Scot, We had a problem at our lab that sounds very similar to yours. It turned out to be dirty power line insulators. We were in the midst of a drought at the time and the rains were not doing a good job cleaning the insulators. The dirt built up to the point that there was conduction through the impurities deposited on the insulators. The bztbztbzt sound, a result of the intermittent making and breaking of the connection through the dirt, was picked up on radios in the parking lot as well as on the spectrum analyzer.
We called the power company, they came by to clean the insulators,and the noise seemed to go away. A couple of things to try when investigating whether dirty - or loose - power line insulators are the culprit: - Set the analyzer trigger to LINE. The noise will bunch up in groups of lines separated by about 1/60 sec - indicates relationship to 60 Hz - If you can, get a bat or large board and hit the power poles one by one while someone else watches the analyzer. The recieved emissions will change in amplitude and pattern in time with strikes to the poles with loose wire/insulators . The power company (PG&E) seemed to know all about this type of problem and was prepared to help. Give yours a call if you suspect power line. Good luck. Tom Cokenias EMC Consultant/Radio Type Approvals >Hello Group - > >Here's a problem for you all to think about and hopefully help solve. > >I have a room in the corner of my building where I perform EMI pre-scans. >Occasionally throughout the day I have a major interference show up on my >analyzer. It does not come from the EUT. This is broadband, from 30 MHz to >1 GHz, at levels up to 70 or 80 dBuv. It is not like a flat carrier but >rather more like PWM driven motor noise. It lasts from 15 seconds to 1 >minute in duration. Then ambient returns to normal. > >We are a typical ITE manufacture. Our Genrad tester, wave solder, air >compressors, motor generators, air conditioners, plumbing waste pumps do >not appear to be the cause although none have been absolutely ruled out. >The frequency and duration would seem to eliminate them however. > >We have swamp on one side, the interstate highway on another, fields on >the third and the town on the last side. The only potential source I can >find is a medical clinic about 350 meters away from us. They are the >nearest building (the next being twice that far) and have an X-ray >machine. Yesterday I went there at lunch and asked was the X-ray in use >that day and they said yes but could not tell me a specific time that it >was used. > >A software engineer here tells me that he has experienced radio problems >in the parking lot outside my corner of the building. When he parks in a >certain spot, his FM radio (the entire band) gets wiped out by a >zhhzhhzhhh kind of noise. Move the car several feet any direction the >problem goes away. This closely resembles what I see on the analyzer. Our >analog guru suggests it is not the x-ray system as x-rays only last a few >seconds. A one minute x-ray would burn you up he says. > >I never noticed this interference in another corner room of my building >diagonally opposite my present location nor have I noticed the FM radio >problem myself. > >Questions: >1. What are the possible sources of such interference? >2. Is the problem radiated or conducted? I suspect radiated as the signal >goes away when you disconnect the antenna from the analyzer. >4. Why does this problem show up in only a small specific location on our >property? >4. What are the potential solutions? > >I will be happy to answer any other questions you think of (if I can) and >I look forward to the interesting comments I will receive from you all. >Thanks in advance. > >Scott Douglas >[email protected]
