An injection and phase-locked super-regenerative NQR spectrometer

Abstract

The details of the design and construction of an injection and phase-locked super-regenerative oscillator (SRO) type NQR spectrometer with signal averaging capabilities and working in the frequency range 15-40 MHz are presented. The injection voltage at the SRO centre frequency f0 is generated from a carrier-suppressed modulator (CSM) which is used to modulate the output of a variable frequency oscillator VFO at f1 with that of a fixed frequency oscillator (FFO) at f2.f1 is set such that f0=f1+f2 or f1-f2. This scheme avoids the problems of leakage radiation leading to uncontrolled injection when one derives the injection signal from a VFO set at f0. Phase-locking is achieved by phase sensitive detection of the f2 component obtained by mixing the SRO output at f0 and the VFO output at f1 and feeding the DC error voltage to a varactor in the SRO. NQR signals can be obtained by demodulating the f2 component. A 'lock box' design concept has been adopted so that any oscillator can be phase-locked in principle and resonance signals obtained directly from the 'lock box'. Typical 35Cl NQR spectra recorded with the spectrometer system are presented along with an assessment of its performance.