrease the number of averages and, consequently, the measurement time of Brillouin optical time-domain analysis (BOTDA) sensors using wavelet shrinkage techniques. Two different wavelet shrinkage techniques, VisuShrink and SureShrink, are applied to denoise signals acquired from measurements in BOTDA sensors. The conventional method to denoise signals in BOTDA sensors is ensemble averaging. Ensemble averaging is a time consuming technique, as it requires many acquisitions of signals to provide an acceptable SNR. To reduce the number of acquisitions, the setup of the BOTDA sensor is modified to denoise acquired signals using VisuShrink or SureShrink before applying ensemble averaging. Experimental results show a significant reduction in the number of averages required to provide an accurate measurement, and consequently, a substantial saving in the measurement time of the sensor. It has been shown that the combination of ensemble averaging with VisuShrink or SureShrink reduces the measurement time of the sensor up to 90%. This reduction in the measurement time enables the implementation of dynamic and fast measurements with BODTA sensors and opens opportunities to target a new range of applications.

Designing Static Fields for Unilateral Magnetic Resonance by a Scalar Potential Approach

We present a method for designing single-sided magnets suitable for unilateral magnetic resonance (UMR) measurements. The method uses metal pole pieces to shape the field from permanent magnets in a target region. The pole pieces are shaped according to solutions to Laplaces equation, and can be designed using a combination of analytical methods and numerical optimization. The design leads to analytical expressions for the pole piece shapes and magnetic field. Here, we develop the method in Cartesian, polar, and spherical coordinates, and discuss the merits of each system. Finite magnet size has a substantial effect on the field quality in many cases, according to our simulations. We found that in order to achieve a compact magnet in which the static field closely matches that specified, a full 3-D design approach is necessary. A magnet designed by our method produces a static field with a constant gradient over a region 2 cm in diameter and 2 mm thick. This leads to a compact cylindrical magnet just over 11 cm in diameter, topped with a single metal pole piece. The design is validated through simulation. The simulated field is found to agree closely with that specified analytically through the design procedure

Accurate estimation of Brillouin frequency shift in Brillouin optical time domain analysis sensors using cross correlation

Current methods of estimating the Brillouin frequency shift in Brillouin optical time domain analysis sensors are based on curve-fitting techniques. These techniques apply the same weight to all portions of the curve and dutifully fit into the peak and noisy ends of the curve. This makes them very sensitive to noise, initialization of fitting parameters, symmetry, and start and stop frequencies. We introduce a method based on the cross-correlation technique to estimate the central frequency of noisy Lorentzian curves, which is more robust to noise and free from initial settings of fitting parameters.

Symbol-wavelength MMSE gain in a multi-antenna UWB system

Ultra wideband (UWB) technology has the potential to provide high speed data services. These speeds can be greatly increased by using multiple input multiple output (MIMO) techniques. Multiple antennas are used to achieve spatial diversity and these antenna elements must be separated properly to have uncorrelated communication signals. The separation can be on the scale of a symbol wavelength ([speed of light]/[symbol rate]). We have designed a printed circular disc monopole and measured the UWB radio channel. Finally, we used the measurements to show the minimum mean squared error (MMSE) performance gain which occurs when the receiver antennas are separated on the scale of a symbol wavelength