Abstract

1. IF/LO Thermal Controller for SMA AntennasD. Y. Kuboa, S.W. Changa, B. N. Kogea, R. WilsonbaAcademia Sinica, Institute for Astronomy & Astrophysics, Hilo, HIbHarvard-Smithsonian Center for Astrophysics, Cambridge, MA 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. We have shown that the IF drift is much less than 1 degree and is therefore negligible even without the aid of thermal stabilization. Thermal stabilization does, however, improve the gain stability of the amplifiers and continuum detectors. Fig 1. Smithsonian Submillimeter Array RF System Block Diagram 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 (6.0 – 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 I 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. Regular calibration is clearly necessary to avoid errors in the position measurement of astronomical sources and to a reduction of the image fidelity. This calibration, however, includes phase corrections for a multitude of sources and is performed only periodically at intervals of approximately 20 minutes. Abstract As part of an ongoing effort to refine the SMA's technical performance we investigated the YIG oscillator's phase to temperature coefficient. Measured lab data shows approximately 1 degree phase per degree C change in YIG oscillator temperature. A typical post YIG multiplier value for observation at 345 GHz is 48, thus a 1 degree YIG phase change results in a nontrivial 48 degrees at the sky frequency. The YIG oscillator temperature varies quite slowly with diurnal variations and weather, and somewhat quickly (~15 minutes) with YIG tuning voltages. The present method to mitigate this phase movement is through regular (~20 minutes) astronomical calibration on bright celestial sources. This method, through the use of interpolation between calibrations, is used to remove the phase variations from a multitude of sources, one being the antenna YIG oscillator. We have developed an active fan speed controller to maintain a constant temperature within the IF/LO electronics enclosure located in the antenna cabin. After struggling with oscillation issues we have obtained some impressive temperature stability results. We are currently in the process of replicating the design for all 8 antennas. Table I. Typical Post Antenna YIG Multiply Ratios Fig 3. Fan controller assembly integrated into enclosure cover. Left and right photos are front and rear views, respectively. The intake baffle was added to prevent excessive cooling of the upper assemblies, the mixing fans were added to prevent thermal oscillation. 4. Fan Controller Performance Fig 4 shows the performance of the fan speed controller mounted on the IF/LO enclosure for antenna 5 (acc5) over a five day interval. The other 7 antennas are unregulated. Note that in addition to a smaller diurnal variation, antenna 5 also operates several degrees lower than the other antennas because of the active cooling. A baffle was added to the upper intake fan to prevent excessive cooling of the upper two assemblies. We encountered thermal oscillation with a very long period of ~40 minutes and finally resolved it with the use of 4 internal mixing fans. These mixing fans are run at 100% speed and circulate the air in a counter-clockwise direction as viewed from the front. One difficulty that we are presently encountering is the determination of the optimum temperature set point which appears to be different for different antennas. The solution may have to be empirically derived separately for each antenna. Fig 2. Antenna IF/LO enclosure attached to cabin wall. Left photo – front cover removed, right photo – internal covers removed. Enclosure size is 36 x 24 x 9 inches (91.4 x 61.0 x 22.9 cm), weight is ~135 LBS (61 Kg). 3. Fan Controller Functional Description A photo of the fan controller assembly is shown in Fig 3. The assembly is entirely self contained, including its own power supply, and can easily be installed onto the IF/LO enclosure by replacing the stock cover. The reference thermistor cable (not shown) extends from the controller board and attaches to the center of the enclosure base plate (just below the lower left corner of YIG-1 in Fig 2). The thermistor (negative temperature coefficient resistor) is biased to produce a voltage which is proportional to temperature. This voltage is amplified, filtered then fed to a commercial Microchip TC648 fan speed controller. The TC648 provides pulse width modulation (PWM) of the +12VDC supply to the intake and exhaust fans. PWM is preferred over linear voltage control to prevent stalling of the fan blades at low rotational speeds. The temperature set point can be adjusted from the outside via a feed-through trimmer potentiometer. Fig 4. Comparison of IF/LO enclosure temperatures for all 8 antennas over a five day interval. 5. Future Plans The next level of thermal stabilization will involve the development of a complementary heater for the YIG oscillator component itself. The YIG oscillator produces a varying amount of heat which is a function of tuning frequency. The YIG frequency is often changed in the middle of the night during the change over from one science track to the next and results in a slow change in the overall assembly temperatures. Contact: Derek Kubo, dkubo@asiaa.sinica.edu.tw March 18, 2008