Receiver dead time

Users of any NMR instrument are well aware of the extensive employment of what is known as pulse sequences. The roots of the term go back to the early days of pulsed NMR when multiple, precisely spaced RF excitation pulses had been invented (17,98-110) and employed to overcome instrumental imperfections such as magnetic field inhomogeneity (Hahn echo) or receiver dead time (solid echo), monitor relaxation phenomena (saturationrrecovery, inversion recovery, CPMG), excite and/or isolate specific components of NMR signals (stimulated echo, quadrupole echo), etc. Later on, employment of pulse sequences of increasing complexity, combined with the so-called phase-cycling technique, has revolutionized FT-NMR spectroscopy, a field where hundreds of useful excitation and detection sequences (111,112) are at present routinely used to acquire qualitatively distinct ID, 2D, and 3D NMR... 

First, either a Lorentzian or Gaussian filter is applied to the FID to reduce the amount of noise. The choice of lineshape will depend on the shape of the frequency domain spectrum, the lineshape is related to how the fluorine spins interact with their environment. The filter linewidth is generally similar to or slightly less than the T2 value (T2 can be estimated from the spectral linewidth). After application of the time domain filter, a fast Fourier transform (FFT) is performed. The resultant frequency domain spectrum will then need to undergo phase adjustment to obtain a pure absorption spectrum. The amount of receiver dead time (time lost between the end of the excitation pulse and the first useful detection time point) will determine the presence and extent of baseline artifact present as well as how difficult phase adjustment will be to accomplish. 

Pulsed NMR measurements have been made as a function of temperature to study the pyrolysis behaviour of oil shales. The method is sometimes referred to as NMR Thermal Scanning. To overcome receiver dead time, a 9O-T-9O90 pulse sequence is used to form the solid echo . The echo signal is then decomposed into a rigid (short relaxation time) and a mobile (longer relaxation time) component. These data are then related to various properties of the system. Parameters that can be extracted from the NMR data relate to the hydrogen content, phase structure, molecular mobility, and free radical content. By measuring the temperature dependence of these parameters... 

The problem of phase correction for NQR spectra presents a particuar problem not found in fixed-frequency spectrometers. The fact that the NQR spectrometer operates at variable frequencies means that for NQR signals, instrumental phase shifts which are not corrected for before final data collection will be present. These phase shifts can be considerable, and are of course also present in echoes, where the receiver dead time problem present for FID signals does not occur. In Table 2, the mathematical results for Lorentzian line shapes are collected as guides to the solution of this problem. 

A) Simulated FID after a 90° pulse and 30 gsec delay corresponding to the receiver dead-time, and the Fourier transform of the FID (B) Simulated quadrupolar echo and its Fourier transforia starting at the echo maximunii... 

Experimentally it is best to use a spectrometer whose electronics have solid state capabilities since it is necessary to have short 90° pulse lengths (typically 5-15 /is, which demands up to 1 kW rf power) and as short as possible a receiver dead-time. Even so, rolling base lines will interfere seriously with lines that are more than 1 kHz wide when single 90° pulses are used, but special pulse sequences are now available" which can substantially eliminate many of these problems. Effects due to spin coupling are normally eliminated by quadrupolar relaxation, but in SF V( S- F) is 251.8 Hz, and this symmetrical species yields" a beautiful binomial septet for its spectrum, the central line having a Tx of 10 s. In the sulfone I /( S-H) is ca. 6 Hz and in the absence of proton decoupling the spectrum is a poorly resolved triplet. As a general rule in NMR Ti = Ti and can be obtained... 

The next step is to calibrate the spectrometer. Most commercial software have automatic procedures for calibrating the parameters of the spectrometer, such as resonance frequency, receiver dead time, receiver gain and pulse length/amplitude. More advanced systems may come with automated shim programmes. Calibrations can be made using test samples. 

