FAST PFN-BASED DRIVER FOR X-BAND RF CIRCULATOR 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. In the context of upgrading our X/Ku microwave EPR spectrometers, consideration is being given to improved high-attenuation radiofrequency (r.f.) switching for receiver protection. As described in a prior subproject (SPID 0035), which presents a proposed circulator r.f. switch for 35GHz, we have developed a suitable means of driving a ferrite-based circulator/isolator waveguide r.f. switch on a time scale compatible with our fast pulse spectrometer requirements. A crucial element in realization of this fast r.f. switch subsystem has been the specification and design of a solid-state high-voltage driver capable of delivering an extraordinarily rapid, high value current transition through the switch?s inductive ferrite element. We are now studying the application of this driver/switch technology to receiver protection at lower frequencies. An estimate of the energy required to switch the X/Ku circulator has been made, and we have calculated the driver voltages and currents which must be attained. In order to adequately protect the receiver, our X/Ku spectrometer application requires that the driver should provide around 250 microjoules of switching energy (as opposed to 87 uJ for the Q-band device), in order to reliably switch the circulator ferrite magnitization state. This implies a rectangular pulse FWHM of approximately 25 nsec with a compliance of 1500 volts and a peak value of approximately 56 amperes. Our previous experience in simulation and evaluation of driver parameters for high-speed switching has demonstrated that a discrete pulse-forming network (PFN) would likely be the preferred solution to the challenging driver performance conditions specified. In the prior 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 might employ, based on our previous experience we will first investigate simulations of the Type A Guilleman form to determine the source and load impedances which best meet our requirements. We will then characterize the desired circulator switch parameters and progress to a more exacting model based on device specifications returned by prospective vendors. On the basis of this information, we will then conduct a final design refinement of the h.v. switch parameters for the X/Ku receiver protection application.