Reduction of line-shape distortion in super-regenerative spectrometers for nuclear quadrupole resonance

Abstract

The shapes of the sideband responses to a nuclear quadrupole resonance line obtained with a logarithmic super-regenerative spectrometer are discussed and compared with those obtained with a new type of spectrometer in which a normally incoherent super-regenerative oscillator is made coherent by injecting an external r.f. voltage into its tank circuit. The phases and amplitudes of the Fourier components of the r.f. output of the locked super-regenerative oscillator are derived and its line shape response is shown to depend on the frequency difference between the locking signal and the r.f. pulses in the super-regenerative oscillator. By controlling this difference, any desired mixture of absorption and dispersion line shapes is obtainable. Other advantages are that the frequency stability is improved, the coherence is independent of quench parameters, and sideband suppression and frequency measurement are both simplified. A practical locked super-regenerative spectrometer is described in which mechanical ganging between the tank circuits of the two oscillators is supplemented by an automatic phase control negative feedback loop. An output signal can also be obtained from the automatic phase control system and it is shown that this enables separate absorption and dispersion line shape responses to be obtained simultaneously. The actual super-regenerative oscillator circuit requires no adjustments in order to maintain full sensitivity over wide tuning ranges. The performance of the spectrometer is illustrated by recordings of 35Cl nuclear quadrupole resonance in KClO3 and in the K form of (PNCl2)4.