BIMA MEMO # 29 Long Baselines at Hat Creek Mundy, Erickson, and Blitz Sept. 3, 1993 The purpose of this memo is to consider two options for implementation of long baselines on the BIMA Array: coaxial cable and fiber optics. The primary goals of the present implementation are: 1) two long baselines, one at roughly 400m N and one at 700m N., 2) all sky coverage, 3) completion of first-order implementation before March 1, 1994, 4) extensible to 9 and 11 antennas cheaply. Secondary goals of the present implementation are: 1) provide groundwork for extension to 1-3 km baselines in future, 2) enable use of analog correlator on long baselines. Given these ground rules, all options will share some costs and features. Some of these costs will not be considered in this discussion; for example, pads need to be built; a trench dug for laying the cable; a civil engineer needs to be consulted about the strength of the road. ----------------------------------------------------------------------------- Generic Coaxial Cable Option: ----------------------------- In this option, the IF and LO signals to the two long baselines are carried on coaxial cables. Both could be connected on 1 5/8" cable; alternatively, the near one could be on 7/8" cable and the far one could be on 1 5/8" cable but the expectation that the 1 5/8" cable will be acquired for $1/meter makes the other cable alternatives unattractive. Current prices for various cable diameters are: 1/2" at $7/meter; 7/8" at $15/meter; 1 5/8" at $40/meter. Generic Optical Fiber Option: ----------------------------- In this option, the IF and LO signals to the two long baselines are carried on optical fibers. Independent cables containing a minimum of 4 fibers each will be run to the two stations. Two fibers are required for complete communication; extra fibers are spares. We will constrain the two cables to have the same physical length to simplify the fix delay requirements (see next section). Generic Variable and Fixed Delay: --------------------------------- If all sky coverage is to be achieved, the current variable delay system must be extended by two steps, 1000 and 2000 nsec and some length of fixed delay added to compensate for the cable length to the outrigger antennas. Nearly all sky coverage can be achieved by adding the 1000 nsec step plus adding roughly 750 m fixed delays to all antennas on the array. This combination moves the phase center closer to the outrigger antennas and permits coverage of the sky from +90 to -30 degrees declination for all elevations greater than 15 degrees. At present we are intending to cost-out the latter option. The extension to a second longer step could be done next Summer. These can be implemented in cable, fiber optics, or digital chips. Ideally the additional long bit would be under computer control and be transparent to the system. As a short-term expedient simplifiction, we will consider an option utilizing manual optical fiber delays, i.e. the observer will switch in/out the longest bit as needed. The advantage of this option is that we can postpone the possibly troublesome implementation of optical switches until next Summer. We will not consider the digital chip implementation in the following discussion because it would exclude future use of the analog correlator and is likely to be too design/labor intensive to complete by March 94. ----------------------------------------------------------------------------- Material Requirements: ---------------------- Antenna Connections Option A: 1 5/8" cable all the way 1800 m 1 5/8" cable ($1/meter) $ 1800 * 4 connector for 1 5/8" cable ($350 each) $ 1400 ----- $ 3200 * Note that this price is uncertain and could be much higher. Option B: 1 5/8" coax to near pad and optical fiber to far 900 m 1 5/8" cable ($1/meter) $ 900 * 2 connector for 1 5/8" cable ($350 each) $ 700 900 m cable with 6-single mode fibers $ 1800 ($2/meter) 2 10GHz laser transmitters Free 2 6GHz receivers Free ----- $ 3400 * Note that this price is uncertain and could be much higher. Option C: Fiber Optic all the way 1800 m cable with 6-single mode fibers $ 3600 ($2/meter) 2 10GHz laser transmitters Free 2 6GHz receivers Free 2 3GHz laser transmitters ($4.6k each) $ 9200 2 3GHz photodiodes ($1.15k each) $ 2300 ------ $15100 Delays: Option I: 6 Coaxial Cable Delay Boxes and 4 fixed delay 6 x 270 m 1/2" cable for 1000 nsec bit ($7/meter) $11340 4 x 750 m 1/2" cable for fixed delay ($7/meter) $21000 switches and amplifiers etc for 6 boxes $ 3000? ------ $35340 Option II: 6 Fiber Delay Boxes and 4 fixed delays 300 m cable with 12-single mode fibers ($4/meter) $ 1200 same 750 m cable with 12-single mode fibers ($4/meter) $ 3000 same 6 3GHz laser transmitters ($4.6K each) $27600 ---- (only 4 if antenna connections are fiber) $18400 6 3GHz photodiodes ($1.15K each) $ 6900 same 2 x 6 optical switches ($500 each) $ 6000 same 2 optical splitters if ant conn. fiber ($1k each) $ 2000 --------- $44700 if combined with optical fiber antenna conn. $37500 Option III: 6 Manual Fiber Delays and 4 fixed delays 300 m cable with 12-single mode fibers ($4/meter) $ 1200 same 750 m cable with 12-single mode fibers ($4/meter) $ 3000 same 6 3GHz laser transmitters ($4.6K each) $27600 ---- (only 4 if antenna connections are fiber) $18400 6 3GHz receivers ($1.15K each) $ 6900 same 2 optical splitters if ant conn. fiber ($1k each) $ 2000 -------- $38700 if combined with optical fiber antenna conn. $31500 ----------------------------------------------------------------------------- Evaluation of Specific Options: ------------------------------- Option AI: 1 5/8" coax and coax delays Total cost:$38,540 Advantages: Cheapest option at the $1/m price for OVRO coax Minimum changes in technology at observatory Disadvantages: Limits bandwidth to current 800 MHz 1 5/8" cables are generally difficult to handle and must be hand-laid into the protective pipe which will be labor intensive Requires 6 more delay boxes comparable in size to current delays plus 4 x 750 m of fixed delays. Where will all of this cable go? It needs to be at constant temperature. Not easily extensible to longer baselines unless go to bigger coax which would be very expensive and delay become expensive since cable is the major cost in coax delays. Labor at Berkeley needed to design and build amplifiers/equalizers for the long delay segment and for the fixed delay segment - this is not currently included in the cost. Expensive to extend delays to one more step or to more antennas - the next step would cost $23K for 6 delay boxes. Potential Problems: Quality of 1 5/8" cable unknown Price of cable uncertain (new cable is $40/meter -- has Scoville verbally agreed to $1/m?) How much manpower is needed to build delays and amp/equalizers? 