That's a great story, Ken! Doug On Mon, 14 Aug 2017 09:29:04 -0500, Ken Javor wrote: Not a question to the group, a fable – a story with a lesson learned. Was at the IEEE EMC Show south of DC this past week, and stuck around to attend a Friday afternoon session (!) One of the presentations concerned a test in which I had played a part, and it reminded me of something interesting that happened there. A lot of you will not have attended, and of those of you who did, perhaps not all had the fortitude to stick it out until the final hours. And, I had made a mental note to disseminate this information post-test, and then promptly forgot, as is my wont these days. There was a spacecraft integration test where they decided for budget/schedule/risk reduction reasons not to move it into a SAC for the EMC portion, but leave it in a high bay, clean room facility where the balance of the testing occurred. The entire facility is just a few miles from a major airport, and it being an industrial plant, there are many mobile radios in use, and those sorts of intermittent transmissions being the most difficult to pin down as ambients, we decided to do an rf survey of the clean room facility and determined that we needed an rf tent to meet our ambient requirements. We were using one of these newer EMI receivers which had the capability to look at large portions of the spectrum at once, as opposed to tuning and dwelling frequency-by-frequency. Using that capability, we could look at the entire launch vehicle command-destruct band (uhf, a 60 MHz wide band just above 400 MHz) and also, separately, over the entire required spectrum at S-band. S-band had to be monitored to ensure payload transmitter compatibility with some launch site communication links operating at close to the same frequency. The command-destruct band was monitored to ensure that unintentional emissions from the spacecraft as a payload did not interfere with reception of that emergency command, in the event the launch had to be terminated after lift-off. The first rf test was ensuring that the spacecraft didn’t emit excessively in the command-destruct band. When the tent was up, we noticed that from time to time the entire noise floor jumped up above our limit, and then settled down. Some of the less experienced engineers took this to be an intentional radio transmission, but as we were looking at a 60 MHz + wide spectrum, this clearly wasn't the answer. It had to be a broadband event, and it turned out to be either people brushing against the tent and depositing charge which then flowed all over, and/or the ventilation in the facility blowing over the tent and causing the material to bow and ripple like a sail, with the same undesirable ESD end result. We dealt with these problems by tying down that which could be tied down to avoid flapping in the breeze, and cordoning off the tent and placing “rf test in progress” signs around the periphery. People being people, even trained engineers and technicians, they completely ignored the roped off area and signs, so that in addition to the restricted zoning, after several violations and required retests, I assumed the responsibility for guarding the perimeter, doing so with much the same fervor as a junkyard dog, and just about as mean by the time it became obvious that all other enforcement had failed. But all that aside, the point of the story is that with a traditional frequency sweep, these accidental discharges would have occurred at random frequencies to which the EMI receiver just happened to be tuned when the ESD event occurred, and it would have been very difficult to discern that these were in fact broadband events as opposed to random keying of transmitters around the plant or at the airport, i.e., attempting to discriminate between pulsed cw and impulsive signals. This is illustrated graphically in the MIL-STD-461G Table II supporting appendix material dealing with the proper use of such EMI receivers. The ability to observe a very wide portion of spectrum jumping up and then receding was a clear signal to the experienced observer of what was happening, and time domain capability, in addition to greatly speeding the test along, also allowed immediate interpretation of the clue, and then eventually, after solving the people problem by making use of the old (junkyard) dog EMC engineer, the entire problem was put to rest. Ken Javor Phone: (256) 650-5261 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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