Recap and corrections for Lecture 1
a) The strength of an electromagnetic pulse depends on its length, and the power of the transmitter. As the power levels are usually defined in decibel ( a logarithmic unit), it is often easier to define the overall strength in the linear unit of “field strength”. The field strength can be derived as follows: Consider the pulse as an RF field leading to precession around the irradiation axis, then (similar to Larmor frequency definition) we obtain:
b) The rotating frame: In NMR spectroscopy we don’t measure absolute frequencies. The (MHz!) NMR signal is sent directly after detection into a mixer, which subtracts a defined reference frequency (MHz) from the signal! The resulting difference signal (Dw= w- wref , 0 +/- sw/2 Hz,) is subsequently digitized! On Varian spectrometers, the reference frequency is given as “sfrq”, which can be modified by the parameter “tof”. Thus, while nuclei precess at MHz frequencies (in the laboratory frame), we detect/digitize/see the signal in the “rotating frame”. A signal at tof/the center of the spectrum, has the same precession frequency as the reference frequency, and we obtain a FID with no frequency modulation, just decaying due to relaxation)
tipangle ?, pulse duration tp,