Program and Abstract Book - SRON

19 th International Symposium on Space Terahertz Technology
ALMA Band 5 (163-211 GHz) Sideband Separating Mixer Design
9-4
B. Billade 1 , I. Lapkin 1 , R. Monje 1 , A. Pavolotsky 1 , V. Vassilev 1 , J. Kooi 2 , and V. Belitsky 1 .
1 Group of Advanced Receiver Development(GARD), Chalmers University of Technology,
Gothenburg, Sweden.
2 California Institute of Technology, Pasadena, USA
Email: bhushan.billade@chalmers.se
The ALMA interferometric radio telescope is under construction by an international
consortia consisting of European countries (ESO), USA, Canada, and Japan. The ALMA
Band 5 will be a dual polarization sideband separating heterodyne receiver covering 163-
211 GHz with 4 - 8 GHz IF. For each polarization, Band 5 receiver employs sideband
rejection (2SB) with quadrature layout based on SIS mixers. In order to achieve the
specified -10dB sideband rejection with reasonable margin, the overall amplitude and phase
imbalance of the quadrature scheme should be better than -2.5dB and 12 degrees
respectively. The major contributors to the imbalance are RF and IF hybrids and the
gain/phase imbalance of the SIS mixers. The amplitude and phase imbalance of SIS mixers
depends on chip fabrication accuracies, mounting, and mechanical tolerances and can not be
predicted accurately pushing for strict constrain on the RF and IF hybrids. In order to
achieve desirable performance, we have performed extensive simulations and optimization
of all the components of 2SB SIS mixer, including 3D EM modeling using Agilent EMDS,
and ADS circuit simulator. Our simulations shows that for the given frequency range, the
RF hybrid can be designed with 0.8 dB amplitude and 3 degree phase imbalance at the best
case, while the IF hybrid employing an unfolded Lange coupler attains 0.7 dB amplitude
and 5 degree phase imbalance. For the SIS mixer, we employ MMIC-like approach where
most of the DSB mixer components are integrated on the same crystal quartz substrate. The
waveguide-to-microstrip transition is done using an E-probe for both LO and RF. In
contrast to split block technique, the probe lies perpendicular to the wave direction (backpiece
configuration) in order to make the mechanical structure more compact; the RF choke
at the end of the probe provides a virtual ground for the RF signal, and the choke is DC
grounded to the chassis. The LO injection is done using a microstrip line directional coupler
with slot-line branches in the ground plane; LO circuitry employs a three stage Chebyshev
transformer to match it to the LO probe. The isolated port of the LO coupler is terminated
by floating wideband elliptical termination [1]. The mixer employs two SIS junctions with
junction area of 3 μm 2 each, in twin junction configuration, followed by a quarter wave
transformer to couple it to the signal probe. This circuitry provides optimum matching of
the SIS junctions at RF frequencies though the problem with such architecture is that there
is no natural cold point to extract the IF. A high-inductance line is therefore used by adding
a layer of SiO 2 . The biasing network for the SIS junction is placed on the 12K plate, which
will be connected to a bias-T integrated with the IF hybrid at 4K.
At the time of the conference we plan to present details of the mixer design and results of
the first experimental verification of the DSB mixer performance.
References:
[1] “High quality micro-strip termination for MMIC and millimeter-wave applications”,
Raquel Monje, V. Vassilev, A. Pavolotsky, V. Belitsky, IEEE MTT-S international
microwave symposium, June 2005.
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