Reference signal delay

In the last method, which is the most commonly used, the multiplier is replaced by an electronic switch controlled by the reference signal. The switch changes the amplification of the signal s(t) from +1 to —1 (for example +1 when the reference is positive and — 1 when it is negative), as shown in Fig. 10.10(a). When there is no phase delay between the... 

Most common lock-in amplifiers can be operated at frequencies ranging from a few Hz up to 100 kHz. This fact is important in analyzing the temporal evolution of optical signals for example, fluorescence decay time measurements. Although this particular application of lock-in amplifiers is beyond the scope of this section, it is instructive to mention that this can be done by tuning the relative phase (the time delay) between the signal intensity and the reference signal provided by the chopper. 

The synchronous demodulator (4) eliminates the quadrature signal. This demodulator multiplies the input signal with a reference signal, which is generated by the sensor drive velocity. The signal conditioner (7) corrects any time delay and phase shift, because the reference signal has to relate in the phase exactly to the velocity of the sensor drive. 

A glycogen solution placed in the emission compartment will scatter light and be used as reference (tf = 0) to determine phase delay and fluorescence demodulation For each measurement, the intensity of the reference is adjusted in order to have it equivalent to the intensity of the fluorescence signal of the sample. Phases and modulations of the fluorescence and scattered light are obtained relative to the reterence photomultiplier or instrumental (internal) reference signal. In fact, two Identical detection electronic systems are used to analyze the outputs of the reference and sample photomultipliers. Each channel consists of an alternative and continuous cunents (AC and AD, respectively). 

Fig. 2.15 Reversed start-stop. Left undelayed reference signal, right delayed reference signal. The laser pulse which released the photon is marked black. With an appropriate delay in the reference channel the time of the photon is measured against the correct laser pulse...

The resulting signal shape is a frequent souree of eonfusion. Sometimes the step at time A is even mistaken for the rise of a fluoreseenee signal. Of course, the reeorded curve is absolutely correct. The photons left of the eutoff point, A, are not lost. They were recorded shortly before the previous reference pulse and appear where they should be, i.e. in the late part of the period, B, at the right end of the photon distribution. The curve can be eentred in the reeorded time interval by adjusting the signal delay in the detector or reference channel. 

Assume that Gp has at least one-sample time delay and the reference signal has sufficient persistent excitation. Since the direct identification method is the same as the open-loop identification method, the prediction error should be the same. Thus,... 

CN] —> I + CN. Wavepacket moves and spreads in time, with its centre evolving about 5 A in 200 fs. Wavepacket dynamics refers to motion on the intennediate potential energy surface B. Reprinted from Williams S O and lime D G 1988 J. Phys. Chem.. 92 6648. (c) Calculated FTS signal (total fluorescence from state C) as a fiinction of the time delay between the first excitation pulse (A B) and the second excitation pulse (B -> C). Reprinted from Williams S O and Imre D G, as above. 

The relaxation rates of the individual nuclei can be either measured or estimated by comparison with other related molecules. If a molecule has a very slow-relaxing proton, then it may be convenient not to adjust the delay time with reference to that proton and to tolerate the resulting inaccuracy in its intensity but adjust it according to the average relaxation rates of the other protons. In 2D spectra, where 90 pulses are often used, the delay between pulses is typically adjusted to 3T] or 4Ti (where T] is the spin-lattice relaxation time) to ensure no residual transverse magnetization from the previous pulse that could yield artifact signals. In ID proton NMR spectra, on the other hand, the tip angle 0 is usually kept at 30°-40°. 

The variable delay can be as simple as an RC network. Often the variable delay line is calibrated directly in terms of lifetime units (nanoseconds). When the reference and comparison signals are in phase the fluorescence lifetimes can simply be read off the calibrated variable delay. 

The current version required recording three different pulse sequences, such as / cross-talk (i.e. an "even signal" in the "odd subspectrum" or vice versa caused by deviation, A/, of an actual 1Jch from the value used in setting the r delays) are minimized.69 It is however possible to obtain edited spectra by using one reference and one up-down sequence. 

The above equation is very similar to the cross-ambiguity function, but here the time-delay is introduced only in the transmitted signal. This form of the transform is more convenient for digital implementation and will be referred to below as a range-Doppler correlation function. 

Practically, the phase delay and the modulation ratio mR of the light emitted by the scattering solution (solution of glycogen or suspension of colloidal silica) are measured with respect to the signal detected by the reference photomultiplier. Then, after rotation of the turret, the phase delay r/ F and the modulation ratio mF for the sample fluorescence are measured with respect to the signal detected by the reference photomultiplier. The absolute phase shift and modulation ratio of the sample are then — [Pg.179]

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