Characterization of Instrumental Phase Stability on the SMA Interferometer D. Y. Kuboa, T. R. Hunterb, R. D. Christensenc, P. I. Yamaguchic aAcademia Sinica, Institute for Astronomy

1
Characterization of Instrumental Phase
Stabilityon the SMA InterferometerD. Y. Kuboa,
T. R. Hunterb, R. D. Christensenc, P. I.
YamaguchicaAcademia Sinica, Institute for
Astronomy Astrophysics, Hilo,
HIbHarvard-Smithsonian Center for Astrophysics,
Cambridge, MAcSmithsonian Submillimeter Array,
Hilo, HI

• Abstract
• Atmospheric water vapor causes significant
  undesired phase fluctuations for the SMA
  interferometer, particularly in its highest
  frequency observing band of 690 GHz. One
  proposed solution to this atmospheric effect is
  to observe simultaneously at two separate
  frequency bands of 230 and 690 GHz. Although the
  phase fluctuations have a smaller magnitude at
  the lower frequency, they can be measured more
  accurately and on shorter timescales due to the
  greater sensitivity of the array to celestial
  point source calibrators at this frequency. In
  theory, we can measure the atmospheric phase
  fluctuations in the 230 GHz band, scale them
  appropriately with frequency, and apply them to
  the data in 690 band during the post-observation
  calibration process. The ultimate limit to this
  atmospheric phase calibration scheme will be set
  by the instrumental phase stability of the IF and
  LO systems. We describe the methodology and
  initial results of the phase stability
  characterization of the IF and LO systems.

Introduction The Submillimeter Array (SMA) is a
collaborative project of the Smithsonian
Astrophysical Observatory (SAO) and the Academia
Sinica Institute of Astronomy Astrophysics
(ASIAA) of Taiwan. The array consists of eight
six-meter diameter antennas with receivers
operating from 180 to 700 GHz and a digital
correlator with 2 GHz of bandwidth. Located on
Mauna Kea, Hawaii, the primary elements of the
SMA interferometer can be reconfigured across 24
pads which provide baselines ranging from 8 to
500 meters. Each antenna cryostat assembly
houses up to 8 receiver inserts consisting of
low-noise superconducting (SIS) mixers. The
inserts can be used in pairs for increased
bandwidth and for polarimetry observations.
Local oscillators (LOs) are used in each antenna
to provide heterodyne mixing from the sky
frequency to the 5 GHz intermediate frequency
(IF). A system block diagram for the IF/LO system
is provided in Fig 1. Photos are provided in
Fig 2. Instrumental phase stability measurements
are discussed herein.
1. IF Functional Description and Impact on Phase
Stability The IF path which carries the 4 to 6
GHz signal begins within the antenna cryostat
just after the SIS mixer (lower left of Fig 1).
This signal is leveled then optically modulated
for transmission over fiber to the main control
building. The received optical signal is
demodulated to electrical, leveled to accommodate
for optical loss variations, then passes through
the 1st down conversion which subdivides the IF
signal into six blocks. A 2nd down conversion
further subdivides the IF into 24 chunks each
centered at 153 MHz and with a bandwidth of 82
MHz (total bandwidth of 1968 MHz). Each of these
24 chunks are fine leveled to accommodate for
channel slope and sky variations then digitized
using 2-bit samplers operating at 208 Msps (upper
right of Fig 1). Phase movements or drifts of
the IF signal do not scale with LO frequency.
I.e., a 1 degree drift of the IF-1 signal
relative to the IF-2 signal appears as a 1 degree
offset between the 230 GHz and 690 GHz signal. We
have shown that the IF drift is much less than 1
degree and is therefore negligible.
3. Experimental Tests Stability of Antenna YIG
oscillator The long term phase stability of the
antenna YIG-1 LO was monitored by sending the
signal back to the control building using the
IF-2 channel (not shown in the figure). This
returned antenna YIG-1 LO was mixed with the MRG
YIG-1 LO to produce a difference frequency of 200
MHz, which in turn was phase compared to the MRG
200 MHz system reference using an HP 8508A vector
voltmeter. The data shown in Fig 3 were taken
during an actual science track at 1 sample/second
and was window averaged over 60 seconds to remove
fast phase variations. These data reveals a high
level of stability of 0.10 degrees peak-to-peak
and 0.025 degrees RMS over the first six hours.
It is significant to note that there is little or
no correlation of the phase to the azimuth
position which implies that the azimuth fiber
tension assembly is functioning as designed.
Translated to the final LO frequency of 225 GHz
the phase number becomes 3.2 and 0.8 degrees
peak-to-peak and RMS, respectively. Assuming a
similar stability for the antenna YIG-2 LO this
number translated to 685 GHz becomes 8.4 and 2.1
degrees peak-to-peak and RMS, respectively.

