A 9-GHz, SINGLE-KLYSTRON, EPR SPECTROMETER USING SUPERHETERODYNE DEMODULATION

Abstract

A 9-GHz electron paramagnetic resonance (EPR) spectrometer, which features superheterodyne demodulation using a single klystron as the source of both the sample cavity and local oscillator (LO) power, is described. It uses a reflection cavity as part of a balanced bridge and operates in the absorption mode. The LO power is derived from the microwave carrier power source by a ferrite single side-band generator driven at a very low intermediate frequency (IF) 0(103–106) Hz. This spectrometer uses triple synchronous demodulation at the microwave, intermediate, and field modulation frequencies to overcome the instability usually associated with superheterodyne instrumentation. The sensitivity of the resulting demodulation system has a stability that is essentially independent of fluctuations in the degree of bridge balance and the instantaneous sample cavity resonant frequency, provided that the microwave power source is simultaneously phase-locked to a high stability, MHz crystal oscillator and to the instantaneous resonant frequency of the sample cavity. The sensitivity has been found to remain within 5% of a constant value for periods in excess of 30 days. No readjustments were necessary during this period because of the system stability inherent in this design. The environmental temperature of the spectrometer was maintained at 20 ± 2 °C. Experimental details are given and the results of a sensitivity study are presented graphically.