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.

https://iopscience.iop.org/article/10.1088/0022-3735/14/7/021/pdf

An injection and phase-locked super-regenerative NQR spectrometer

The starting frequency within the range of the oscillator used, the width of each frequency step and the number of measurements within each step can be chosen. The computer produces the desired frequency which is compared with the actual spectrometer frequency shown by an 8- (or 7-) display digit frequency counter. After digital-to-analogue conversion via an add-and-hold circuit, the difference frequency signal tunes a free-running oscillator; this synchronises a super-regenerative oscillator, which is sensitive to the NQR absorption. The spectra obtained are calibrated to within +or-3 (or+or-10) Hz and are exactly linear with respect to frequency.

A pocket computer frequency-controlled NQR spectrometer

A new and improved method for continuous frequency calibration of the super‐regenerative oscillator spectrum is described. The arrangement employs a Hewlett–Packard Model HP5345A reciprocal frequency counter capable of measuring the carrier frequency of rf pulses. The counter gate time is externally controlled via the quench pulses. High‐resolution measurements are obtained through the use of the frequency‐averaging mode provided by the counter. Three successive digits are displayed on a two‐channel recorder using a digital‐to‐analog converter along with the lock‐in output. This method provides automatic and continous high‐accuracy frequency calibration of the spectrum which is independent of the rf level, quench frequency, and FM amplitude.

Automatic continous frequency calibration for super-regenerative oscillators used in nuclear quadrupole resonance.

A digital system for a programmable frequency synthesiser is described. It performs a staircase frequency sweep with a step of 1 Hz and digitally programmed bidirectional square-wave frequency modulation in an NQR spectrometer with a frequency-locked super-regenerative oscillator. The system is particularly convenient when using digital averaging techniques. NQR spectra parameters are not influenced by frequency modulation even when extremely large frequency modulation deviations are used. Parameters such as the sweep rate and the amplitude of frequency modulation deviation are stable, accurately fixed and independent of the running frequency. NQR spectra of 35Cl in p-dichlorobenzene and 65Cu in Cu2O were plotted against frequency by an X-Y recorder.

M Ostafin

Papers

Digital frequency modulation in an NQR spectrometer