rd  - 1962 - ENC Conference

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J ,/ On the Use of Amplitude Modulation in Measuring Large Chemical Shifts and in Producing Sidebands for Spin Decoupling Experiments. L. G. Alexakos and C. D. Cornwell Department of Chemistry, Univsersity Of Wisconsin Madison, Wisconsin Two common methods for generation of sidebands used in measurement of frequency separations in NMR spectra are a) modulation of the field by injection of an audio voltage in the sweep coils and b) frequency modulation of the RF generator. The first of these is generally satisfactory at low audio frequencles~ but fails above about I000 c/s~ probably because of attenu~tlon of the modulating field by eddy currents in the probe. A further inconvenience associated with this method is the need for using widely different modulating voltages at different modulation frequencies on account of the inductive reactance of the modulation coils. Method (b)~ frequency modulation of the RF generator, may be used at higher audio frequencies. With this method~ however# an error can arise if the carrier frequency is shifted by the modulation. If the measurement is made by placing sidebands on either side of the line 3 this error can become very serious at higher modulation frequencies 3 since relatively large frequency deviations are required for the generation of sldebands of adequate amplitude when the modulation frequency is high. Thus with the frequency modulation system of the varian ~310C Spectrometer# we have observed a substantial shift in the position of a fluorine resonance line as the amplitude of the modulation voltage is changed. The change in average frequency of the RF generator as measured with a frequency counter was found to be well over I00 c/s. A similar observation was made with the ~II spectrometer. We have employed pure amplitude modulation of the RF generator for the measurement of splittings over a very wide range of modulation frequencies~ with excellent results. The circuit arrangements used in the V4310C and V~311 are shown in Fig. I (a) and (b) respectively. The modulating voltage is introduced at the grid of a multiplier stage, at a point well isolated from the ascillator itself. Sidebands~ generated in this manner show little change in amplitude as the modulation frequency is changed from 20 c/s to 60 kc/sec at a constant modulation voltage. (The lower frequency limit could presumably be reduced by increasing the sizes of the coupling capacitors.) As would be expected for pure amplitude modulation I no effect is detectable in the frequency as measured by a counter except for the expected decrease in apparent frequency observed when the percentage modulation approaches I00~ at which point some of the cycles are "missed" by the counter. Frequency spllttings from several hundred cycles to over 40 kc/sec have been measured with this method with excellent reproducibility. Consistent
-2- results are obtained even with a large percentage modulation and a very distorted modulation envelope, since the carrier frequency remains strictly constant even though the apparent average measured by a counter decreases.
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rd  - 1962 - ENC Conference

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