Precise comparison of experimental and theoretical SNRs in CO2 laser heterodyne systems

A detailed comparison of experimental and theoretical SNR in an IR laser heterodyne system has been made with three different signal analyzers. Good agreement, considerably better than a factor of 1.5, is reported. Accurate allowance was made for transmission in the receiver optics, the effective quantum efficiency of the detector due to shot noise domination by the local oscillator, and for coherent speckle effects across the collection aperture. The evaluation of SNR with a surface acoustic wave spectrum analyzer and digital integrator is described in some detail. As an illustration an absolute measurement of backscattering strength in the atmosphere from an airborne equipment at altitudes up to 13.1 km is provided.     

A Raman microprobe study of electrical treeing in polyethylene

The chemical nature of electrical tree growth in a blend of high and low density polyethylene has been studied by confocal Raman microprobe spectroscopy. The observed spectra, which are easily perturbed by the Raman probe beam, can be described in terms of three components, the relative intensities of which vary from place-to-place on the sample. Throughout the body of the tree, the usual Raman bands of polyethylene are seen, but superimposed upon a pronounced fluorescent background. This suggests a degree of material degradation throughout the structure, even where there is no visible evidence of electrical discharge damage or ageing. The individual channels that make up the fractal structure of the tree can then be divided into two distinct categories. Within the core of the tree, their Raman spectra are made up of two elements; fluorescence, plus the G and D bands of sp² hybridized carbon. Here, the tree channels are best thought of as hollow tubules surrounded by a carbonaceous shell. At the tips, the channels are characterized simply by the Raman bands of polyethylene superimposed upon a reduced fluorescent background, irrespective of their size. That is, in this region, the tree channels are simply hollow tubules within the dielectric. The transition between these two structures occurs relatively abruptly. These observations are related to the treeing process.     

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Radio Echo Sounding of Pine Island Glacier, West Antarctica: Aperture Synthesis Processing and Analysis of Feasibility From Space

Airborne radio echo sounding of the West Antarctic Ice Sheet over Pine Island Glacier was performed in the austral summer of 2004/2005 under the National Science Foundation's West Antarctic Ice Sheet Initiative. The British Antarctic Survey flew its newly developed 150-MHz ice-sounding radar over Pine Island Glacier and collected approximately 35 000 km of sounding data. The synthetic aperture radar (SAR) technique was applied to process those data in order to enhance radar signatures from the bed. As a matter of fact, airborne ice-sounding radar systems are generally affected by surface clutter returns, masking the echoes of internal layers and ice-bedrock transition at a large depth. Focused and unfocused (Doppler filtering) SAR techniques were compared, and their respective advantages/disadvantages were analyzed. Enhancement of bedrock detection at a large depth (> 2000 m) through SAR processing is demonstrated. Finally, a simulation analysis was performed for assessing the feasibility of ice-sheet sounding from space. It is shown that the gain in bed detection threshold is marginal in the satellite sounding geometry. Airborne radar, Antarctica, ice sounding, satellite remote sensing, subglacial topography, synthetic aperture radar (SAR) processing, West Antarctic Ice Sheet.     

Secondary emission formulas

A minor revision is made to the author's `A New Formula for Secondary Emission Yield' (ibid., vol.36, no.9, p.1963-7, September 1989) based on the work of A. Shih and C. Hor reported elsewhere in this issue (ibid. vol.40, no.4, p.824-9, Apr. 1993)     

Transmitting, receiving, and scattering properties of antennas

In most modern textbooks on antennas, it is stated that receiving antennas scatter as much as they absorb under matched-load conditions. This is incorrect, as was shown 50 years ago. The reason for this discrepancy is an apparently incorrect use of the equivalent circuits. In this tutorial paper, a simple example of a two-element Yagi antenna and a test dipole in the far field is analyzed for transmission, reception, and scattering, through the use of the symmetric impedance matrix. It is shown that the correct equivalent circuit for the scattering case is more complicated, owing to the existence of more sources than in the transmitting case. Through a numerical example, it is also shown that the scattered power may be larger, equal to, or smaller than the absorbed power.     

The theory of bandpass sampling

The sampling of bandpass signals is discussed with respect to band position, noise considerations, and parameter sensitivity. For first-order sampling, the acceptable and unacceptable sample rates are presented, with specific discussion of the practical rates which are nonminimum. It is shown that the minimum sampling rate is pathological in that any imperfection in the implementation will cause aliasing. In applying bandpass sampling to relocate signals to a base-band position, the signal-to-noise ratio is not preserved owing to the out-of-band noise being aliased. The degradation in signal-to-noise ratio is quantified in terms of the position of the bandpass signal. For the construction of a bandpass signal from second-order samples, the cost of implementing the interpolant (dynamic range and length) depends on Kohlenberg's sampling factor (1953) k, the relative delay between the uniform sampling streams. An elaboration on the disallowed discrete values of k shows that some allowed values are better than others for implementation     

The diversity of potential cheese ripening characteristics of lactic acid starter bacteria: 1. Resistance to cell lysis and levels and cellular distribution of proteinase activities

By reference to subcellular fraction markers, the resistance to lysis of 23 strains of Lactococcus lactis subsp. cremoris, 30 strains of L. lactis subsp. lactis and five strains of Streptococcus thermophilus and the levels and distribution of proteinase activity in the strains were determined. Strains of L. lactis subsp. cremoris were readily lysed by transfer to hypotonic buffer after treatment with lysozyme alone, whilst strains of L. lactis subsp. lactis and S. thermophilus could be efficiently lysed in this way only after treatment with a combination of lysozyme and mutanolysin. With a few notable exceptions, those strains which gave the fastest rates of acid production also generally presented higher levels of cell surface proteinase, as determined by activity on fluorescein isothiocyanate-labelled β-casein. The highest levels of cell surface proteinase detected were found for strains of L. lactis subsp. cremoris. However, the levels of total proteinase activity in the lactococcal strains did not correlate with the rate of acid production in milk, some slow acid-producers yielding similar or greater total proteinase levels than fast acid-producers. Homology to DNA probes for the lactococcal cell surface proteinase gene and to the conserved region encoding the serine proteinase active site was shown by the fast acid-producing lactococcal strains, but not by most of the slow acid-producing lactococcal strains or by the strains of S. thermophilus. A significant proportion of the total proteinase activity was recovered in the subcellular fractions in which high levels of cytoplasmic marker enzyme activity were found. The total proteinase levels detected in strains ofL. lactis subsp. lactis showed a greater range of variation than in the strains of L. lactis subsp. cremoris. High levels of total proteinase activity were found in the slow acid-producers despite the strains having been grown in the presence of yeast extract. For many of the strains, the levels of proteinase released from the cell surface during cell wall degradation with lytic enzyme treatment were higher than those found using whole cells, suggesting that a significant amount of proteolytic activity was either inaccessible to substrate or present in an inactive form.