A new crossed-loop resonator (CLR) structure is described that However, since the isolation is dependent on the tuning, uses two orthogonal resonators to isolate the EPR signal from the phase noise cannot be reduced merely by critical coupling. microwave source. Resonators of this type are usually referred to Short-term instabilities of the source frequency (phase) as bimodal, which is descriptive of cavity or distributed element cause variations in the reflected microwave power. These resonators. However, it is more useful to think of the new resonator variations are detected as noise superimposed on the change as two virtually independent lumped-element resonators that have in reflection caused by the resonance phenomenon, which a common sample volume where the loops of the two resonators reduces spectrometer sensitivity and signal-to-noise ratio meet orthogonally. The first resonator excites the spins, and the (S/N). This phase noise limits the use of reflection spectrossecond resonator (or resonant section of the CLR) acts like an copy primarily to EPR absorption (amplitude) measureantenna that detects only the signal caused by the spin system. In ments. Long-term instabilities in source frequency or tuning this manner, the very phenomenom that is being studied is used to separate the desired EPR signal from the microwave source cause baseline drift. Magnetic field modulation commonly and performs the function of the circulator. The signal coupled is used to improve the stability and sensitivity of CW EPR into the second resonator is due to the spin system. The phase operation. However, field modulation can cause passage efnoise of the source is reflected from the first resonator back toward fects that can seriously distort the spectra and can add microthe source and is efficiently isolated from the second resonator phonic noise. In addition, the resonator must be constructed and the EPR signal. This resonator structure eliminates the need in a manner that allows penetration of the field modulation. for the circulator, simplifies the spectrometer circuit, virtually For pulse measurements such as electron spin echo (ESE), eliminates source phase noise in the detected signal, and allows saturation recovery (SR), and Fourier transform EPR, the dispersion spectra to be measured with the same high signal-totransient response of the reflection mode resonator to the noise (S/N) as absorption spectra. In addition, for pulse experimicrowave pulse causes an appreciable ''dead time'' during ments, this resonator greatly decreases the dead time of the instruwhich an important part of the EPR signal is lost. This paper ment and allows measurement of a portion of the signal previously inaccessible with other resonators. The CLR is rugged, easy to describes a new type of crossed-loop resonator that greatly tune, does not have to be critically coupled, and is easily adjusted decreases the impact of source phase noise and provides a to maintain source isolation when the sample is changed. α§ 1996 means of eliminating magnetic field modulation in absorp-Academic Press, Inc.
tion and dispersion EPR and decreases instrument dead time in pulsed EPR applications.