Onsala Space Observatory
B. Billade, V. Belitsky, A. Pavolotsky, I. Lapkin, E. Sundin, D. Meledin, V. Desmaris, D. Dochev, S.-E. Ferm – all are with the Group for Advanced Receiver Development (GARD), Department of Radio and Space Science and Onsala Space Observatory, Chalmers University of Technology, SE 412 96, Gothenburg, Sweden. J. Kooi is with the CSO, California Institute of Technology, Pasadena, CA, USA.
Disclaimer: since the time this web-page was created (June 2009), a lot of progress has been made. As a result, the Band 5 Cold Cartridge Assembly has been fully tested and proved its compliance to ALMA specifications. Please visit the ALMA Band 5 Cartridge development page to see the results of the performance tests.
The ALMA Band 5 is an EC Framework Program 6 (FP6) infrastructure enhancement project aiming at the development and design of a prototype receiver cartridge fulfilling specifications of ALMA Band 5. After the prototype cartridge is accepted, the project considers to supply 6 production cartridges for the ALMA Project for integration into the ALMA frontend receiver. Group for Advanced Receiver Development, OSO, Chalmers University, is responsible for cold cartridge assembly (CCA). STFC Rutherford Appleton Laboratory, UK, are our partners in the ALMA Band 5 project and responsible for development of the Band 5 warm cartridge assembly (WCA) and the local oscillator (LO) sources.
The Band 5 cartridge is a dual ‑ polarization receiver with the polarization separation performed via orthomode transducer (OMT). The receiver is based on sideband rejection quadrature layout (2SB) and employs modular design with SIS DSB waveguide mixers , covering 163-211 GHz with 4 ‑ 8 GHz intermediate frequency (IF) band. The major challenge with Band 5 mixer design is that, there is a very limited space inside the cartridge. The ALMA Band 5 cartridge is the lowest frequency channel of the ALMA frontend that utilizes all ‑ cold optics and thus has the largest mirrors amongst the other ALMA bands with cold optics. The Band 5 optics mirrors together with its support structure leave very little room for placing the receiver components, such as the corrugated horn, OMT, the mixers for both polarizations and the IF system and thus calling for specific technical and design solutions for the layout of the cartridge demanding a very compact design. Furthermore, the arrangement of the components in the cartridge is such that we have to direct the IF output of the mixers pointing down along the cartridge axis. In such a configuration, the mixer design with a spilt block technique becomes impractical and does not fit inside the cartridge. We have found that the only possible solution is to use a mixer block configuration with waveguide back piece . This design allows very compact design of the mixer block and also IF output pointing in desirable direction. Furthermore, to avoid extra cables and hence RF losses, all the components in the chain are directly attached to each other with SMA connectors. This design requires a custom made IF hybrid in order to fit the distance between the SMA connectors of the 2SB mixer IF outputs.
Since there is limited cooling capacity of the ALMA frontend cryostat at 4K stage, we can only allow 36mW of heat dissipation at 4K stage, which puts constrain to integrate the DC bias circuitry into the mixer. In our design, the DC biasing to the mixer is done using a bias circuitry placed on 15K plate of the cartridge. The IF hybrid connected to the combined IF/DC output of the 2SB mixer have an integrated bias-T, and the DC biasing is thus achieved through the output SMA connector of the mixer.
The mixer chip is fabricated on a 90 µm thick crystalline Z-cut quartz substrate with dimensions 310 µm wide and 2640 µm long. The mixer chip contains along with the SIS junctions most of the DSB components integrated on the same substrate: the chip comprise of an E-plane probes, waveguide-to-microstrip transition, for both the LO and RF, an RF choke at the end of the probe provides virtual ground for the RF/LO signals. We use the same probe with impedance of around 40 Ω for both the LO and RF.
The RF probe is followed by a LO coupler and two SIS junctions, and the IF is extracted between the RF and LO waveguides using a high impedance line (Fig. 2). The LO probe is followed by a three stage Chebyshev transformer to match the probe impedance to the LO coupler input, and the reflected signal at the isolated port of LO coupler is terminated using wideband floating elliptical resistive termination . The elliptical termination has the sheet resistance (12 Ω), the same as that of the impedance of the LO coupler input.
The shape of the E-plane probe is optimized for a broad band performance using Agilent Electromagnetic Design System (EMDS), a full 3D EM solver. Probe’s real impedance is 42Ω with ± 4% variation across the entire RF band and imaginary impedance of the probe varies between +j5 Ω to –i2 Ω. A hammer type RF choke provides a virtual ground for the RF/LO signal applied between the end of the probe and the choke, which excites microstrip mode between the top conductor layer (LO/RF) and the bottom ground (choke) layer. The thickness of the silicon dioxide layer used for the micro strip line is 350 nanometres. In order to achieve broad band performance from the mixer we use two SIS junctions in twin junction configuration  with SIS junctions having the size off 3 μm˛ each and RnA product of 30. The transmission line length between the two junctions is optimized such that the imaginary part of the twin junction configuration is tuned out. In this configuration the LO coupler serves two purposes: first, it couples the LO signal to RF with weak ‑ 18 dB coupling and, secondly, it transforms the probe impedance from 40 Ω to the input impedance of the twin junction circuitry.
