A versatile Q-band electron paramagnetic resonance spectrometer

The design, construction, and operation of a Q-band (34.5-35.5 GHz) electron paramagnetic resonance (EPR) spectrometer system that is versatile to accommodate several sample irradiation and signal detection configurations is described. The spectrometer system that was developed includes a microwave bridge, synthesizer array, receiver, and signal processing unit. The main feature of the microwave bridge is the use of frequency translation technology to translate the fundamental microwave frequency, nominally 33 GHz, to spectrally pure multiple time-locked closely-spaced frequencies, nominally centered about 35 GHz, from an array of time-locked microwave synthesizers. A spectral purity of all incident spurious products < -65 dBc has been achieved. The design strategies to this end are described. The main features of the receiver are low-noise, low intermodulation products, and synchronous down- conversion to a nominal 1 GHz first intermediate frequency (IF). This first IF can then be downconverted either to baseband or to a nominal second IF of 12.8 MHz. The noise figure from the sample port of the circulator to the output of the IF preamplifier is approximately 7.5 dB. The main feature of the signal processing is direct analog-to-digital (A/D) conversion of the 12.8 MHz IF using a time-locked sub-sampling (TLSS) approach for subsequent analysis on a computer. EPR information at multiple microwave frequencies of interest can now be collected in a single sweep of the DC magnetic field. The automatic frequency control (AFC) system in both single-frequency and multiple-frequency environments is described.