This year, an article in Review of Scientific Instruments described the design, construction, and characterization of an X-band multiquantum electron paramagnetic resonance (MQEPR) microwave bridge, with MQ electron-electron double resonance and MQ electron-nuclear double resonance capabilities. The main feature of the bridge is the use of double-balanced mixers as double-sideband modulators to generate multiple irradiation fields with variable frequency separation. The microwave source is a low phase-noise Gunn diode oscillator, the frequency of which is translated by a nominal 300 q f MHz. This approach, called double-sideband/fixed filter (DSB/FF), allows the use of fixed-bandpass microwave filters to reduce incident spurious products to at least -70 dBc. Each frequency is amplified separately to avoid system-generated intermodulation (IM) sidebands in the incident irradiation. As a result, the dominant source of system intermodulation is the nonlinearity in the receiver system, consisting of a low-noise amplifier (LNA) and a double-balanced signal mixer. A detailed analysis of receiver-generated IM products is presented. The use of a loop-gap resonator with a high resonator-efficiency parameter, , and low Q is essential to achieve a balance between microwave power and system IM sidebands. It is shown that, even at maximum incident power, the levels of these sidebands can be reduced to 51 dB below the MQEPR response by switching out the LNA. This permits the extension of MQEPR applications into systems where high power is required. The operation modes of the bridge are briefly described. Alternative bridge designs are considered and compared with the DSB/FF design. It is found that the DSB/FF approach gives the best overall performance with greater flexibility and compatibility with multiple operation modes.
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