Page 134 Sixth International Symposium on Space Terahertz Technology An Integrated SIS Mixer and HEMT IF Amplifier S. Padin, D.P. Woody, J.A. Stern, H.G. LeDuc, R. Blundell, C.-Y.E. Tong and M.W. Pospieszalsid Abstract— Design details are given for a 205- 270 GHz fixed-tuned SIS receiver in which the SIS mixer and HEMT IF amplifier are integrated into a single compact unit. The mixer and IF amplifier are connected with an inductor which provides the reactive part of the optimum input impedance for the HEMT. This simple coupling circuit gives an IF bandwidth of '4 GHz. The re- ceiver has a DSB noise temperature in the range 35-80 K over the 205-270 GHz local oscillator band and 0.5-4.5 GHz IF band. I. INTRODUCTION In a millimeter-wave SIS receiver, the SIS mixer and HEMT IF amplifier are usually separate units con- nected with a 50 S1 coaxial line. The mixer is at- tached to the 4 K stage of a refrigerator while the IF amplifier is typically mounted 10 cm away on the 12 K stage [1, 2, 3}. The mixer may have an isolator at its IF port or a matching network which transforms the mixer IF port impedance to 50 SI [41. The IF amplifier also has a matching network which transforms 50 n to the optimum input impedance for a 13EMT [5]. SIS mixers usually operate with conversion loss so the IF contribution to the receiver noise is sig- nificant. IF amplifier noise temperatures increase roughly linearly with frequency and low-noise am- plifiers typically have bandwidths of about an oc- This work was supported by the National Sci- ence Foundation under Grant AST 93-14079 and by NASA Grant NAGW 4030. S. Padin and D.P. Woody are with the California Institute of Tedinology, Owens Valley Radio Obser- vatory, Big Pine, CA 93513 USA. J.A. Stern and H.G. LeDuc are with the Jet Propulsion Laboratory, California Institute of Tech- nology, Pasadena, CA 91109 USA. IL Blundell and C.-Y.E. 'Thug are with the Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 USA. M.W. Pospieszalslci is with the National Radio Astronomy Observatory, Charlottesville, VA 22903 USA. tave [6]. This leads to a compromise between re- ceiver noise and IF bandwidth which is responsible for the popularity of the 1-2 GHz IF band. Increas- mg the receiver IF bandwidth requires the devel- opment of either a wideband low-noise IF amplifier or a mixer with enough gain to precede a noisier amplifier. The first approach is difficult because it requires a low-loss network to simultaneously match a H:EMT input to 50 fi and provide the optimum input impedance for the BEMT over a bandwidth of several octaves. Increasing the mixer gain also causes problems because SIS mixers with gain tend to saturate [71. We have explored a new approach in which the SIS mixer and first IF amplifier are integrated into a single unit with a very simple coupling network between the two circuits. In this case there is no impedance transformation through 50 Q. The mixer is designed to provide an IF port impedance that is real and equal to the real part of the op-, timum input impedance for a HEMT at a partic- ular IF The imaginary part of the required input impedance is provided by an inductor. This simple coupling network provides the HEMT with a gener- ator impedance which is close to the minimum noise impedance (Z opt ) over a wide range of frequencies. In the standard receiver configuration the coupling network contains a coaxial line and this sverely lim- its the bandwidth of "noise matching" of the SIS junction and HEMT. The simple coupling circuit in the integrated re- ceiver has lower loss than the isolator, 50 11 line and IF amplifier input matching network in the standard. receiver configuration. This reduces the IF contribu- tion to the receiver noise in the integrated receiver. Also, in the integrated receiver the IF amplifier is cooled to 4 K which gives some improvement in noise compared with a receiver where the IF amplifier is at 12 K. Integrating the mixer and IF amplifier sub- stantially reduces the size of the receiver and this is an important advantage for multiple receiver sys- tems such as dual polarization receivers and focal plane arrays. The main disadvantage of the inte- grated approach is that the mixer and IF amplifier