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

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.