Pulse programmer

K. Kose, T. Haishi 1998, (Development of a flexible pulse programmer for MR] using a commercial digital signal processor board), in Spatially Resolved Magnetic Resonance, eds. P. Blunder, B. Blu-mich, R. Botto, E. Fukushima, Wiley-VCH. 

Note that you can t just use any number of transients. Many experiments require a multiple of a base number of transients to work correctly. This is due to the needs of phase-cycling which we won t describe here - once again, check other text books if you want to find out more about this. Generally you will be safe if you choose a multiple of eight as this covers most of the commonly used phase cycles although there are many experiments that can use multiples of two or even one. If in doubt, check the pulse programme or ask someone who knows. 

First, it is useful to understand what we mean by 1-D and 2-D experiments. If you consider a normal proton spectrum, it is plotted in two dimensions (chemical shift on the x axis and intensity on the y), so why is it called 1-D In fact, when NMR started, it wasn t because there was no need to distinguish it from what we now call 2-D. The dimensions that we are talking about are the number of frequency dimensions that the data set possesses. To try to understand we need to explain the basics of the pulse programme. If we take a simple example (e.g., 1-D proton) we can represent the pulse sequence in Figure 8.1. 

The individual pulse programmer operates at a clock frequency of 160 MHz, and the digital control signals can be generated with the minimum width and increment of 25 and 6.25 ns, respectively. 

In order to implement the phase-tunable DDS, the phase wheel is also shifted according to phase modulation and phase cycling. The digital signal carrying the amount of the additional phase shift is given to DDS (I) by the pulse programmer. 

Response (FIR) filters have been implemented in the FPGA. Finally, the in-phase and quadrature signal pair is stored in a memory, also built inside the FPGA, according to the acquisition phase indicated by the pulse programmer. 

The basic components of the solid state spectrometer are the same as the solution-phase instrument data system, pulse programmer, observe and decoupler transmitters, magnetic system, and probes. In addition, high-power amplifiers are required for the two transmitters and a pneumatic spinning unit to achieve the necessary spin rates for MAS. Normally, the observe transmitter for 13C work requires broadband amplification of approximately 400 W of power for a 5.87-T, 250-MHz instrument. The amplifier should have triggering capabilities so that only the radiofrequency (rf) pulse is amplified. This will minimize noise contributions to the measured spectrum. So that the Hartmann-Hahn condition may be achieved, the decoupler amplifier must produce an rf signal at one-fourth the power level of the observe channel for carbon work. 

Any spectrometer system capable of doing complicated two-dimensional NMR work will have a sufficient data system and pulse programmer. The... 

TD-NMR and HR-NMR spectrometer systems have a majority of components in common. All spectrometers consist of a magnet, magnet temperature sensors, magnet heater power supply, RF frequency synthesizer, pulse programmer, transmitter/amplifier, sample probe, duplexor, preamplifier, receiver, and ADC, all controlled by a computer. In addition to these items a HR-NMR has several other requirements which include an electromagnetic shim set, a shim power supply, and a second RF locking channel tuned to the resonance frequency of Li. The second RF channel is identical to that of the observed H channel. Figures 10.9 and 10.10 show the basic setup of TD-NMR and HR-NMR spectrometers, respectively. 

The phase cycle of both pulse programmes may be expanded four-fold or two-fold by the use of CYCLOPS [20] or 2-step CYCLOPS [21], respectively, to suppress quadrature images in the F2 dimension. 

The operating concept of PFT NMR can be recognized by following the arrows in Fig. 2.34, going from pulse programmer via probehead and computer to plotter. 

Figure 8.2.14 Schematic of the hardware set-up for acquisition of multiple signals with a single receiver channel. The TTL lines from the spectrometer are controlled from within the pulse programme and determine the position of the switch...

Lin L-J, Cordell GA (1986) Application of the SEMEPT Pulse Programme in the Structure Elucidation of Coumarinolignans. J Chem Soc Chem Commun 5 377... 

The pulse programmer is flexible and controlled by a simple scripting language that makes it easy to implement new pulsing schemes with phase cycling and multiple pulse channels. 

Fig. 12. The phase shifter/mixer unit. For each of the four rf channels the TTL logic box contains two monoflops that may activate a mixer-driver through a triple OR gate. The third input of each OR gate accepts gate pulses from the pulse programmer whose lengths are specified by software on a raster of 100 ns. The two monoflops are fired by needles from the pulse programmer and allow finely adjusted rf pulses of duration 0.3-1.5 and 0.5-3 /us, respectively, to be generated.

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