EXPLANATION OF DANA'S FIBER TESTS ON THE 12M TELESCOPE... MOTIVATION: For the VLBI observations using the 12m telescope it is desirable to send a clock signal to the feed cone which has a constant delay relative to the signal launched from our hydrogen maser. But we know full well that this is not directly possible- the delay in the optical fiber carrying this signal will change both with temperature and to some degree with fiber bending as the telescope tracks objects or moves from one object to another. The next best thing is to measure the one-way delay change and report that measurement result along with data from the telescope at frequent intervals so the delay changes can be compensated out. But all we can really do is to measure round trip delay changes and hope to calculate one-way changes from this. This is pretty simple if we can find a matched pair, otherwise it gets harder. So we would like to identify at least one pair of single mode fibers in our bundle, whose delay changes due to antenna tracking etc are pretty well matched. If we can find such a pair, then we're in business just that simply. But this may not happen, in which case life gets tougher. It may then prove necesary to measure individual fiber delays and use a more complicated calculation to figure the uplink delay changes when the two fibers in final use are not matched. This is the purpose of the temporary system that I'm presently checking out in the RX Lab. SYSTEM DESCRIPTION: This system uplinks a signal on one fiber, then turns it around and splits it into four separate signals which are donwlinked on any desired test group of four fibers. The idea is that if the returned signals on any two fibers show adequately matched delay variations, we've found what we need. Otherwise I'll need to do a somewhat more complicated set of measurements to establish independent values for pairs of uplink and downlink fibers. HW DESCRIPTION: The main elements of the system are two big aluminum plates full of RF components and with temperature stabilizing devices. The massive plates are used to provide lots of thermal inertia to make it easier to avoid rapid temperature changes, which could affect the measurement electronics and masquerade as delay changes. The slightly smaller plate is at the 12m antenna, as close as is practical to the feeds. The larger plate will be down in the operations building, near where the fiber bundle is brought in. 12m SITE NEEDS: The "hilltop" plate needs to be in or near the feed cone, above the joints about which tracking motions occur so that the fiber bundle to this plate is exposed to the full normal gamut of bending, twisting, temperature, etc. This plate will be accompanied by several small power supplies (or one larger one if preferred), plus two small electronic units for the temperature regulation system. The plate and all this other stuff needs to be well-protected from the weather. The plate will need easy access because a full search of the fibers in the bundle will require changing and/or swapping of fiber connections at least several times (possibly many times). The antenna has a reinforced flat plate at the "bottom" of the movable part, currently with a large coverable hatch and a smaller oblong hole for cables. I'm thinking that mounting my stuff on this plate might suit my needs best, assuming that it's permissible for us to drill mounting holes in this plate. The antenna part of the system will need 120VAC single-phase power, in the neighborhood of 100-200 watts, from a UPS capable of keeping the system alive in the face of the usual interruptions that occur at the AO. It will also have five optical fibers connected at any given time, although the entire bundle of single-mode fibers must be available at the plate (terminated in FC/APC connectors). Additionally I should really have RS232 access to the temperature controller for monitoring and tweaking the controller parameters. It would be sufficient if this were just brought down into the pedestal, but a remote link to the operations bldg would be really nice. The software for doing this expects to be driving an RS232 port on its host. OPERATIONS BUILDING NEEDS: Here I just need space, AC power, access to 10 MHz derived from our hydrogen maser, and access to the single-mode fiber bundle (with FC/APC terminations). The space should be large enough to comfortably accommodate the big "downstairs" plate plus a small CW synthesizer and associated power supply, a small data acquisition device, two small electronics packages for the downstairs temperature controller, and a laptop PC. It would be best if the space were air conditioned, but I could probably live without if needed. AC power requirements will also be modest, probably under 200 watts. Here especially it is essential that the power come from a reliable UPS. THE FUTURE: Assuming that all this works out, the "hilltop" plate will be replaced with a similar but perhaps somewhat smaller plate. This new plate will contain just two fiber link components (instead of the current five) for the delay monitoring task, plus possibly the two fiber link transmitters for bringing RF down from the feeds. The new hilltop plate might also include the VLBI phase CAL pulse generator, which we will be purchasing from an outside supplier.