Phase modulation waveform

Figure 20.14. Square-wave phase modulation waveform used by Drapcho et al. (Reproduced from [14], by permission of the Society for Applied Spectroscopy copyright 1997.)...

Siegel et al. showed that enhancement of the CT can also be obtained using hyperbolic secant (HS) pulses to invert selectively the STs [74], Unlike the DFS waveform, whose frequency sweep is generated by a constant rf-pulse phase while modulating the amplitude, the HS pulse utilizes both amplitude and phase modulation, yielding an enhancement exceeding that obtained by DFS or RAPT [61, 74, 75]. Most recently, the pulse sequence called wideband uniform-rate smooth truncation (WURST) [76] was introduced to achieve selective adiabatic inversion using a lower power of the rf-field than that required for the HS pulses [77,78]. One of its applications involved more efficient detection of insensitive nuclei, such as 33S [79]. 

FT/ICR experiments have conventionally been carried out with pulsed or frequency-sweep excitation. Because the cyclotron experiment connects mass to frequency, one can construct ("tailor") any desired frequency-domain excitation pattern by computing its inverse Fourier transform for use as a time-domain waveform. Even better results are obtained when phase-modulation and time-domain apodization are used. Applications include dynamic range extension via multiple-ion ejection, high-resolution MS/MS, multiple-ion simultaneous monitoring, and flatter excitation power (for isotope-ratio measurements). 

In a CPA, because a large secondary dispersion is added to the stretcher, the spectrum is mapped to the temporal waveform in the amplifier. For that reason, the phase change added to the pulse shaper cannot cause a large change in the pulse shape in the CPA. Self-phase modulation depends on the temporal amplitude transition. Therefore, the phase perturbation can be regarded as almost linear in the amplifier. Thus, the inverse of the spectrum phase difference between the acquired and the target phase is added to the pulse shaper. 

Illustrated in Fig. 10.151, this system uses DC-to-AC conversion through phase modulation of two square wave high-frequency signals to create an output waveform. The waveform then is filtered to remove the carrier signal and feed the load. 

The basic transmission principles used today for frequency modulation falls into two categories. One type, direct FM, is the modulation process where the frequency of the transmitter oscillator varies in accordance with the amplitude of the modulating signal. The other type, indirect FM, obtains a fi equency modulated waveform by phase modulating the carrier. 

Drapcho et al. [14] demonstrated a very elegant way of performing multifrequency PA measurements. Rather than using a PM waveform composed of the sum of several sinusoidal waves, they drove the phase-modulation mirrors with a square wave, as shown schematically in Figure 20.14. A square wave with frequency / hertz is composed of the odd harmonics of / ... 

We saw in Chapter 20 that phase modulation could be achieved by dithering the interferometer mirror with a waveform that was the sum of several sinusoids. 

Chen, L. Wang, T. C. L. Ricca, T. Marshall, A. G. Phase-modulated stored waveform inverse Fourier transform excitation for trapped ion mass spectrometry. Anal. Chem. 1987, 59, 449 54. 

Phase and modulation information up to Np/2 harmonics are present when Appoints per waveform are sampled. Letfy be the maximum frequency sampled, then... 

Kay It is external coincidence. Both are required. If you activate phytochrome A (phjA) and cryptochrome 2 cry 2) in wild-type plants at the end of the day in short days this has no effect on flowering because the phase of CO expression is such that it is at too low a level. It really seems to be entrainment regulating CO waveform. This is where Bunning in the 1940s was brilliant in proposing how this could work. This story will get even prettier when we fulfil the prediction that this light is leading to some post-translational modulation of CO protein or potentially a CO partner. 

One of the most convenient methods for determining lifetime, as well as one of the best suited to low-cost applications, involves using frequency-modulated excitation (49). Upon excitation by a frequency modulated source, the finite lifetime of the emitter causes a phase-shift and demodulation of the emission relative to the excitation waveform as shown schematically in Fig. 6. 

Figure 6. A schematic representation of a modulated excitation wave form (solid line) and the time delayed modulated emission waveform (dashed line). The parameters Atp and AA represent the change in phase and the change in modulation amplitude of the two wave forms.

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