Fig 3 Antenna 4 YIG-1 round trip phase stability
data (left) and corresponding azimuth position
(right). YIG tuning was changed at 390 minutes.
Peak phase drift seen over 6 hours was just over
0.10 degrees. RMS value over this same interval
was 0.025 degrees.
4. Experimental Tests Stability between MRG
YIG-1 and MRG YIG-2
Fig 2 Antenna cabin photos. Left , cryostat
with optics cage. Gunn oscillator and multiplier
assemblies are located on the top of the optics
cage. Water vapor radiometer optics are just
visible on upper left. Center, IF/LO electronics
showing IF-1/IF-2 (left) and YIG-1/YIG-2 (right)
assemblies. Right, azimuth fiber optic tension
assembly. The yellow fibers pass through the
center of the azimuth encoder and are pulled
taught by a spring loaded assembly.
Since the antenna YIG-1 and YIG-2 LOs are locked
to MRG LO-1 and LO-2 located in the control
building, it is necessary to characterize the
phase stability between these MRG LOs. This was
accomplished by locking MRG LO-1 to 7.000 GHz and
LO-2 to 7.200 GHz and mixing the two to produce a
difference of 200 MHz. This 200 MHz difference
was then phase compared to the MRG 200 MHz system
reference. Fig 4 shows the phase stability plot
over a limit period of only one hour. This test
was conducted in the morning after a science
track and was affected by the air conditioning
compressor turning on at approximately 45
minutes. Nevertheless a variation 0.30 degrees
peak-to-peak and 0.058 degrees RMS was seen
during the first 45 minutes. Assuming that each
MRG YIG is drifting by equal amounts we assign an
RMS stability of 0.041 degrees to each.
2. LO Functional Description and Impact on Phase
Stability The LO system begins with a 10 MHz
crystal oscillator phase locked to a GPS
reference. A set of 109 MHz and 200 MHz
references are generated then optically modulated
for transmission over fiber to the antenna. A
common tunable LO (5.5 8.5 GHz) denoted as MRG
YIG-1 PLL in the lower right of Fig 1 is
optically modulated and power divided to each of
the 8 antennas. Each antenna receives these LOs
on fibers A and C and are demodulated to
electrical then leveled to accommodate for
optical loss variations. The antenna YIG-1 PLL
phase locks to the 200 MHz and LO-1 to produce an
output at either LO-1 /- 200 MHz. Following the
YIG-1 output is a harmonic mixer which produces
multiple harmonics (M) and mixes them with the
Gunn oscillator output operating in the 80 to 120
GHz band. The Gunn output is followed by a final
fixed multiplier whose value is denoted as N.
Table 1 provides example LO frequencies for some
typical tuning frequencies. It becomes obvious
from the table that a small movement in the
antenna YIG phase translates to a non-trivial
phase movement at the final LO. For example, a
1.0 degree phase movement at the antenna YIG
output translates to 32, 48, and 84 degrees at
the final LO for 230, 345, and 690 GHz,
respectively, which is clearly too large because
it will correspond to significant errors in the
position measurement of astronomical sources and
to a reduction of the image fidelity.
Fig 4 Stability of MRG YIG-1 verses MRG YIG-2.
Note air conditioning compressor turn on at 45
minutes.
5. Preliminary Conclusions These early results
have produced an antenna YIG stability of 0.025
degrees RMS. Adding to this number in RSS (root
sum squared) fashion 0.058 degrees RMS from the
MRG YIG produces a stability of 0.063 degrees
RMS. Translating up to the final LOs of 225 and
685 GHz produces 2.02 and 5.29 degrees RMS of
phase variation, respectively. The phase
variation seen between these two LOs and
therefore between the two observed signals is
5.66 degrees RMS which corresponds to a
contribution to the reduction in sensitivity of
less than 1. Further tests are required to
characterized the performance of the remaining 7
antennas. We would ultimately be interested in
staring at a 230 GHz and 690 GHz beacon over
several hours to establish the phase stability of
the overall system working together.