Modulation source amplitude

For many applications, there may be some advantage in employing phase modulation 54,85) instead of the usual amphtude modulation. In the latter technique the path of the radiation from the source to the detector is blocked and opened periodically by a chopper (cf. Fig. 20 and Section 4.3). For phase modulation, the chopper is removed from the spectrometer and the fixed mirror of the Michelson interferometer is moved back and forth about its mean position with a certain frequency. In contrast to the interference modulation (see Section 4.2), the amplitude of the mirror motion is small, being a quarter of the wavelength of the light. For the analogue Fourier transform or interference modulation, the amplitude of the mirror has to have many wavelengths in order to achieve a reasonable resolution... 

As the amplitude of the modulating source increases, the frequency of the carrier increases. 

Luminescence lifetimes are measured by analyzing the rate of emission decay after pulsed excitation or by analyzing the phase shift and demodulation of emission from chromophores excited by an amplitude-modulated light source. Improvements in this type of instrumentation now allow luminescence lifetimes to be routinely measured accurately to nanosecond resolution, and there are increasing reports of picosecond resolution. In addition, several individual lifetimes can be resolved from a mixture of chromophores, allowing identification of different components that might have almost identical absorption and emission features. 

Extended X-ray absorption fine structure (EXAFS) A technique for observing the local structure around a metal centre, using X-rays from a synchrotron source. The atom of interest absorbs photons at a characteristic wavelength and the emitted electrons, undergoing constructive or destructive interference as they are scattered by the surrounding atoms, modulate the absorption spectrum. The modulation frequency corresponds directly to the distance of the surrounding atoms while the amplitude is related to the type and number of atoms. In particular, bond lengths and coordination numbers may be derived. 

EPR spectra were recorded with a Varian E9 X-band spectrometer using field (100 kHz) and light (13 or 83 Hz) modulation with phase-sensitive detection at the modulation frequencies (19). Typically, the field modulation amplitude employed ranged from 20 to 40 gauss, the microwave power from 0.1 to 0.5 mW. Measurements were performed on frozen solutions of the porphyrins at about 100 K using the standard Varian variable temperature accessory or at about 10 R with an Oxford Instruments helium gas cryostat. Light sources used for photoexcitation were a 1000 W Xe arc source powered by a Photochemical Research Associates Supply with electronic modulation... 

Assuming that the rate of change of the optical retardation introduced by the interferometer is the same for all the input radiation frequencies (which is normally the case), each individual value of v in the broadband source output contributes a different value of /m in the a.c. component of the detector output (see Figure 3.20). These contributions are, of course, summed by the detector. The combined detector output (see Figure 3.21) arising from the simultaneous measurement of all modulated input signals, is a sum of cosine functions. Such a sum is a Fourier series, and the amplitudes of the individual... 

Mcadams [McAdams, 1984] hypothesizes that die tracing of resonant amplitude by frequency modulation contributes to die distinctness of the sound in the presence of competing sound sources. 

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