Swept-frequency continuous-wave signal

In a continuous-wave NMR (CW NMR) experiment the HY frequency is slowly scanned and each nucleus resonates at its specific frequency, which is a function of the examined nucleus and the electron density that surrounds it. This experiment can be long and tedious because if high resolution is desired, the frequencies must be swept very slowly. If the concentration of the examined nucleus is low, a large number of repeated sweeps may be needed to obtain a detectable signal. 

Answer The spectrometer was sweeping the magnetic field to find a lock signal, so the resonant frequencies were changing rapidly during the collection of the FID. One observes the same ringing pattern when continuous wave spectra are swept too fast. If one tries to collect several transients under these conditions, one obtains many copies of the spectrum superimposed at different frequencies. The field sweep can be turned off if a lock is not going to be used. 

The most widely used and in many cases most sensitive way of recording EPR spectra is continuous-wave (CW) EPR. In this experiment, microwave with constant frequency is irradiated with relatively weak power (typically between 1 jlW to 200 mW) and the magnetic field is swept to Irring the transitions into resonance. As broadband microwave detection by diodes is used, the detection bandwidth has to be limited by other means to avoid accumulation of excessive noise over a wide frequency range. This is achieved by low-frequency (typically 100 kHz) modulation of the magnetic field with an amplitude of 0.01-1 mT and phase-sensitive detection of the signal component modulated with this frequency. As a result, the derivative of an absorption lineshape is measured, which is better resolved than the absorption lineshape itself The technique is disadvantageous if all features of the absorption lineshape are much broader than the maximum modulation amplitude that can be technically achieved. 

The technique we have discussed so far is the continuous-wave (stationary) method. For a constant magnetic field, the frequency of the oscillator, that generates a field with an amplitude of about lO" T, is slowly swept and the signal is recorded as a function of frequency. An NMR spectrum in the frequency domain is thus obtained. 

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