Hie FID of polystyrene (PS) at room temperature shown in Figure 12(a) is a quickly decaying Gaussian function (with the initial part being unobservable due to the finite receiver dead time), which corresponds to an equally Gaussian-shaped, broad spectmm in the frequency domain. 

Dead time A very short delay introduced before the start of acquisition that allows the transmitter gate to close and the receiver gate to open. Density matrix A description of the state of nuclei in quantum mechanical terms. 

The NMR experiment performed is the simplest one imaginable. The H nuclei in the sample are irradiated with a single square pulse of RF energy ( 1 W) lasting about 10-12/rs. After waiting a dead time of 10/rs, to allow the coil to physically stop oscillating from the power input it experienced, the receiver is... 

Another important feature regarding the whole receiver chain (including the probe and the preamplifier) is the total dead time which must be kept as short as possible in order to allow measurements of NMRD profiles of solids with extremely fast decaying FIDs. 

The distance over which pneumatic signals can be transmitted is limited by the volume of the tubing and the resistance to flow. The dynamics of pneumatic systems can generally be approximated by a first order lag plus a dead time (Sections 7.S and 7.6). Tubing may be made of copper, aluminium or plastic, and is normally of S mm ID. Pneumatic receivers can be in the form of indicators, recording devices and/or controllers. 

Adjust the receiver s dead time (the time between the end of the pulse and the beginning of acquisition of the signal) to minimize pulse breakthrough, which is manifested by a baseline roll. Certain spectrometers carry out this operation in the following way. First, a spectrum is recorded and phased as usual. Then, a software command calculates the dead time such that the first-order (left) phase control equals zero in a repeated spectrum. 

The value of t (which is the total time of the pulse and the window after a single pulse) should be set as short as possible in order to increase the resolution. The pulse length, the dead time of the receiver, and the sampling time should be included in It. In our work, we set the r value to about 3 ps for the BR-24. For the MREV-8, on the other hand, we set t to be less than 2.5 ps. 

In nonreversed start-stop systems the TAC is started with the laser pulse. When a photon is detected, the TAC is stopped and the signal proeessing starts. Signal processing takes some time, here indicated as TAC/ADC dead time . After the processing is completed the TAC does not convert another photon pulse until it has received the next start pulse from the laser. For the eounting loss, two eases have to be distinguished ... 

The electronic components needed for processing the detector signals have evolved into standardized modular units, and are relatively simple to select. One important factor when using multi-channel analyzers is the analyzer dead-time. Typically, a multi-channel analyzer receives a pulse, digitizes that pulse, and stores... 

One technique that is often used to alleviate the effects of receiver recovery is the solid echo sequence (Figure 7.2(a)) [18], where a second 90° pulse (phase shifted by 90° with respect to the first) is applied to the system after an interval that is slightly longer than the dead time. This generates an echo... 

Relaxation data on quadrupolar halogen nuclei may be obtained either from pulsed NMR studies or from line width measurements. The lower limit of T.j and T2 values that can be measured with a pulse spectrometer is set by the effective dead time of the receiver system after a pulse and by the width and amplitude of the pulse. At present... 

An open volume, high isolation RF system suitable for pulsed NMR and EPR spectrometers with reduced dead time has been described. It comprises a set of three RF surface coils disposed with mutually parallel RF fields and a double-channel receiver (RX). Theoretical and experimental results obtained with a prototype operating at about 100 MHz have been reported. Each surface RF coil (diameter 5.5 cm) was tuned to 100.00 0.01 MHz when isolated. Because of the mutual coupling and the geometry of the RF coils, only two resonances at 97.94 MHz and 101.85 MHz were observed. These were associated with two different RF field spatial distributions. In continuous mode operation the isolation between the TX coil and one of the RX coils (singlechannel) was about —10 dB. By setting the double-channel RF assembly in subtraction mode the isolation values were optimised to about —75 dB. The described system was selected as a model for potential applications in solid-state NMR and in free radical EPR spectroscopy and imaging. 

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