NATIONAL RADIO ASTRONOMY OBSERVATORY Green Bank, WV MEMORANDUM April 9, 1997 TO: R. Fisher, R. Lacasse, M. MkKinnon, C. Niday, R. Norrod, T. Weadon FROM: R. Lacasse SUBJ: Minutes of Interference Protection Group 4/08/97 Meeting In attendance: Fisher, Lacasse, McKinnon, Niday, and Weadon RF: Changes to minutes of previous meeting? All: No. RF: Comments on IPG web page? All: none. MM: Interference web page text has been updated. Observers will be advised of its availability. Address of page: http://www.gb.nrao.edu/~sfore Progress on Action Items: Interference reports: MM: No call-outs. Had 1 inquiry, from Ed Murphy, about 1421.57MHz birdie that seemed to originate in the lab. He is will to use some of his observing time to help track it down. TW: Norrod and Weadon spent time looking for this. Due to its intermittent nature it has been very difficult to find. Source is still unknown. RF: Every should think about what type of equipment would be useful in tracking this type of emission down. MM: The SETI group has also complained that interference at the 140' has gotten considerably worse since we've moved into the new building. However, they have not shown anything quantitative. Storage for equipment MM: Talked with Fleming. No cabinets available. Might it be better to find space in electronics. RL: Some equipment, like antennas may take up a lot of space. There is room in the anechoic chamber control room for many of the things we will need there. Other equipment could be stored in lab if space can be found. **Action item for Weadon and Lacasse: Identify storage locations for equipment Device evaluation document RL: Niday and Lacasse used this document recently to evaluate measurements of a workstation. Only suggestion would be to add a procedure for calculating Effective Radiated Power. Cable Entry into Anechoic Chamber TW: Fleming said that we could save money on the 208V 3-phase filters if we buy them with the shielded room for the GBT control building. Don't know when this purchase will occur. RF: Lets give it a month or two and then get them ourselves if nothing happens. Also, some of the wiring could be put in place now. **Action item for Niday: Check on whether 208V is available close by, and what can be done to get it wired into the chamber. TW: A cable entry plate into the anechoic chamber is in place. However, it is a "quick and dirty." The desired plate is awaiting parts. TW: Cal and Sig/Ref fibers are in place. Transceivers are on order to get signals from the 140' to the mock-up room. Still need transceivers to get from mock-up room to anechoic chamber control room (ACCR). Cost is about $600 for a pair. **Action item for Weadon: Order transmitter/receiver pair to get from mock-up room to anechoic chamber control room. RF: Suggest that TTL version of Sig/Ref in the mock-up room be buffered and permanently wired to transmitter to the ACCR. TW: Single mode fiber to ACCR is in place, but may still require splicing. Not sure how this is routed to the 140'. MM: There are two single mode fibers to the 140'; one from the mock-up room and one from the ACCR. Monitor station design: RF: No progress MM: Same. Indoor Range design: RL: Rough block diagram done. Have studied 3GHz link requirements. Have ordered catalogs from several vendors, and identified some components which we want to buy. However still waiting on additional catalogs before making commitments. Have found an amp from Mini- Circuits that covers our entire bandwidth (30MHz to 3GHz). Will need a power meter to monitor power into the 3GHz link, as it is very sensitive to input level. Power splitters that cover the entire band of interest have been difficult to find; the only exception are resistive splitters which offer little isolation. RF: Suggest buffer amp and pad after splitter to keep signals from the power splitter or spectrum analyzer out of the spectral processor. Status of technician help for IPG work: TW: He (Chuck Niday) is in attendance. RFI testing of feed arm servo: RL: Have what we need for testing with a spectrum analyzer. Still need some components to run signals to the Spectral Processor. Trying to get components that will be useful for the rest of the group's work. May be some problems with closing up PCD's rack to test; there are lots of computers in the room, some of which are used to control the servo. TW: Should be able to get around this since these computers are on the site LAN. List of GBT equipment to be tested: RF: No update. TW: The LO rack can probably be tested next. Need to coordinate with McKinnon as well. IPG tasks for Samantha Fore: MM: Still looking for good ideas. She has already completed all tasks originally specified. TW: May be some work in electronics she can handle. Standardization of Documentation: RF: Next meeting we will go through documentation done to date and make suggestions. QZ Incident: MM: One radio service supplier has squealed on his competitor that the required antenna was not installed. It is impossible to visit all sites to verify compliance with our requirements. This emphasizes the necessity of getting a good monitoring station on the air ASAP. RL: Might be possible to request photograph of antenna installation. Why no call-outs? RF: We have received no call-outs for interference. Why? MM: Since group started, most observations have been at frequencies where there is little interference. Also pulsar observers have learned to adjust their observing around known sources of interference. RF: Suggest we visit telescopes after installations to have a look at the interference environment. Sun Workstation Measurements: RL: RFI measurements of the new workstation Rigel (Sparc Ultra 1) have been made by Niday and Lacasse. The interference is 25 to 40 db above the CCIR limit throughout the range measured (100MHz to 1000MHz). Interference from the monitor was less by 10 to 20 db, but Brandt has pointed out that it would probably be worse if a complicated screen were displayed. (When measured the display was in monitor mode, with some lines of text on it.) CN: A mini shielded room to isolate some components of the system might be useful in isolating the worst sources. Also noticed that some in-house designs do not comply with low-emission design techniques. A case in point is right angle traces on PC boards; chamfered 45 degree corners are simple and much better. RL: Some education may be required. RL: Big question on workstation interference is where do we go from here? RF: Suggest we invite Norrod to next meeting to discuss this topic. It can't be solved by IPG alone. **Action item for Lacasse: Brief Norrod and invite him to the next meeting. MM: Limits imposed by CCIR are very reasonable goals, which we should strive to meet if not exceed. In one band, the limit corresponds to 150 Jy, and observers are trying to observe milli-Jansky sources. RF: Agreed. Next meeting: