Sweep pulse “chirp

The frequency sweep resulting from a linearly varying GD(v) of the pulse in Figure 7.4c is called a chirp. This pulse is much longer that the respective 20 fs FTL pulse of the same bandwidth (Figure 7.4b). As any optical material shows dispersion... 

As a consequence of the spectral modulation, the temporal width of the laser pulse is modified and a linear frequency sweep (chirp) is introduced. The modulated pulse duration is... 

Figure 7 Parts of 1,1-ADEQUATE spectra of cholesteryl acetate acquired with 13C offset at 100 ppm and covering the sweep with of 200 ppm. (A) rectangular (B) inversion and composite chirp pulses. From Ref. 28, reproduced by permission of John Wiley and Sons.

From equation (8) three modes of adiabatic spin inversion using RF pulses may be defined as (a) amplitude modulated pulses, e.g. I-BURP [13], G3[16], I-SNOB [17], (b) frequency modulated pulses, e.g. chirp [18,20], tangential sweep [20,21] and (c) both amplitude and frequency modulated pulses, e.g. the hyperbolic secant [22] or WURST (Wide band Uniform Rate Smooth Truncation) [23] pulse. 

Figure 1 Diagram of the chirped-pulse, Fourier transform microwave spectrometer showing the generation of the the linear frequency sweep which is bandwidth multiplied and amplified. The inset shows the pulse in both time and frequency space. The pulse is broadcast into the chamber through a double-ridge microwave horn and the free induction decay (FID) is collected with another microwave horn. The FID is downconverted to the 0.5 - 11.5 GFIz range before being recorded by the oscilloscope.

Chirped pulse amplification is achieved using a pulsed traveling wave tube amplifier (TWTA, Amplifier Research 1000TP8G18) with peak power or 2 kW (7-18 GHz). The final pulse is shown as an inset in Figure 1. The spurious signals in the pulses we create using this technique are at least 20 dB lower in power than the instantaneous sweep frequency across the full 11 GHz range of the pulse. 

An adiabatic pulse is a special type of shaped pulse where either a frequency or a phase sweep occurs during the pulse duration. Adiabatic pulses are discussed in detail in section 5.3.1. So far the simulations involving the Bloch module have not considered the exact time related frequency sweep of a shaped pulse yet it is this factor that determines if each point in the pulse shape obeys the adiabatic condition. Using an adiabatic chirp pulse Check its 4.3.33 and 4.3.3.6 will examine various aspects of adiabatic pulses starting with the time evolution and the graphical representation of the amplitude and phase modulation. 

Figure 5.72. Zero-quantum interference in selective ID TOCSY spectra, (a) Partial spectmm and the corresponding regions of ID TOCSY recorded with 80-ms DIPSI-2 (b) without zero-quantum suppression and c) with the zero-quantum dephasing scheme shown in Fig. 5.74 employing adiabatic smoothed CHIRP pulses with 40 kHz sweep widths and durations of IS and 10 ms.

Figure 8.38. Regions of 400 ms NOESY spectra recorded (a) without and (b) with the inclusion of the zero-quantum filter shown in Fig. 8.37. The ZQC suppression employed a 20 ms adiabatic smoothed CHIRP pulse with a 40 kHz frequency sweep.

Figure 10.10. The simulated inversion profiles of (a) 20% smoothed CHIRP, (b) WURST-20 and (c) 25-ps haid 180° pulse. For both (a) and (b), a 60 kHz sweep was employed over 0.5 ms pulse with 7Bi(max/...

If several different masses are present, then one must apply an excitation pulse that contains components at all of the cyclotron frequencies. This is done by using a rapid frequency sweep ( chirp ), an impulse excitation, or a tailored waveform. The image currents induced in the receiver plates will contain frequency components from all of the mass-to-charge ratios. The various frequencies and their relative abundances can be extracted mathematically by using a Fourier transform which converts a time-domain signal (the image currents) to a frequency-domain spectrum (the mass spectrum). 

Very recently, the WURST-QCPMG sequence was developed to achieve uniform excitation of quadrupolar nuclei across very wide bandwidth using frequency-sweep chirped pulses [86]. The WURST (wideband uniform-rate smooth truncation) pulses provide broadband excitation. Integrating the WURST pulses with QCPMG results in the WURST-QCPMG method, which has proven to be a highly efficient approach for acquiring ultra-wideHne NMR spectra [74]. 

---