An improved super-regenerative spectrometer for nuclear quadrupole resonance spectroscopy.

Abstract

A super-regenerative spectrometer has been constructed which possesses effective sideband suppression together with automatic gain control and direct frequency calibration of the recorder chart. An analysis of the super-regenerative circuit shows that the gain of the circuit depends mainly on the length of the damping period; consequently, by making the damping period independent of quench frequency, relatively little gain change occurs when the latter is varied in order to suppress the sidebands. The gain may therefore be simultaneously controlled by changing the damping period and damping efficiency by a feedback loop which maintains constant noise output. Frequency measurements are greatly simplified by providing circuitry which operates a separate marker pen on the recorder chart whenever the spectrometer frequency zero-beats with a set of harmonics from a standard frequency source. A small delay in the marker channel then ensures that sideband zero-beats are not printed when sideband suppression is in use; otherwise, the quench frequency is made a small multiple or sub-multiple of the quench frequency. As an example of the applications of the new instrument part of the 35Cl spectrum of PCl5 at 77°K is shown, with direct calibration of the frequency scale; signals are clearly resolved which have been missed by previous workers.