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Analysis, design and realization of an RF driving system for space-based Acousto-Optical Tunable Filter applications
Book - Dissertation
ALTIUS (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere) is a three-channel spaceborne spectral imager bound to fly aboard a PROBA satellite. The ALTIUS project (satellite and instrument) is developed under the supervision of the European Space Agency (ESA). The ALTIUS instrument will make hyperspectral images of the limb of the Earth. To do this, the instrument will use different techniques such as direct limb viewing, star occultation as well as sun occultation. It will measure in the visible, near-infrared and ultraviolet spectral domain. In the visible and near-infrared channel an AOTF (Acousto-Optical Tunable Filter) will be used for the selection of the appropriate optical wavelength. These AOTFs allow to scan at a fast rate through the complete spectral band. An AOTF is based on a birefringent crystal that gets an amplified Radio Frequency (RF) signal injected via a transducer. The latter converts the RF energy into soundwaves, creating an optical filter effect in the crystal. Based on different models found in literature, a theoretical transducer concept, together with a properly matched external impedance network, is calculated and simulated in this study. Also the relationship between the optical diffraction efficiency and electrical bandwidth of the impedance matching network is examined.Depending on the optical range (visible, near-infrared or ultraviolet), RF frequencies from 40 MHz up to 250 MHz are needed to drive the AOTFs. RF generation techniques available today for space applications, are not capable of generating frequencies exceeding 200 MHz. In this work, an in-depth trade-off study is performed of different RF generator techniques, taking into account the technical requirements for ALTIUS. A Phase-Locked Loop (PLL) based analog solution, a Direct Digital Synthesizer (DDS) integrated in an FPGA, and several others are discussed. The PLL-based RF generation technique is proposed for ALTIUS. It will be demonstrated that this technique is capable of generating the required high frequencies in a stable and accurate manner.For the visible and near-infrared channel, low-power space grade control electronics to drive the AOTFs are developed, each with their own specifications for resolution, sensitivity, frequency range, electrical and optical performance. For the ultraviolet channel a Fabry-Pérot system will be used to select the optical wavelengths instead of an AOTF. Nevertheless, the investigation on how to develop a high frequency RF chain in the ultraviolet for space flight is interesting as this could be useful for future (space) applications, and it is thus described in this work.All three RF systems are developed such that they can survive multiple years in a space environment (temperature, radiation, vibrations, EMI/EMC-demands). To achieve this, specific Electrical, Electronic and Electro-mechanical (EEE) components have been selected, the PCB-design was carried out in accordance to 'space qualified' ESA standards and extended test programs were executed. After the concept study and the design of the channels, also prototyping and on-ground testing were performed in preparation of building flight models and integration in the instrument in a later stage.During the design of the three RF chains, the focus was on low-power consumption, low-volume and survivability in space. The PLL RF generator module is first subjected to stand-alone tests, and is then used in integrated tests per channel (combination RF generator, RF amplifier and AOTF). All tests are discussed in this work.Besides ALTIUS, the designed PLL-based RF chains, and especially the UV RF chain, are also interesting for other (space) projects. In this work four additional applications are discussed, three of which can be transformed to space qualified instruments on short notice.The concept of using an AOTF in a spectrum imager in space applications is innovative. The development of suitable RF systems implies some technological challenges and/or difficulties such as high frequencies, broad frequency spectrum, limited power, stability and quality of the generated signal and temperature control. All these challenges have been tackled effectively and are discussed extensively in this work.