100 kHz Homodyne, 35 GHz Reflection‐Cavity Spectrometer with Phase‐Lock of the Reference Signal and Low Frequency Field Modulation

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Abstract
A 35 GHz, reflection‐cavity type microwave spectrometer is described. We use a 35 GHz ferrite modulator in the signal arm to produce a double sideband (±100 kHz) suppressed carrier signal to reduce the 1/f microwave crystal noise. Solid metal cavities and low frequency modulation of the externally applied magnetic field are used to reduce leakage from the cavity, thereby reducing susceptibility to microphonics and allowing more freedom in the choice of cavity modes suitable for a wide variety of experiments with metallic or dielectric samples. This instrument employs a 35 GHz ferrite phase shifter to phase lock the microwave signal in the reference arm of the bridge for observation of absorption signals An analogous scheme for phase‐locking at dispersion signals is also suggested. A narrow passband, low noise, active filter, with a Q factor of approximately 106, is used as part of a feedback loop around the 100 kHz receiver. This minimizes the possibility of saturation of the receiver due to less than critical coupling to the microwave cavity. Practical advantages of the present unit in low temperature spectroscopy, particularly with regard to helium bubbling and helium level change, are stressed. Comparative sensitivity tests of this unit with the standard 200 Hz magnetic field modulation systems show that, at cavity power levels smaller than 5 mW, a factor of ten improvement can be attained. At higher microwave power levels the improvement available is hampered by microphonics in the bridge. The theoretical analysis of the microwave bridge operation, of various key components, and of the electronic system is presented in the text and in the appendices.

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