Design of a side-band-separating heterodyne mixer for band 9 of ALMA

Abstract

A side-band-separating (SBS) heterodyne mixer has been designed for the Atacama large millimeter array (ALMA) 602-720 GHz band, as it presents a great improvement over the current double-side-band configuration under development at the moment. Here we present design details and the results of extensive computer simulations of its performance. The designed SBS mixer exploits waveguide technology. At its core it consists of a quadrature hybrid, two LO injectors, and three dumping loads. The entire structure has been analyzed in a linear circuit simulator with custom code written to accurately (verified by HFSS finite element simulations) model the hybrid structures. This technique permitted an optimization of the dimensions and the study of the consequences of deviations from the ideal situation. It is estimated that the tolerances in several of the components should be kept at less than 3 μm.

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17th International Symposium on Space Terahertz Technology
WE1-4
Side-band-separating heterodyne mixer for band 9
of ALMA.,
F. P. Mena*, A. M. Baryshev*, J. Kooit, G. Gerlofsma*, C. F. J. Lodewijkt, R. Hesper*, and W. Wild*
*Space Research Organization of The Netherlands and Kapteyn Institute, University of Groningen,
Landleven 12, 9747 AD Groningen, The Netherlands
tcalifornia Institute of Technology,
MS 320-47 Pasadena, California 91125, USA
tKavli Institute of Nanoscience, Delft University of Technology,
Lorentzweg 1, 2628 CJ Delft, The Netherlands
Abstract— In this paper we will present the realization of a
side-band-separating (2SB) heterodyne mixer for the frequency
range from 602 to 720 GHz. This frequency range corresponds
to band 9 band of ALMA. The mixer, in brief, consists of a
quadrature hybrid, two LO injectors, two SIS junctions, and
three dumping loads. All the parts were modeled and optimized
prior construction. The fabricated SSB mixer exploits waveguide
technology and has been constructed in the split-block technique.
We used state-of-the-art CNC micromachining which permitted
to obtain details as small as 70 pm with tolerances of 2 pm.
Finally, we will present the performance of the SIS junctions that
will be used in the 25B mixer.
I. INTRODUCTION
Of the receiver bands that are currently being constructed
for the Atacama Large Millimeter Array (ALMA), only band
9, operating at the highest frequencies (602-720 GHz) uses
double side-band mixers. Despite of the well known ad-
vantages of a side-band-separating (2SB) mixer, it has not
been implemented in band 9 as the involved dimensions
are prohibitory small. However, the current state-of-the-art
micromachinning technology has proved that the complicated
structures necessary for this development are attainable [1],
[2]. We have already reported a complete design of a 2SB
mixer for band 9 together with a full simulation of the RF
components [1]. The purpose of this article is to present the
physical realization of such mixer including the corresponding
IF circuit. We also present the testing results of the SIS
junctions that will be used in the mixer. The results show a
good homogeneity which are desirable for a good performance
of the complete mixer.
II. 2SB MIXER BLOCK
Recently, we proposed a design for the 2SB mixer for
ALMA band 9 which is reproduced in Fig. la.[1] The idea
behind the design is to have a compact unit which will contain
all the main components: defluxing magnets, DC biasing
board, and the IF circuit filtering. The realization of such block
is presented in Fig. lb. It was obtained via state-of-the-art
CNC micromachining. In Fig 2 we show a detailed view of
the core of the mixer consisting of a quadrature hybrid, two
LO injectors, two SIS junctions, and three dumping loads. The
Fig. 1. (a) Proposed design of a 2SB mixer for ALMA band 9. (b)
Micromachinning realization of the mixer. (c) Mixer with the IF circuit board
and magnets.
quality of the finishing is excellent and meets the required
tolerances.[2]
III. IF CIRCUIT
For the 4 to 8 GHz IF circuit we have opted for the use of
parallel coupled suspended microstrip lines. This is a compact
