Re: EMI Site Interference

George . Waters Thu, 26 Mar 1998 21:32:06 +0000

     The duration, and your location could point to a vehicular source, for 
     example a high power CB, or radio telephone in a vehicle.

______________________________ Reply Separator _________________________________
Subject: EMI Site Interference
Author:  [email protected] at PMDF
List-Post: [email protected]
Date:    3/26/98 13:14

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]

Date: Thu, 26 Mar 1998 13:14:35 -0500
From: [email protected] (Scott Douglas)
Subject: EMI Site Interference
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Re: EMI Site Interference

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