ULTRA-WIDEBAND 170 AND 240 GHZ CW SOURCE SWEEPING SYSTEMS

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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. Verification and tuning procedures related to bridge setup for ESR spectroscopy are, in general, greatly facilitated by swept-frequency measurement of transmitted/reflected power. Swept-frequency evaluation methods are especially useful in the millimeter-wave regime, and even more so when quasioptical techniques are employed, mainly due to the numerous interacting adjustments and enhanced quasioptical bridge susceptibility to extra-path reflections. As useful as swept measurement capability is, off-the-shelf test and measurement instrumentation providing phase-locked swept-frequency capability at even the lower millimeter-wave regime (i.e., below 100 GHz) is inordinately expensive and, above that frequency range, appears to be available only on an even more expensive custom-built basis. For this reason, ACERT had not previously invested in millimeter-wave swept-measurement capability for its 170 and 250GHz c.w. spectrometer bridges. Last year, ACERT acquired significantly improved millimeter-wave sources of center frequencies at 170 and 240 GHz for its high-field c.w. quasioptical ESR spectrometers. These units generate approximately 12dB more output power than the previous c.w. sources and provide phase-locked operation over a relatively broad frequency range of fc ¿ 5 GHz. In view of the significantly higher output power available from these sources, an extensive program of bridge and resonator design upgrades was undertaken for optimized bridge operation in inductive-reflection mode. During the course of this development work, it became evident not only that swept-frequency measurement capability would be indispensable, but also, conveniently, that these extended-range c.w. sources might be pressed into service for such incidental use. Our work to date in this area has resulted in an interim 170GHz swept-frequency system which has proven extremely useful in the setup and fine-tuning of the H.F. reflection bridge. We have found that, although the swept output power is not actively leveled, the source multiplier stage bias levels may easily be tuned to provide better than 1.2 dB flatness at fc ¿ 1.0GHz and approximately 3dB over the entire source tracking range from 165 to 175GHz. This performance level is more than sufficient for general bridge and resonator tuning procedures. Given the encouraging results thus far, we plan to continue this project over the coming year with similar investigation of the new 240GHz tracking c.w. source for swept measurements. During the coming year, we plan to continue with the design, construction, installation and programming of a simplified operator interface for switching between normal c.w. operation and the swept-frequency diagnostic modes.