unit containing the IF match, DC-break, bias tee, and EMI
filter. The advantage of such planar structure has already been
demonstrated and selected to be used in various astronomical
instruments (see, for example, Ref. [3]). The already mounted
circuit can be seen in Fig. 1. It has to be noted that, for this
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Fig. 3. Simulated transmission of the IF circuit.
RF+LO
Port 2
1F/DC
(b)
High-impedence
line
Choke
structureProbe
1F/DC
811
S12
Su hammer
S„ rectangular
S12 hammer
" S12 rectangular
-12
-16
17th International Symposium on Space Terahertz Technology
WE1-4
Fig. 2. (a) Detailed view of the RF components of the mixer. (b) LO injector.
(c) RF quadrature hybrid. (d) Room for the mixers.
filter to work, the ground plane directly underneath the filter
has to be removed. Previous to fabrication, the dimensions
were optimized for a good performance in the 4 to 8 GHz
frequency range. As an example of this results, in Fig. 3, we
show the transmission of this IF circuit.
Iv. JUNCTIONS
For the SIS junctions we selected the design that contains
both RF and IF matching. The exact layout is similar to the
one presented by Risacher et al. [4] and is shown in Fig. 4. The
chosen design was simulated [5] and its dimensions optimized
for a maximum transmission and minimum reflection between
ports 1 and 2 (see Fig. 4). We compared both a hammer
type and a rectangular choke structure. The best reflection and
transmission for both configurations are shown in Fig. 4. Given
these results, a rectangular choke structure was fabricated.
The SIS devices were fabricated on a quartz substrate. First,
a Nb monitor layer is deposited, after which an optically
defined ground plane pattern of Nb/Al/A10x/Nb is lifted off.
Junctions are defined by e-beam lithography in a negative e-
beam resist layer and etched out with a SF6/02 reactive ion
etch (RIE) using AlOx as a stopping layer. The junction resist
pattern is subsequently used as a lift off mask for a dielectric
layer of Si02. A Nb/Au top layer is deposited and Au is etched
with a wet etch in a KI/I2 solution using an optically defined
mask. Finally, using an e-beam defined top wire mask pattern,
the layer of Nb is etched with a SF6/02 RIE, finishing the
600640680
720
f (GHz)
Fig. 4. (a) SIS design with a rectangular choke structure. The figure also
shows the cavity were the junction will be placed. (b) SIS design with a
hammer type choke. (c) Results of the simulations for the two types of choke
structures.
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0.0002
No H-field
H-field
0.0000
-0.0002
17th International Symposium on Space Terahertz Technology
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-6 -3
0
V (mV)
Fig. 5. IV curves of 12 different junctions intended to be used in the 2S5
mixer. Notice the good repeatability necessary for a good performance of the
mixer.
fabrication process. After dicing and polishing, we measured
the resistance of the individual devices as indicated in Fig. 5.
The results show that the chosen fabrication procedure results
in a good reproducibility of the devices. This is a key element
for the future performance of the mixer.
V. CONCLUSIONS
In this article we have presented the current status of the
development of a side band separating mixer for ALMA band
9. Currently, all of the components have been simulated,
fabricated and tested individually. The assembling is a going
on task that will be completed in the next months.
ACKNOWLEDGEMENT
We gratefully acknowledge funding from NOVA, the
Netherlands Research School for Astronomy, and the FP6
program of the European Union.
REFERENCES
[1] F. P. Mena et al.. Design of a side-band-separating heterodyne mixer
for band 9 of ALMA, 16th Proc. THz Space Conference (2005).
[2] Radiometer Physics GmbH. http://www.radiometer-physics.de/ .
[3] J. Kooi et al., Heterodyne Instrumentation Upgrade at the Cahech
Submillimeter Observatory, SPIE, MIllimiter and submillimiter detectors
for Astronomy II, Vol 5498, pp 332-348 (2004).
[4] C. Risacher et al., IEEE Microwave and Wireless Components Letters,
vol. 13 (2003).
[5] Microwave Studio, http://www.sonnetusa.com/.
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