Transmitter attenuation

Having determined the necessary pulse duration, the transmitter power must be calibrated so that the pulse delivers the appropriate tip angle. This procedure differs from that for hard pulses where one uses a fixed pulse amplitude but varies its duration. For practical convenience, amplitude calibration is usually based on previously recorded calibrations for a soft rectangular pulse (as described below), from which an estimate of the required power change is calculated. Table 9.3 also summarises the necessary changes in transmitter attenuation for various envelopes of equivalent duration, with the more elaborate pulse shapes invariably requiring increased rf peak amplitudes (decreased attenuation of transmitter output). 

Demonstration of this important Lambert-Beer-Bouguer law (often presented as empirical) is very simple and instructive for the purpose of this study. It relies on the basic idea that each layer of the medium contributes to a modification of the transmittance (attenuation) in a multiplicative manner. Everything is said with this sentence because it is implicit that the layer thicknesses are additive. Nevertheless, a mathematical derivation is worth giving. The scheme in Figure 7.4 shows the effect on the beam of a first layer with thickness A/12, spanning from distance /, to I2, which is mathematically expressed as... 

The attenuation of electromagnetic radiation as it passes through a sample is described quantitatively by two separate, but related terms transmittance and absorbance. Transmittance is defined as the ratio of the electromagnetic radiation s power exiting the sample, to that incident on the sample from the source, Pq, (Figure 10.20a). 

Attenuation of radiation as it passes through the sample leads to a transmittance of less than 1. As described, equation 10.1 does not distinguish between the different ways in which the attenuation of radiation occurs. Besides absorption by the analyte, several additional phenomena contribute to the net attenuation of radiation, including reflection and absorption by the sample container, absorption by components of the sample matrix other than the analyte, and the scattering of radiation. To compensate for this loss of the electromagnetic radiation s power, we use a method blank (Figure 10.20b). The radiation s power exiting from the method blank is taken to be Pq. 

The use of ultrasonic energy is different in on/off switches and in transmitters. Switches act on the attenuation of the acoustic signal in the gap between two crystals, while transmitters measure the time of flight of the ultrasonic pulse. 

Regulation of transmitter release does not rest solely on the frequency at which nerve impulses reach the terminals. Early experiments using stimulated sympathetic nerve/end-organ preparations in situ, or synaptosomes, indicated that release of [ HJnoradrenaline was attenuated by exposure to unlabelled, exogenous transmitter. This action was attributed to presynaptic adrenoceptors, designated a2-adrenoceptors, which were functionally distinct from either aj- or )S-adrenoceptors. Later experiments have confirmed that ai-adrenoceptors comprise a family of pharmacologically and structurally distinct adrenoceptor subtypes. 

As with many neurons (e.g. NA) there are presynaptic autoreceptors on the terminals of dopamine neurons whose activation attenuate DA release. Although most of these receptors appear to be of the D2 type, as found postsynaptically, D3 receptors are also found. It is possible that in addition to the short-term control of transmitter release they may also be linked directly to the control of the synthesising enzyme tyrosine hydroxylase. It seems that autoreceptors are more common on the terminals of nerves in the nigrostriatal (and possibly mesolimbic) than mesocortical pathway. 

Using the bi-static concept can reduce all of the above mentioned problems. The spatial separation of the transmitter and receiver antennas leads to significant attenuation of the direct path signal. In addition, the near-far target problem is reduced, while the target signal can be... 

The factor (q/a ) may be obtained from independent optical and geometrical measurements, since q is the transmittance of the attenuator used for the incident beam, a is the solid angle of detection of scattered light and is the thickness of sample. 

Spectra of solid samples are usually recorded in the units of reflectance (R) or percent reflectance (%/ ), which is analogous to percent transmittance in that reflectance equals the ratio of the reflected radiation to the incident radiation. With diffuse reflectance, the reflected signal is attenuated by two phenomena absorption (coefficient k) and scattering (coefficient s). Lollowing the Kubelka-Munk theory, these two coefficients are related to the reflectance of an infinitely thick sample, by... 

The International Union of Pure and Applied Chemistry recommends that the definition should now be based on the ratio of the radiant power of incident radiation (Pq) to the radiant power of transmitted radiation (P). Thus, A = log(Po/P) = log T. In solution, Pq would refer to the radiant power of light transmitted through the reference sample. T is referred to as the transmittance. If natural logarithms are used, the quantity, symbolized by P, is referred to as the Napierian absorbance. Thus, B = ln(Po/P). The definition assumes that light reflection and light scattering are negligible. If not, the appropriate term for log(Po/P) is attenuance. See Beer-Lambert Law Absorption Coefficient Absorption Spectroscopy... 

An infrared spectrum is a plot of percent radiation absorbed versus the frequency of the incident radiation given in wavenumbers (cm ) or in wave length ( xm). A variation of this method, diffuse reflectance spectroscopy, is used for samples with poor transmittance, e.g. cubic hematite crystals. Increased resolution and sensitivity as well as more rapid collection of data is provided by Fourier-transform-IR (FTIR), which averages a large number of spectra. Another IR technique makes use of attenuated total reflectance FTIR (ATR-FTIR) often using a cylindrical internal reflectance cell (CIR) (e.g. Tejedor-Tejedor Anderson, 1986). ATR enables wet systems and adsorbing species to be studied in situ. 

GABA is a major CNS inhibitory neurotransmitter, which, among other effects, may attenuate catecholaminergic systems. VPA (perhaps by inhibition of this transmitter s degradation by GABA transaminase), CBZ, and lithium have all been reported to enhance GABA activity (see also the section Mechanism of Action in Chapter 12). 

Many techniques are based on this principle and can be used for the analysis of all types of samples. The spectrum obtained from reflected light is not identical to that obtained by transmittance. The spectral composition of the reflected beam depends on the variation of the refractive index of the compound with wavelength. This can lead to specular reflection, diffuse reflection or attenuated total reflection. Each device is designed to favour only one of the above. The recorded spectrum must be corrected using computer software. 

These are similar to nephelometers, except that they measure the attenuation of a light beam due to the combined effects of absorption and scattering by the sample. The instrument consists of a light source, a collimator and a photo-detector. The most common application is the measurement of smoke density in chimney stacks. In this case the optical surfaces exposed to the smoke are kept clean by flows of clean air. The density of the smoke is expressed in terms of per cent opacity, per cent transmittance or optical density, where ... 

The practical value of the attenuation index is evident from the above considerations. Because of the linear dependence, any transmittancy data on commercial sugar liquors can now be reported as attenuation indices at some reference concentration. This need not be the concentration actually used in making the measurement. It appears that either a or a can be used for this purpose. 

It is important to appreciate that particular values for the wavelength exponent, n, and the attenuation index, a, at some one wave length will characterize the entire transmittancy curve in the visible spectrum of the sugar solution. When a sugar liquor is treated with a solid adsorbent, the changes in the spectrophotometric curves in the visible spectrum are adequately defined by changes in the values of n and a (see Table I). This procedure promises to be an effective tool, both in research and in applications.31... 

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