750 m of 1/2" coax has an attenuation of 54 db at high freq which needs to be equalized Where are we going to put all of the delay cable? Option BI: 1 5/8" coax to near pad, fiber to far, coax delays Total cost:$38,740 Advantages: Gets us started into optical fiber technology Cheapest option if 1 5/8" coax is priced at more than $1/meter Disadvantages: Mixed system of coax and fiber leaves future direction muddled Plus same disadvantages as AI Potential Problems: Same as AI Options AII or BII: options A or B with optical fiber delays AII Total Cost:$47,900 BII Total Cost:$44,500 Advantages: Gets us started into optical fiber technology Fiber delay are compact - delays for 12 antennas can be done in two boxes comparable in size to the current delay boxes Fiber delays are cheaply extended to more antennas or longer bits Potential bandwidth on fiber is 3 GHz Building block toward redoing array in optical fiber Disadvantages: Costs more than pure coax options Optical switches not well known quantities Requires ramp up to optical fiber technology Limits bandwidth to current 800 MHz 1 5/8" cables are generally difficult to handle and must be hand-laid into pipe which will be labor intensive Potential Problems: Can switchable optical fiber delays be built in time? Same coax cable questions as for AI and BI Options AIII or BIII: options A or B with manual fiber delays AIII Total Cost:$41,900 BIII Total Cost:$38,500 Advantages: Same as AII or BII easier and faster to implement as a first step toward AII or BII Disadvantages: Same as AII or BII Requires software to queue observer for manual switching Potential Problems: Same coax cable questions as for AI and BI Option CII: Optical fiber all the way Total Cost:$52,600 Advantages: Gets us started into optical fiber technology All off the shelf items so little electronic development work Potential bandwidth on fiber is 3 GHz Easy to extend optical fiber connections to array antennas and have 3 GHz potential bandwidth all the way to the receiver. Cable is thinner and easier to work with than 1 5/8" coax Disadvantages: More expensive than coax options Potential Problems: Can optical fiber delays be built on March timescale? Troubles ramping up to optical fiber technology? Possibility for unrepairable cable breaks Option CIII: Optical fiber with manual fiber delays Total Cost:$46,600 Advantages: Same as CII Delays easy to build Disadvantages: More expensive than coax options Manual switching of large delay segment requires observer intervention Potential Problems: Troubles ramping up to optical fiber technology? Possibility for unrepairable cable breaks ----------------------------------------------------------------------------- Conclusions: ------------ The primarily coax cable options AI at $38,740 and BI at $38,940 are the cheapest options for implementing the long baselines and utilize coax technology which is well established at Hat Creek. In that sense, these are the safest and easiest options. However, these options share a number of significant drawbacks: (a) 1 5/8" cable is required. This cable is heavy and bulky to work with (b) the required delays are space consuming - where can we put them? (c) extension to more antennas or one more delay step is expensive (d) extension to longer baselines is expensive just in the costs of 1/2" cable for delays, and outrageous if 1 5/8" cable needs to be purchased at market rates. (e) we do not take our first step into optical fiber technology -- the technology needed for future dual polarization or multi-receiver observations. (f) equalizer boxes for the variable and fixed delays need to be designed/built at Berkeley, in the queue with other high priority items for the 9 element array. Mixed cable and optical fiber options which use optical fiber delays, such as options AII, BII, AIII, and BIII, are intermediate in cost between the pure coax and pure fiber options but there is no compelling reason to take this halfway step to optical fiber. In these options, one incurs all of the dangers of taking on the new technology while incurring little-to-no benefit from not going to all optical fibers. The price differential between these options and comparable full fiber options is $5K. The optical fiber options CII and CIII are the most expensive options by a modest margin but they are also the most forward looking of the options. If in the future we wish to expand the IF bandwidth or return multiple IFs for dual polarization work, optical fiber links are going to be needed. The proposed options CII and CIII can be viewed as the first step toward a optical fiber IF system capable of 3 GHz bandwidth. The major drawback of these options is in the potential problems to be experienced in ramping up to the new technology. We have gained some experience with the previous testing of an optical fiber link on the existing array and we are prepared to take-on this larger implementation. The current plan involved only off-the-shelf technology. Our plan is to proceed with options CIII with the intention of making observations with two outrigger antennas by March 1, 1994. We then intend to continue work toward completion of the computer controlled delay boxes by September 1994 (option CII). Depending on the outcome of the observations we may proceed to add a second longer bit to the delays during the upgrade to option CII. Most of the components can all be assembled and tested in Maryland and installed at Hat Creek without extensive commitment of time by Berkeley personnel and without a large amount of array engineering time.