Abstract
The hypothesis is made that the dispersion electron paramagnetic resonance (EPR) spectrum can yield a higher signal-to-noise ratio than the absorption spectrum in diagnostic examinations if phase noise in the bridge is under control. The rationale for this hypothesis is based on the observation that the dispersion spectrum becomes more intense than the absorption spectrum at high incident powers. The rationale is dependent on optimization of high microwave efficiency (Λ; mT/W1/2) and low-quality factor (Q-value) sample resonators as well as the use of microwave sources with reduced phase noise. Microwave frequencies from 1.2 to 94 GHz are considered. Although the dispersion display appears to be observable with an adequate signal-to-noise ratio for most EPR research initiatives, a weakness of microwave bridges for studies at high incident microwave power was identified. Spurious leakage of incident microwave power through the circulator, thereby bypassing the probe leading to the resonator, can result in a decreased signal-to-noise ratio in both absorption and dispersion because of phase noise. For dispersion EPR with low Q-value sample resonators, this leakage is the primary contributor to phase noise at the receiver. In this work, we focus on the design of microwave reflection bridges and discuss possible methods to ameliorate this source of noise.
Highlights
In continuous wave electron paramagnetic resonance (EPR) spectroscopy, the dispersion component of the signal is seldom observed because of high levels of phase noise arising from the microwave source
This paper began with an observation from the literature that in selected circumstances the continuous wave EPR dispersion signal can be significantly higher than the absorption signal
The hypothesis was made that there could be a major opportunity for improvement of signal-to-noise ratio in diagnostic applications by observation of the dispersion signal at high microwave power
Summary
In continuous wave EPR spectroscopy, the dispersion component of the signal is seldom observed because of high levels of phase noise arising from the microwave source. At sufficiently low non-saturation power levels, the absorption and dispersion modes can be converted through a Hilbert transformation Detection of both yields 21/2 increase in the signal-to-noise ratio, which can be helpful when using oxygen-sensitive probes with narrow lines. At lower field modulation frequencies, the phase noise of the source is elevated, which can reduce the signal-to-noise ratio of the dispersion signal This problem is exacerbated when high quality factor (Q-value) sample resonators are utilized. Previous phase noise results from W-band (nominally 94 GHz) experiments are examined in the context of a hypothesis that microwave leakage through the directional coupler that interfaces the sample resonator probe to the EPR bridge can affect results in the dispersion mode. Recommendations are made to reduce phase noise at the receiver in low-frequency bridges (1.2 GHz)
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