FAST PFN BASED DRIVER FOR FERROMAGNETIC R F SWITCH

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Successful realization of the Center's high-power 35 GHz FT-ESR pulse spectrometer mandates inclusion of fast, high-attenuation radiofrequency (r.f.) switching for receiver protection. As described elsewhere (Subproject: Development of Receiver Protector for 35 GHz Pulsed ESR), we have previously developed a prototype ferrite-based circulator/isolator waveguide r.f. switch for receiver protection in the 35 GHz frequency regime. A crucial element in realization of this fast r.f. switch subsystem is our design and assembly of a solid-state high-voltage driver capable of delivering an extraordinarily rapid, high value current transition through the switch¿s inductive ferrite element. In order to adequately protect the receiver, this ultrafast-switching application requires that the driver must provide a rectangular pulse of FWHM 20 nsec with a compliance of 1200 volts and a peak value of approximately 25 amperes. Our previous experience in simulation and evaluation of driver parameters for high-speed switching has been utilized in determining that a pulse-forming network (PFN) would be the preferred solution to the challenging driver performance conditions specified. In the preliminary design analysis, we noted that the PFN's inherent charge/discharge symmetry could also be exploited to provide the needed complementary reset pulse. Of the possible PFN topologies in combination with the various characteristic impedances which we simulated for evaluation and optimization, we have determined that the Type A Guilleman form at 36 ohms best meets our requirements. When coupled to a suitably fast h.v. switch, we observe that our prototype 3-stage PFN driver causes the circulator/isolator to switch incident r.f. energy in a notably short interval of 10 ¿ 20 ns. We are presently characterizing the 35GHz receiver protection requirements in terms of the spectrometer bridge r.f. parameters and, on the basis of this information; we will then conduct a final design refinement of the h.v. switch parameters for this application.