Rokeakh, A.I., and M. Yu. Artyomov. “Continuous Wave Desktop Coherent Superheterodyne X-Band EPR Spectrometer.” Journal of Magnetic Resonance 338 (May 2022): 107206.
This article describes a desktop X-band superheterodyne spectrometer designed to measure EPR spectra in the vicinity of g = 2. The spectrometer operates in the vicinity of the 9.2 GHz frequency in the power range of about 40 mW–1 nW. The device uses Time Locking concept with digital quadrature detection of intermediate frequency (100 MHz) and the synthesis of all signals used in the device from a single driving oscillator. To register CW spectra, a TE102 rectangular cavity with the capability of matching and frequency tuning is used. The microwave part of the spectrometer is implemented in the form of two non-tunable modules based on microstrip technology and commercially available microwave MMICs. Two modes of operation are provided – both with PLL of microwave sources with a highly stable driving oscillator as a reference and an AFC system without frequency modulation. All signal processing after digitization of the intermediate frequency is performed digitally using FPGA resources. It is possible to record EPR spectra both with magnetic ﬁeld modulation at various frequencies and without modulation. The bandwidth of the receiving path can be selected from the widest 46 MHz to about 1.5 kHz, which allows the spectrometer to be used both for recording weak signals and for recording fast-ﬂowing processes. The spectrometer uses a magnetic system on permanent magnets Sm-Co with magnetic ﬁeld uniformity of about 3 * 10À5. Changing the magnetic ﬁeld in permanent magnets is done using scanning coils which provides a ﬁeld change of >800 G (p-p). The control of the magnetic ﬁeld induction in the magnet gap is performed by a specially designed precision Hall effect magnetometer, which provides a resolution of 1 mG with an absolute error of no more than 0.2 G. A personal computer for spectrometer control is connected via Ethernet. The spectrometer is stable and easy to use. The high performance of the spectrometer is conﬁrmed by the recorded spectra.