Chris,
Fluorescent tubes are noisy anyway, but the old passive ballasts are
starting to be replaced with active "electronic" ballasts. Those electronic
widgets actually run at frequencies down near what you're using.
Electrically they can be EXTREMELY noisy. The US has no real restrictions.
As far as what you observe, the hand can act as a shield, or as an enabler
which would enhance a picked up signal putting it right into your
electronics.
What's important is that your circuit is very susceptible when exposed and
not when closed. That implies marginality in your design. You should take
a look at just how much you reject and whether that is adequate for your
needs.
- Robert -
Robert A. Macy, PE m...@california.com
408 286 3985 fx 408 297 9121
AJM International Electronics Consultants
619 North First St, San Jose, CA 95112
-----Original Message-----
From: Chris Maxwell <chris.maxw...@nettest.com>
To: EMC-PSTC Internet Forum <emc-p...@majordomo.ieee.org>
List-Post: emc-pstc@listserv.ieee.org
Date: Thursday, September 06, 2001 9:22 AM
Subject: Noise from flourescent light ballasts?
>
>Hi all,
>
>We have personally experienced an interesting phenomenon. We build a
>certain circuit that detects a 20Khz tone. This circuit is housed in a
>product that has an EMI spray coated case. One of our engineers noticed
>that our techs on the manufacturing floor were having a difficult time
>setting up units on the floor. The tone detect circuit kept getting set
>off for apparantly no reason.
>
>However, back in the engineering lab, we have no problems.
>
>So we did a little experiment. We took a unit out to the manufacturing
>floor and opened its case (which is how they work on them in
>manufacturing).
>
>We held it up near the lights. The tone detect circuit went crazy.
>
>We put the unit down on the bench. No problem.
>
>But, if you leave it opened up on the bench and hold your hand over the
>board, the circuit goes off again.
>
>When we close up the case work, all of these problems go away.
>
>So, our obvious solution is to make a modified casework with "tweaker
>holes" so that manufacturing personnel can tweak the amplifier pots with
>the casework closed. But.... I'm still curious. What's causing the
>interference?
>
>I was wondering if flourescent light ballasts could be giving off an
>emission in the KHz range. (Maybe that's why holding it to the lights
>sets it off.) But what about the hand waving? If I assume the ballasts
>are giving off emissions, can I also assume that the human body can
>change the local field pattern?
>
>Am I barking up the wrong tree? Maybe its the LAN cables in the
>ceiling?
>
>Anybody else have a similar experience or some insight into this?
>
>Thanks,
>
>Chris Maxwell | Design Engineer - Optical Division
>email chris.maxw...@nettest.com | dir +1 315 266 5128 | fax +1 315 797
>8024
>
>NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA
>web www.nettest.com | tel +1 315 797 4449 |
>
>
>
>
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