Broadband IF Circuits for Superconductor Isolator Superconductor (SIS) Mixers for the Heterodyne Instrument for the Far Infrared on the Herschel Space Observatory and for Sideband Separating Mixers

This thesis describes the development and manufacturing of the IF circuits for two different heterodyne receivers used for sub-mm radio astronomy. The first project is the broadband 4-8 GHz IF circuitry for the mixer units for band 2 of the Heterodyne Instrument for the Far Infrared (HIFI). This instrument will be operating onboard of ESA�s Herschel space observatory. The band 2 mixer units for HIFI, built by KOSMA, implement low noise SIS mixers in a very compact, modular design. The complete IF path in the mixer units consist of the mixer block (with the SIS mixer chip) and the bias-T, which is needed to apply a bias voltage to the SIS-junction and to extract the IF signal. The detailed investigation of the IF performance of the mixer block must consider the geometric capacitances of the SIS-junction and the tuning structure as well as the inductance of the bond wires used to establish the contact between the mixer chip and the SMA output connector. The calculations presented in this thesis show that the design can be used for IF frequencies up to 12 GHz. The bias-T, which is implemented in the mixer unit, is a dedicated, new development with high performance and high reliability complying with the special requirements for a spaceborne instrument. These include the survival under high mechanical stress such as vibration during the launch and extensive thermal cycling to cryogenic temperatures during the tests of the mixer unit and the instrument. An excellent (S21 > -0.25 dB) IF signal transmission of the bias-T is achieved by special high reliability connections in the signal path. Different coefficients of thermal expansion are compensated by the mechanical flexibility of the connections. These special parts have been developed, manufactured, integrated and tested within this work in a three phase design cycle (Demonstration Model(DM) - Qualification Model (QM) - Flight Model (FM)). In addition to the high transmission, the bias-T provides filter circuits, which effectively protect the signal path from electromagnetic interference (EMI) entering the mixer unit via the unshielded bias lines. The mixer units have passed all performance and environmental tests and are presently integrated into the HIFI instrument. The second project is the development and fabrication of very broadband nonuniform couplers, which can be used as -3 dB 90°-hybrids on the IF side of sideband separating mixers with large IF bandwidth. By the smooth (e.g., nonuniform, not stepped-uniform) change of the even- and odd-mode impedances along the wave propagation direction, uncontrolled reflections are avoided, which is indispensable to achieve a large bandwidth. Strong power coupling (-3 dB, e.g., 50%) is achieved with broadside coupled lines in a homogeneous stripline configuration, which ensures TEM mode propagation. This TEM mode propagation and the smooth distribution of the electrical parameters allow to achieve a large bandwidth. The impedances can be calculated by Fourier transformation of the power coupling bandshape. In order to determine the actual geometry (e.g., stripline width and spacing) from even mode line impedances, a design procedure has been developed, which is presented in this thesis. It includes an appropriate mix of analytic calculations and an interpolation scheme which uses results from a commercial 2D EM-field solver. This procedure has been successfully applied in order to design various couplers, which have been manufactured and tested. By introducing mode launchers in the vicinity of the coupler ports, the TEM mode purity could be improved, which led to an increased coupler bandwidth of 1.9-9.3 GHz.