• SSC98-VII-2 • • Feasibility Considerations for High Temperature Superconducting Transponders in Communication Satellites • Author: Nicholas Seet Contributors: Matthew Owings, Brian Fudge, Mikhail Pinelis, Brian hleib • Faculty Advisor: Dr. Patrick Little Harvey Mudd College • Claremont, CA, USA 91711 • ABSTRACT year, and much of this paper is derived from that project. • This paper provides a comprehensive, background-rich framework around which a • fully developed study of the use of 2. APPROACH superconducting transponders can be generated • for a specific communication satellite. The By replacing the satellite's transponder approach divides results into two categories: with a HTS version, the low ohmic loss that signal benefits, and physical constraints. Each superconductivity provides improved signal-to­ • is realized in terms of metrics and methods noise ratio (SNR), and increases bandwidth. warranted by the current state of superconductor This improvement comes at the cost of the • technology. active cooling system required to bring the The paper describes methods by which electronics down to superconducting signal perfonnance of superconducting temperature (70 K). The two distinct fields in • transponder components may be approximated; which the integration of HTS components also methods of evaluating heat generated by affects the satellite are thus in signal • such a system; and finally, the selection of an perfonnance and physical characteristics. appropriate etyogenic cooling system. • 3. PRINCIPLES 1. INTRODUCTION Superconductors are functionally • With the proliferation of defined as materials that conduct current with communication satellite applications, from negligible resistance. These materials only act • digital television to global mobile connectivity, as superconductors at very low temperatures, great demand has been placed on the strictly traditionally no greater than 20K. However, in allocated bandwidth that satellite manufacturers 1986, a class of ceramics was discovered that • may purchase from the FCC. In order for acts as superconductors at temperatures vendors to maximize bandwidth usage, various approaching 90K. This relatively high • signal modification approaches have been temperature makes these materials attractive implemented, such as data compression, or since it is readily achieved using cryogenic • multi-phase transmission, but a method through techniques. Furthermore, HTS materials offer which additional bandwidth can be gained significant advantages over more standard remains the holy grail of the satellite materials in the field of electronics and signal • communications industry. processing; these potential advantages are This paper evaluates the methods by briefly discussed below. • which the feasibility of the use of High One potential use of superconducting Temperature Superconductor (HTS) materials is in filters. Filters are devices that Transponder for a particular communication attenuate all frequency components not • satellite can be determined. These methods contained in a particular range of frequencies were applied to a Harvey Mudd College called the passband. The frequency content of • (Claremont, CA) Clinic project for Space the passband is output to the next component in • Systems/Loral during the 1997/1998 school the system. However, to gain an idea of how • I • • filters work, we must first understand what a ambient environment, thus requiring some sort • resonator does. of cooling method. The second drawback is that A resonator is a component in which fITS materials exhibit non-linear characteristics, • electromagnetic energy of a particular frequency as discussed below. is stored, that is, resonates. Resonators are For all superconducting materials, there • typically described in terms of their quality is a critical temperature below which they must factor, abbreviated Q. This dimensionless be reduced in order to behave as parameter is proportional to the amount of superconductors. However, reducing the • energy stored in a resonator divided by the temperature of the materials below the critical amount of energy dissipated. The higher the Q temperature is not sufficient for superconducting • factor, the more efficient the performance of the behavior; the magnetic field around the material resonator. Typical microstrip circuits fabricated must be held below a critical value as well. with normal metals have a Q factor in the low Furthermore, since electric current produces a • hundreds. Filter designs such as waveguides magnetic field this property limits the amount of may have a Q factor of a few thousand. current a superconductor can support. The • However because of their extremely low result is that there is a limit on the power resistance, superconducting resonators can have handling capabilities of a superconducting • Q factors in the tens of thousands. component. These properties are summarized in A microwave filter consists of a series Figure 1 (paul K., Chen M. IEEE Spectrum., of resonators, each tuned to a slightly different May, 1998 p49) • frequency in the filter's passband. The more resonators used in a filter with a given passband, T • the sharper the drop-off in signal strength at the edges of the filter, and the greater the ofiband rejection. However, simply adding more and • more resonators to a filter to increase performance is problematic in that each • additional resonator adds to the insertion loss, defined as the loss of signal strength in the J • passband due to the insertion of the filter into the system. The lower the Q factor of a resonator the greater the insertion loss. The • insertion loss must be kept under a particular value for acceptable performance of the system. • Furthermore, adding additional resonators to a filter increases the weight, size, and complexity Figure 1 States of a Superconductor. Zero­ of the filter. • resistance (inside Sl), transition state Thus, the advantage of using (between Sl and S2), normal conducting superconducting materials in filters is their (beyond S2). • superior Q factors as compared to standard materials. For a given maximum insertion loss These properties make fITS components fundamentally nonlinear, since doubling the • in a filter one can use a much greater number of superconducting resonators than normal metal input power may not result in a doubling of the resonators. This gives the filter far superior output power. This problem must be resolved • performance in terms of oftband signal for each specific filter before it can be used in attenuation. Further advantages stem from the the transponder. • fact that superconducting filters give comparable performance to normal metal filters, but in a 4. COMMUNICATION SATELLITES much smaller package. Using superconducting • materials can thus save both size and weight, The most general description of the two very precious commodities on satellites. functionality of a communication satellite • fITS materials have two primary transponder is captured in Figure 2 . drawbacks. The first is that they must be • operated at temperatures much below that of the • 2 • • • • • The primary reason for using IITS components in communication satellites is to • improve the signal processing characteristics of the satellite. Thus a large part of any feasibility • Power analysis is quantifying these improvements, and Amp determining if they are worth the difficulties that result from the use of IITS materials in Figure 2 Transponder Block Diagram • space. The incoming signal, weak due to • propagation losses and atmospheric reflections, 5.1 Research must be amplified before being processed by all subsequent blocks. The preamplifier must thus While high temperature • superconductivity has been studied for many provide low noise amplification and exhibit good linearity over the entire uplink frequency years, its application to microwave • range. communications components is fairly new. A After amplification, the signal must be literature search was conducted to identifY the • converted to the downlink frequency range by state of the industry developments and academic the frequency downconverter block. Since this research. block is dealing with the entire bandwidth of the Attempts were made to find • signal, good linearity of the mixer is necessary; components that were exact matches for the otherwise, the frequency converter would components in a generic transponder. In some • produce inter-channel modulation in the output cases, parts were found in the correct frequency signal. range and were space-qualified. In other cases, At this point, the signal is divided into parts were from other frequency ranges or were • channels for individual processing. Each one-of-a-kind items. Contact with industry and channel has its own input multiplexer block. research representatives confirmed under what • This block is a band-pass filter with a center situations extrapolations could be made. frequency tuned to that channel's frequency. The commercial market for IITS • Each filter must have vel)' strong stop-band microwave components is mainly limited to rejection, as well as a fairly flat pass-band and filters and amplifiers for the cellular base station group delay response. industry. These components are for the 800 • Each channel also has its own power MHz and 1900 MHz frequency ranges. A amplifier block. Once the signal has been number of companies were found that • filtered, it is amplified at high power for manufacture base station preselect filters. retransmission. It is important