Publications by authors named "Fred L Walls"

5 Publications

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Sensing bacterial vibrations and early response to antibiotics with phase noise of a resonant crystal.

Sci Rep 2017 09 22;7(1):12138. Epub 2017 Sep 22.

Applied Chemicals and Materials Division, National Institute of Standards and Technology, Boulder, CO, 80305, USA.

The speed of conventional antimicrobial susceptibility testing (AST) is intrinsically limited by observation of cell colony growth, which can extend over days and allow bacterial infections to advance before effective antibiotics are identified. This report presents an approach for rapidly sensing mechanical fluctuations of bacteria and the effects of antibiotics on these fluctuations. Bacteria are adhered to a quartz crystal resonator in an electronic bridge that is driven by a high-stability frequency source. Mechanical fluctuations of cells introduce time-dependent perturbations to the crystal boundary conditions and associated resonant frequency, which translate into phase noise measured at the output of the bridge. In experiments on nonmotile E. coli exposed to polymyxin B, cell-generated frequency noise dropped close to zero with the first spectra acquired 7 minutes after introduction of the antibiotic. In experiments on the same bacterial strain exposed to ampicillin, frequency noise began decreasing within 15 minutes of antibiotic introduction and proceeded to drop more rapidly with the onset of antibiotic-induced lysis. In conjunction with cell imaging and post-experiment counting of colony-forming units, these results provide evidence that cell death can be sensed through measurements of cell-generated frequency noise, potentially providing a basis for rapid AST.
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http://dx.doi.org/10.1038/s41598-017-12063-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5610186PMC
September 2017

Merits of PM noise measurement over noise figure: a study at microwave frequencies.

IEEE Trans Ultrason Ferroelectr Freq Control 2006 Oct;53(10):1889-94

National Institute of Standards and Technology (NIST), Boulder, CO, USA.

This paper primarily addresses the usefulness of phase-modulation (PM) noise measurements versus noise figure (NF) measurements in characterizing the merit of an amplifier. The residual broadband (white PM) noise is used as the basis for estimating the NF of an amplifier. We have observed experimentally that many amplifiers show an increase in the broadband noise of 1 to 5 dB as the signal level through the amplifier increases. This effect is linked to input power through the amplifier's nonlinear intermodulation distortion. Consequently, this effect is reduced as linearity is increased. We further conclude that, although NF is sometimes used as a selection criteria for an amplifier for low-level signal, NF yields no information about potentially important close-to-carrier 1/f noise of an amplifier nor broadband noise in the presence of a high-level signal, but a PM noise measurements does. We also have verified experimentally that the single-sideband PM noise floor of an amplifier due to thermal noise is -177 dBc/Hz, relative to a carrier input power of 0 dBm.
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http://dx.doi.org/10.1109/tuffc.2006.121DOI Listing
October 2006

High spectral purity microwave oscillator: design using conventional air-dielectric cavity.

IEEE Trans Ultrason Ferroelectr Freq Control 2004 Oct;51(10):1225-31

National Physical Laboratory, New Delhi, India.

We report exceptionally low PM noise levels from a microwave oscillator that uses a conventional air-dielectric cavity resonator as a frequency discriminator. Our approach is to increase the discriminator's intrinsic signal-to-noise ratio by use of a high-power carrier signal to interrogate an optimally coupled cavity, while the high-level of the carrier is suppressed before the phase detector. We developed and tested an accurate model of the expected PM noise that indicates, among other things, that a conventional air-dielectric resonator of moderate Q will exhibit less discriminator noise in this approach than do more esoteric and expensive dielectric resonators tuned to a high-order, high-Q mode and driven at the dielectric's optimum
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http://dx.doi.org/10.1109/tuffc.2004.1350949DOI Listing
October 2004

Conjugate regenerative dividers.

IEEE Trans Ultrason Ferroelectr Freq Control 2004 Mar;51(3):271-6

Time & Frequency Division, NIST, Boulder, CO, USA.

We discuss a novel design of a self-starting regenerative divider that permits division by 3, 4, 5, 6 ... instead of the usual 2. This is accomplished by having the loop oscillate simultaneously at two harmonically related conjugate frequencies, e.g., at nu/4 and 3nu/4. A prototype of the divide-by-four circuit has been constructed for an input frequency of 400 MHz. This divider exhibits very low phase noise, Fourier frequency (1 kHz) = -162 dBc/Hz and Fourier frequency (100 kHz) = -170 dBc/Hz, which is approximately 9 dB lower than that of its constituent parts. Simple modifications of the feedback loop of this circuit enabled it to divide by 3, 5, and 8. Operation at higher division ratios appears feasible under certain conditions.
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http://dx.doi.org/10.1109/tuffc.2004.1295406DOI Listing
March 2004

Interpreting anomalously low voltage noise in two-channel measurement systems.

IEEE Trans Ultrason Ferroelectr Freq Control 2002 Jan;49(1):11-9

Physics Department, The University of Western Australia, Nedlands.

In this work we 1) analyze and give a theoretical explanation for the anomalously low cross-spectral density of voltage fluctuations that is observed when two thermal noise sources with matched intensities are coupled to the inputs of two-channel phase modulation (PM) or amplitude modulation (AM) noise measurement systems (NMS), 2) empirically evaluate spectral resolutions of different types of measurement systems, and 3) discuss noise measurement techniques involving cross-correlation signal processing. Our work shows that the statistical uncertainty, which sets the ultimate spectral resolution in the thermal noise limited regime, is approximately the same for both systems. However, in practical terms, the non-stationary nature of the noise, the temporal separation of calibration and measurement, and the difficulty of reproducing the calibrations for two measurements make it extremely difficult to resolve noise that is more than 10 dB below the noise floor in a single channel NMS. In a two-channel NMS, however, the calibrations of the two channels are carried out simultaneously, and one can take full advantage of a large number of averages and make reproducible noise measurements with resolution 10 dB below the noise floor of a single channel NMS.
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http://dx.doi.org/10.1109/58.981379DOI Listing
January 2002