Central Development Laboratory
SIS MIXER DEVELOPMENT
Our present mechanically tunable SIS mixers give acceptable
double-sideband performance in the bands 68-90 GHz, 90-115 GHz, 130- 170
GHz, and 200-260 GHz, although there is room for substantial improvement
in the 68-90 GHz band. Fixed-tuned SIS mixers are used in the 260-300 GHz
band, and are used in the 8-beam 230 GHz receiver now in use on the
12-meter telescope. The best mixers of the present fixed-tuned design
have low noise in the 200-260 GHz range, comparable with the best tunable
mixers, but worsen rapidly from 260-300 GHz.
Over the last 2 years, we have been developing mixers of a new design
which are intended to be tunerless, balanced, sideband-separating mixers
with internal IF amplifiers. These features are needed in order to achieve
the best possible sensitivity for the Millimeter Array.
The present performance of SIS mixer receivers, as measured in the
laboratory in observatories around the world, is shown in a graph of SIS mixer receiver noise temperature.
Work in SIS receivers has been carried out by A. Kerr, S.-K. Pan, N.
Horner, K. Crady, F. Johnson, A. Marshall, and M. Wharam. SIS wafers have
been fabricated at the University of Virginia by A. Lichtenberger, and at
JPL by R. LeDuc.
The ultimate goal of the work described below is to produce a balanced,
image-separating mixer with integrated IF amplifiers for the MMA. In order
to achieve this, we are developing separately the required circuits and
Modified on [an error occurred while processing this directive]
by Vince Summers.
- Sideband Separating Mixer:
In spectral line measurements, atmospheric noise in the unwanted
sideband can be eliminated by using an image separating scheme. At
the 12-m telescope, the antenna temperature at the zenith is
typically 60K at 230 GHz. For a good SIS receiver, eliminating the
noise in the image band would decrease the overall system noise by
about 30%, thereby halving the integration time required to attain a
given sensitivity. Since we would like to use such a system on the
MMA as well, we need a system which has wide IF bandwidth and
requires no mechanical tuning. We have chosen to usea phasing type
image separating mixer to achieve this goal. Such a mixerhas been
designed by NRAO and fabricated by JPL on a 1 x 2 mm quartz chip,as
shown below. The chip contains an input quadrature hybrid, an LO
power splitter, LO couplers, and two broadband SIS mixers. The two
IF outputs are combined in an IF quadrature hybrid (not on the mixer
chip), at whose output connectors the downconverted upper and lower
sideband signals appear separately.
Initial test results for single-sideband noise temperature and
sideband separation are shown below.
On the wafer containing this mixer, the terminating resistors on
the wafer from which this mixer was fabricated were 70 ohm rather
than 50 ohm terminations due to the difficulty of process control
for very thin layers. In addition, there is some LO leakage
underneath the chip which results in the LO phases not being exactly
right and thus degrades the sideband separation. Nevertheless, the
results are excellent, with more than 9 dB image rejection across
the band and noise temperature comparable to that of a single mixer
element. Another wafer is in process which is expected to have the
correct terminating resistance, and a new mounting block has been
designed which allows insertion of appropriate microwave absorber
material to reduce the waveguide mode coupling. This should result
in much improved performance over the first prototype.
- Balanced Mixer:
A balanced mixer for 200-300 GHz has been designed. Like the
sideband separating mixer, it uses two single mixers to build on a
single chip a mixer which will require 17 dB less LO power than a
single-ended design. It will also greatly reduce the contribution of
LO noise to the receiver noise. This will make design of a good LO
system much easier. A diagram of the balanced mixer is shown below.
A prototype of the balanced mixer has been assembled and tested,
and the results are shown in the plot below. This particular mixer
chip was tuned slightly higher than the target band of 211-275 GHz,
but it works well.
- Integrated IF amplifier:
In collaboration with S. Weinreb at the University of
Massachusetts, we have begun the design of an integrated IF
amplifier which will permit the MMA goal of 8 GHz instantaneous
bandwidth per sideband to be realized. The MMIC amplifier chip
employs a grounded gate first stage and is expected to give good
performance over the 4-12 GHz target band.