FREQUENCY-STEPPING SYSTEM FOR 95 GHZ ESR SPECTROMETER

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. For both DEER (double electron?electron resonance) and extended-coverage ELDOR at 95GHz, a method of dual-stepped-frequency operation is desired. The ability to conduct DEER investigations is important because there is significant biological motivation in expanding distance measurement techniques developed at conventional frequencies at ACERT to the high frequency regime. The unique ACERT 2D-FT ESR 95GHz 1kW pulse ESR spectrometer holds the promise of being able to access a wide range of samples including macroscopically-aligned biological specimens, single-crystals with goniometry, and microliter-sized samples. The enhanced orientation selection of high-field ESR spectrum is valuable in structural studies, with both DQC and DEER techniques presently under consideration for this purpose. The implementation of DEER requires, however, irradiating spin systems sequentially at two frequencies. Extended-coverage ELDOR at 95GHz may be accomplished by either field-jump or frequency-stepping techniques, but the desirability of making DEER distance measurements at 95GHz gives us added incentive to implement a dual-frequency capability in order to address both types of experimental work. For our high-power 95GHz pulse spectrometer, the addition of stepped-frequency operation is facilitated by the ca 500 MHz instantaneous bandwidth of the existing transceiver/power amplifier chain. Moreover, the transceiver reference oscillator section in our instrument was designed with an external input which directly accommodates an external dual-frequency source. Consequently, adding an external system for stepped-frequency work is straightforward. Our realization of the external dual-frequency system is based on a fixed reference PLDRO at the nominal 95GHz center frequency, plus a synthesized offset providing, after SSB conversion and multiplication, a 95 ¿ up to 0.4 GHz signal at the transceiver's output port. A 10ns microwave switch is employed in the frequency reference module to rapidly select either frequency source, with shaping of the actual millimeter-wave pulses performed within the transceiver following frequency multiplication. Timing for the frequency selection switching is provided by a spare output line from our existing 95GHz spectrometer high-resolution (PBHR) timing system. Design of the external dual-frequency source is complete and the components, including the frequency-specific PLDRO, have been ordered and received. Construction, evaluation and experimental use of the step-frequency source is scheduled for this summer.