Pulse noise generated

RS232 serial port or IEEE488 interface bus). The plug-in cards use the computer power supply which tends to contain noise generated by all of the digital pulses generated in the computer. Higher bit conversion would not introduce improved measurement accuracy because of this noise. 

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. 

The principle of this pulse method and its general equations are easily extended to the case of several components in a mixture. The method was used by Lindholm et al. [24] to determine the quaternary isotherms of the enantiomers of methyl- and ethyl-mandelate on the chiral phase Chiral AGP. One of the serious roadblocks encountered in the use of the pulse tracer method is that the amplitudes of most of the system peaks decrease rapidly when the plateau concentration increases. Since the signal noise increases in the same time, it becomes rapidly impossible to make any accurate measurements of the retention time of these peaks. On the basis of fundamental work by Tondeur et al. [114], the origin of this variation of the relative intensity of the system peaks was explained by Forss n et al. [47], who then derived an effective rule to determine the composition of a perturbation pulse that generates system peaks that are detected easily. The concentrations of the components in the injected perturbation pulse should... 

Unfortunately, pulse piekers have some severe drawbacks. One is that the minimum rate reduetion faetor is about 10. Repetition rates around 40 or 20 MHz are thus not available. Moreover, there is a leakage in the intermediate pulses of the order of 1%. For the first suppressed pulse the leakage can be even higher. Finally, most pulse piekers generate RF noise, which is synchronous with the pulses and ean severely impair the differential linearity of a TCSPC system. 

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