Carrier-wave frequency

These results led us to analyze the relationship between carrier-wave frequency and power density. We developed a mathematical model (6) which takes into account the changes in complex permittivity of brain tissue with frequency. This model predicted that a given electric-field intensity within a brain-tissue sample occurred at different exposure levels for 50-, 147-, and 450-MHz radiation. Using the calculated electric-field intensities in the sample as the independent variable, the model demonstrated that the RF-induced calcium-ion efflux results at one carrier frequency corresponded to those at the other frequencies for both positive and negative findings. In this paper, we present two additional experiments using 147-MHz radiation which further test both negative and positive predictions of this model. 

Values of P. used in previously reported experiments on calcium- ion efflux from chick-brain samples at carrier-wave frequencies of 50, 147, and 450 MHz. 

Specific values of P. used in previously reported experiments on calcium-ion efflux from chick-brain samples are listed in Table I under the carrier wave frequency used in the experiment (3,4,1 ). In addition, values of P. which result in the same E in the sample have been calculated for the other carrier frequencies using Equations (3) and (4). For example, 3.64 mW/cm2 at 50 MHz, 0.83 mW/cm2 at 147 MHz, and... 

The relationship between effective power densities at different carrier frequencies can be viewed in a different manner by plotting carrier frequency versus incident power density (Figure 3). Lines connecting P. values which produce the same internal electric field intensity at 50, 147, and 450 MHz are as shown. This figure serves to illustrate the smaller range of values as carrier-wave frequency is increased from... 

Keeping the average electric field intensity the same within a spherical model of chick-brain in buffer solution at different incident carrier wave frequencies requires that incident power density be changed with frequency to compensate for the change in complex permittivity and wavelength with frequency. The resulting Equations (3) and (A) relate corresponding values of P. at carrier frequencies of 50, 1A7, and A50 MHz. 1... 

Modulation Process of superimposing the analytical signal on a carrier wave. In amplitude modulation, the carrier wave magnitude varies according to the variations in the analytical signal in frequency modulation, the carrier wave frequency varies with the analytical signal. 

In many telemetry systems the PCM signal is frequency modulated on to a carrier wave. This is termed frequency shift keying (FSK). In other arrangements, the output of the transducer is converted into fixed step changes of the phase of the modulating signal. A device for this purpose is termed a modulator/demodulator or modem. 

Microwave switches are beam-breaker-type point sensors with an accuracy of 13 mm (0.5 in.) and with pressure and temperature ratings up to 28 bar (400 psig) and 300°C (600°F). Pulse-type radar gauges have ranges up to 200 m (650 ft) and are accurate to 0.5% FS, whereas frequency-modulated carrier wave (FMCW) units have errors from 1 to 3 mm (0.04-0.125 in.). Their pressure and temperature ratings are up to 80 bar (1,200 psig) and up to 400°C (750°F). 

Frequency modulated carrier wave (FMCW) radar transmitters. (Courtesy of Thermo Measure Tech Inc.)... 

Figure 2. Comparison of (A) lr Cd from freshly isolated chick cerebral hemispheres exposed to a weak radiofrequency field (147 MHz, 0.8 mW/cm2), amplitude-modulated at low frequencies, and (B) 45C2 efflux changes from exposure to far weaker electric fields (56 V/m) in the same frequency spectrum from 1 to 32 Hz. The peak magnitude of the efflux change is similar for the two fields, but opposite in direction. For the radiofrequency field (A), the unmodulated carrier wave U had no effect when compared with controls C. Field gradients differ by about six orders of magnitude between (A) and (B) (22, 23).

The first term in Equation (11) is the unmodulated carrier wave, of amplitude kjE, and the second term is a signal at the modulation frequency, of amplitude k2E2 m, generated by the nonlinear dependence of E on E. oc... 

The field intensities, or wave amplitudes, that have produced changes in calcium-ion efflux at typical carrier frequencies and at typical modulation frequencies may be used to estimate the amount of nonlinearity that would be required to conform with observed results. Adey (1 1) has reported that a relatively small internal electric field intensity, on the order of 10"7 V/cm, at 16 Hz is sufficient to alter the binding of calcium ions in brain tissue. The internal field intensity of the carrier waves shown in Table 1 are on the order of 10-2 V/cm. From these observations, the ratio of the amplitudes of the first two terms in Equation (11) is... 

