Development of a Sideband Separating SIS Mixer Technology for MM-Wavelengths

Doktorsavhandling, 2003

This thesis presents work done in developing instrumentation for mm-wave radio astronomy. The main subject of the thesis is a sideband separating (2SB) receiver for 85-115 GHz based on a quadrature scheme with two identical SIS mixer junctions pumped by a LO applied with 90° phase difference. The design is based on a layout where the quadrature scheme is implemented using waveguide based and integrated on chip components. This type of scheme requires balanced power division for both LO and RF channels. The key component in the mixer is a waveguide to microstrip double probe transition used as a power divider to split the input RF signal and to provide transition from waveguide to microstrip line. The double probe transition enables the integration of all mixer components on a single compact substrate. The design also involves coupled lines directional couplers to introduce the LO power to the mixer junctions with the required phase delay. Additional SIS junctions are used to provide termination load for the idle port of the couplers. The design of all mixer components is presented in detail together with different techniques used in the calculations. The measurement methods specific for the 2SB mixer are described and illustrated with examples from real mixer measurements. Extensive mixer tests were carried out to characterize the mixer by measuring its single sideband noise temperature and sideband separation. The best single sideband noise temperature is below 40 K with a sideband suppression ratio above 12 dB for upper and lower sidebands. A method to deduce the IF chain noise contribution vs. the IF frequency by using the SIS junction as a hot/cold load is also presented. This work addresses also specific problems and challenges encountered during the measurement of the 2SB mixer.