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lecture4 - Today's Lecture 4 Wed Oct 9 Data Collection a...

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A. S. Edison University of Florida 2006 Today’s Lecture 4) Wed, Oct 9: Data Collection a. Digitization and Spectral Width b. Fourier Transform c. Quadrature detection
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A. S. Edison University of Florida 2006 What happens next? 1) A sample with a single resonance (e.g. water) is placed in a strong static (B 0 ) magnetic field and the sample reaches equilibrium. 2) An rf B 1 field is applied at a frequency 10 Hz away from the single resonance in the sample. The B 1 field is applied along the x-axis long enough to rotate the magnetization vector from the z-axis into the x-y plane. Then the B 1 field is turned off.
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A. S. Edison University of Florida 2006 Precession and relaxation Mx My Mz Mz Mx My t t t
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A. S. Edison University of Florida 2006 Detection The sample rotates in the x-y plane at a frequency of 10 Hz and relaxes in the x-y plane (T 2 ) and along the z-axis (T 1 ). The rotating magnetization induces a current in the same coil which produced the B 1 field pulse. This current is amplified and detected in the spectrometer. However, the analog signal is converted to digital by sampling the data at select time points:
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A. S. Edison University of Florida 2006 Time Domain Signal The time domain signal is collected point by point and can be thought of as a vector d = d 0 , d 1 , d 2 , …, d M-1 with M points. To make the math work, the count begins at 0. For example, here is a 10 Hz signal sampled for 0.2 seconds with 0.01 sec spacing between points.
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