Instrumentation for remote control of the unloaded quality factor, Q0, of a loop-gap resonator is described. The value of Q0 can be lowered by weak inductive coupling of the resonator to a loop of wire that contains a carbon resistor. Replacement of the resistor by a PIN diode permits remote control. Theoretical analysis and experiments are carried out in a context of electron paramagnetic resonance (EPR) spectroscopy at X band. Equivalent circuits are solved numerically and predicted performance confirmed experimentally. Two applications are demonstrated: (a) superfine control of critical coupling of the incoming transmission line to the resonator in excess of −80 dB, and (b) dynamic Q spoiling for reduction of the dead time after a microwave pulse. Possible degradation of EPR system performance by shot noise from the PIN diode is considered. Scant literature suggests that it is very low, and noise from this source could not be detected experimentally.

Articles you may be interested inThe description of the multiquantum effects in electron paramagnetic resonance spectroscopy using the Bloch equationWe describe 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 300rAf 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. 4 detailed analysis of receiver-generated IM products is presented. The use of the loop-gap resonator with a high resonator efficiency parameter, A, 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 brietly 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. 0 I995

Two low phase-noise Gunn diode X-band oscillators intended for use in electron paramagnetic resonance (EPR) spectroscopy are described. In the first, a 250-mW MA49159 Gunn diode oscillator (M/A-COM, Burlington, MA) is mounted in a coaxial transmission line that is closely coupled to a TE011 transmission cavity that in turn is loosely coupled to the output transmission line. The output power is 50 mW and the phase noise is −145 dBc/Hz at 100 kHz offset. In the second, two such coaxial assemblies are used with 500-mW MA49110 diodes for increased power. The output power is 150 mW and the phase noise is −150 dBc/Hz at 100-kHz offset. These phase noise values are in the range of 24–29 dB better than the specification for a normal high quality klystron used in commercial spectrometers.

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