To obtain envelope equations, one expresses the field in terms of an envelope by factoring out the carrier wave at a chosen reference angular frequency wr with the corresponding wave-vector Air = A1(0,0,wr) ... 

Up to the scaling factors An this sum represents a periodic spectrum in frequency space. If the spectral width of the carrier wave Au>c is much smaller than the mode separation ojr, Eqn. 7 represents a regularly spaced comb of laser modes with identical spectral line shapes, namely the line shape of C(u>) (see Fig. 1). If C(oS) is centered at say uic then the comb is shifted from containing only exact harmonics of u)r by uic. The center frequencies of the mode members are calculated from the mode number n [23,24,21] ... 

On the other hand, there are measurements in which signal and noise cannot be directly filtered and the signal has to be transposed onto a carrier wave to be shifted away from the noise frequencies (modulation). Then an amplifier is tuned to the frequency of the carrier wave and the amplified original signal is finally recovered (demodulation). The use of a chopper in optical spectrophotometers is a common example of this process [i]. Ref [i] Horowitz P, Hill W (2001) The art of electronics. Cambridge University Press, Cambridge... 

An alternative method for excitation of nuclei over a range of chemical shifts is by irradiation with a weak, noise-modulated radio-frequency, instead of with strong r.f. pulses. In one realization of this method, protons were irradiated with repetitive sequences of noise that was truly random,162 and, in another,163 fluorine nuclei were excited by pseudo-random noise generated by amplitude modulation of the r.f. with maximum-length sequences of pulses from a computer or shift register (a series of flip-flop devices connected by feedback loops). With the carrier wave suppressed, the latter process is equivalent to phase modulation of the r.f. by+7r/2 radians when the pulse is turned on, and by —ir/2 radians when it is turned off. This method is identical with that used in most broadband, heteronuclear, noise decouplers, except that greater power is required for decoupling. 

Modulation is defined as the changing of some property of a carrier wave by the desired signal in such a way that the carrier wave can be used to convey information about the signal. Properties that are typically altered are frequency, amplitude, and wavelength. In AAS. the source radiation is amplitude modulated, but the background and analyte emission are not and are observed as dc signals. 

Figure Ila shows how an ideal cosine amplitude modulation of the RF carrier wave could be approximated by a rectangular RF pulse scheme, which is much easier to implement. Such a scheme comprises of pulses with alternating phases of 0° and 180° and is referred to as FAM. As was already mentioned, the modulation frequency should be tuned such that Vm matches tq, at least during part of the excitation. Due to the sample spinning, the quadrupolar splitting of many crystallites will pass through the v n value. It was shown that mismatches between and the powder i/qS do not create large phase distortions and simultaneous adiabatic and direct coherence transfer processes result in relatively pure MQ SQ transfers. By pure we mean that no significant phase dispersions are observed when looking at the transfer of each crystallite separately.

A very effective signal sensitivity enhancement scheme for MQMAS as well as for MAS is the DFS introduced by the group of Kentgens.The underlying principles of DFS and FAM are similar and they have been exphcitly dealt with already. In a nutshell, a cosine amplitude modulated RF carrier wave irradiates the sample at two frequencies, namely, ioq -F w, and aio — Larmor frequency of the spins and is the frequency of modulation. This means that if satellite transitions of a static spins-1 system will be simultaneously... 

We can learn more from this analogy between radio systems and NMR spectrometers. In radio, the desired information in the audio frequency range is combined with the carrier wave which is in the rf range. This process of combining the lower frequency information with the carrier is called modulation. The modulated carrier wave is sent out... 

In this method, quartz piezoelectric crystals with a fundamental frequency near 10 MHz are glued to a specimen with flat and parallel opposite faces (Fig. 1). With suitable electronics (Fig. 2), high-frequency, phase-coherent pulses are gated from a continuous carrier wave and applied to the quartz transducer, which both transmits pulses and receives reflected pulses. From the round-trip transit time, t (typically 5 tsec), and the specimen length, / (typically 1 to 2 cm), the sound velocity is z = ll/t. Elastic-stiffness constants are computed by C - where p is mass density. Different waves (longitudinal, transverse, extensional, torsional, etc.) are generated by various transducers, and are related to different elastic constants. 

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