1. Characterization of Instrumental Phase Stability on the SMA Interferometer D. Y. Kubo a, T. R. Hunter b, R. D. Christensen c, P. I. Yamaguchi c a Academia Sinica, Institute for Astronomy & Astrophysics, Hilo, HI b Harvard-Smithsonian Center for Astrophysics, Cambridge, MA c Smithsonian 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 1 st down conversion which subdivides the IF signal into six blocks. A 2 nd 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. 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. 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 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. 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. 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.