Electron paramagnetic resonance Q-band bridge with GaAs field-effect transistor signal amplifier and low-noise Gunn diode oscillator

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Abstract
A Varian Q‐band E‐110 microwave bridge for electron paramagnetic resonance (EPR) spectroscopy has been modified by addition of a low‐phase noise Gunn diode oscillator of our own design, a low‐noise GaAs field‐effect transistor microwave signal amplifier, and a balanced mixer requiring high input power (10 mW) at the local oscillator port. The oscillator has previously been found to have −129 dBc/Hz phase noise, 22 dB lower than for the original klystron. Noise measurements indicate that the microwave amplifier and mixer reduce the overall receiver noise figure by 24.6 dB, a very significant improvement. It is shown that reduction of both phase noise and receiver noise are required in order to achieve full improvement in signal‐to‐noise ratio over the full range of available microwave power. Spectra of 1.6×10−6 M 15N‐perdeutero TEMPONE (1‐oxyl‐2,2,6,6‐tetramethyl‐4‐piperidone) and of 10−6 M Mn2+ are shown in order to demonstrate sensitivity.