Synchroniser frequency

As we relax in preparation for and pass into sleep, the active desynchronised awake EEG characterised by the low-amplitude (5-10 pV) high-frequency (10-30 Hz) beta waves becomes progressively more synchronised giving larger (20-30 pV) and slower (8-12 Hz) alpha waves, and then even slower (1-4 Hz) and bigger (30-150 pV) delta waves. This so-called slow-wave sleep is interrupted at intervals of some 1-2h by the break-up and desynchronisation of the EEG into an awake-like pattern. Since this is accompanied by rapid eye movements, even though sleep persists and can be deeper, the phase is known as rapid eye movement, REM or paradoxical, sleep. It is a time when dreaming occurs and when memory may be secured. 

The state of conscious awareness, with orientation of self in time and space, depends on hnely tuned and accurately co-ordinated activity in multiple neuronal networks in the brain (Park Young, 1994). Such activity involves parallel processing in many cortical and subcortical pathways including arousal and memory systems (Chapters 3 and 4) and systems involved in mood (Chapters 5 and 18) and utilises an orchestra of many neurotransmitters. The whole ensemble appears to be synchronised by high frequency (40+ Hz) oscillatory electrical activity which binds the component parts together (Llinas et ah, 1998 Tallon-Baudry Bertrand, 1999). 

The angles a and P define the orientation of the sample relative to the Bo-field cor denotes the rotation frequency. For the REDOR reference experiment, the rotor-synchronised spin-echo experiment for the S nuclei (cf. Figure lA), the dipolar Hamiltonian integrated over one rotor period Tr averages to zero... 

Despite numerous applications, conventional CRAMPS still remains one of the most demanding solid state NMR experiments as it requires the use of specially prepared spherical samples to minimise radiofrequency inhomogeneity effects and the careful calibration and setting of pulse widths and phases. Further modifications of the experiment that do not require the complicated and extended set-up procedures have been suggested recently. These are known as rotor-synchronised CRAMPS, which combines a new multiple pulse sequence [21], and its modification which uses a standard WHH-4 sequence at ultrafast MAS frequencies (e.g. 35 kHz) [22]. 

For spinning frequencies slower than or equal to the characteristic RF frequency (ujr < ujc with 1/ > k), k/v is less then 1. Since n = 1, 2 and k is an integer (it is an index), only when k/v equals 1 or 1/2 elements iT -uk/v exist that can contribute to Ho. This corresponds to the synchronisation conditions with Tr/Tc = = 1,2. For all other t /tc ratios the only element contributing... 

In common with all living matter, the brain is subject to rhythmic changes in its functional state. The most common, and most marked rhythms are the 24-h rhythms , so called because they are synchronised by the 24 h light-dark cycle of the environment (for examples of such rhythms see Richter, 1964 Mills, 1966 Friedman and Walker, 1968). Many of these rhythms can be shown to exist solely as a result of environmental factors, and disappear in the absence of any external time cue (Zeitgeber). Others are endogenous rhythms, which under normal circumstances are conditioned to an exact 24 h cycle by external Zeitgebers. However, in the absence of time cues, these rhythms do not disappear, but revert to their true frequency, which only approximates to 24 h. These are the true circadian rhythms. 

Correlation down to 100 ns is usually enough to resolve diffusion times and intersystem crossing. Nevertheless, cross-correlation data at a shorter time-scale can be obtained by using two TCSPC modules with synchronised macrotime clocks (see Fig. 5.120). Synchronisation can be achieved by using the Sync signal, i.e. the laser pulse repetition frequency, as a macro time clock for both modules. This synchronisation works up to about 100 MHz, so that times down to 10 ns can be correlated. 

The amplified rf power is fed to a step recovery diode housed in a microwave structure with output in an intermediate frequency range, typically low GHz. That range is not critical and can be chosen to be compatible with existing or low cost equipment used in other fields, e.g. radars and satellite downlinks. The present authors, for example, chose the band 11-15 GHz as it permitted the use of low cost coaxial components, synchroniser and YIG oscillator for the locking steps. 

Figure 3.7 Schematic diagram of a phase-sensitive detector at 20 MHz. The transformer is replaced by an active circuit in the HP8709A synchroniser, and probably most high precision configurations. The phase error voltage output is amplified and used in the spectrometer to control the YIG oscillator magnetic field and hence lock the YIG source frequency to the synthesiser frequency. An identical device locks the Gunn MMW source to the YIG frequency (Adapted from Connor )...

The important point is that the beat frequency carries the phase relationship between the synthesiser and the YIG oscillator outputs. The function of the synchroniser is to reduce that phase difference to zero by sending a control signal to the YIG oscillator to adjust its frequency until that zero phase difference is achieved. At that point the YIG oscillator will be phase-locked to the synthesiser and thus have its characteristic stability and resettability, viz. 25 X 0.1 Hz resolution. 

The next stage is to lock the Gunn oscillator that is the actual spectral source for the measurement at, let us suppose, 62.520 GHz. This is achieved in the same way the YIG oscillator output is used to drive a varactor multiplier diode held in a MMW structure that couples a fraction of the signal from the Gunn spectral source. The beat frequency at around 20 MHz, between the 5th harmonic of the YIG oscillator at 62.500 GHz and the Gunn oscillator at around 62.520 GHz, is passed to a second synchroniser. 

A much simpler expression for the peak height is obtained, however, if a digital modulation of the form shown in Figure 4.2B is applied in which Av(r) jumps between peak positive and negative values Av with an intermediate pause at zero. This is synchronised to waveform A and detected by waveform C. If the source is tuned to the peak frequency Vq its modulated frequency simply jumps back and forth between the line peak and a symmetrical offset on each side, generating a square wave of angular frequency 2(o and amplitude ... 

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