The mixer block consists of two parts, a mixer back piece and a middle piece. In the mixer back piece, the mixer chip is glued to the block using wax, a 50 to 15 W IF transformer produced on 500μm thick alumina substrate, the IF signal from the mixer chip is extracted using a bond wire with a single layer capacitor employed at the IF side to compensate for the inductance of the bond wire and the mixer circuitry in order to achieve good IF matching. The two mixer back pieces should be used in the 2SB configuration and are exactly identical to facilitate production, however the mixer chips used have mirrored layout. The middle piece consists of a 900 RF hybrid and an in phase LO splitter. In order to suppress the Josephson current, the middle piece also holds magnetic concentrators, and the magnetic coils sit in a copper heat sink. This assembly is connected to the middle piece, using a fiberglass in between, to avoid heat leak from the coils to the mixer block.
Fig. 4 shows the picture of the mixer back piece with the chip mounted. In this configuration the mixer chip is installed perpendicular to the direction of E-field in the waveguide. The quartz substrate, used for the chip, extends into the full height for both LO and RF waveguides and even further; this enables a better thermal contact with the mixer block
The cavity created above the chip at the two ends caused resonance at around 197 GHz. Since the structure is electrically large and thus very complicated to simulate as the whole; we could not see this resonance effect in our simulations, while simulating each component separately. This resonance will also change the behavior of the RF chock structure. Fig. 4 right shows the modified mixer back piece, where the top cover height is reduced above the chip, at the two ends. This moves the resonance frequency of the cavity outside the RF band. Fig 5 shows the noise temperature measurements after reducing the height of the cavity about the chip.
For the reasons discussed in earlier sections and , ALMA band 5 design requires a custom made IF hybrid. Since we can not put the SIS bias circuitry at 4K plate, an SIS DC bias-T circuitry is also integrated into the IF hybrid. In order to minimize the RF losses and eliminate series resistance in the SIS junction DC bias circuitry, we use all Niobium superconducting IF hybrid employing Lange-coupler. The pitch between the input SMA connector is pre-defined by the distance between the two output SMA connectors of the 2SB mixer block. The IF hybrid has been fabricated in-house and uses a 500 µm thick Alumina substrate. In order to avoid possible substrate mode, we use separate substrate of alumina for the Lange coupler itself and for connecting lines: one for the Lange coupler, two for 50 Ohm connecting lines and two for bias-Ts. The interconnection between the different substrates is done using bond wires. Fig. 6, shows the layout of the IF hybrid. DC biasing of the mixer use EMI protection filter bulkhead connectors.
Current status of the mixer development: we have produced two 2SB mixers which perform very weel in the ALMA Band 5 prototype cartridge, please see the test results at the web-page for Band 5 CCA.
We present the design of ALMA Band-5 (163-211 GHz) mixer, measurement results of DSB mixer. The mixer design uses on ‑ chip LO injection circuitry employing a -18 dB microstrip – slot-line directional coupler and a high performance elliptical termination for the isolated port of the LO coupler. The DSB measurement of ALMA Band 5 mixer shows results with the noise temperature following the the line of three times of the quantum noise (3xHf/k) and is below 30K across the band. The 2SB mixer measurements have been installed and tested as part of ALMA Band 5 Cold Cartridge Assembly.
 V. Belitsky, I. Lapkin, B. Billade, E. Sundin, A. Pavolotsky, D. Meledin, M. Strandberg, R. Finger, O. Nyström, D. Henke, V. Desmaris, M. Fredrixon, S.-E. Ferm. “Prototype ALMA Band 5 Cartridge, Design and Performance”, to be published in the Proceedings of the 20th International Symposium on Space Terahertz Technology, Charlottesville, 20-22 April 2009.
 R. Monje, V. Belitsky, V. Vassilev, A. Pavolotsky, ”SIS Mixer for 385-500 GHz with On-Chip LO injection”, Proceedings of the 18th International Symposium on Space Terahertz Technology, pp. 44-49, Pasadena, USA, March 21 ‑ 23, 2007.
 R. R. Monje, Vessen V. Vassilev, Alexey Pavolotsky and Victor Belitsky, “High Quality Microstrip Termination for MMIC and Millimeter-Wave Applications”, IEEE MTT-S International Microwave Symposium, ISSN: 01490-645X, pages 1827-1830, Long Beach, California, June 12-17, 2005.
 Belitsky V. , Tarasov M.A., "SIS Junction Reactance Complete Compensation", IEEE Trans. on Magnetic, 1991, MAG- 27, v. 2, pt. 4, pp. 2638-2641.
Site maintained by Magnus Strandberg