Re: Noise

["Clarence Dittman" <Clarence.Dittman@tdh.state.tx.us>]

Date sent:      	Tue, 1 Sep 1998 14:58:55 -0500 (CDT)
Send reply to:  	radsafe@romulus.ehs.uiuc.edu
From:           	olga fernandez-flygare <olga@ucla.edu>
To:             	Multiple recipients of list <radsafe@romulus.ehs.uiuc.edu>
Subject:        	Noise

First, check your AC power lines.  Does UCLA ring the class bells by 
superimposing a high frequency signal on the 60 Hz AC power lines?

While working in a research lab in the 60's at the University of Texas (now 
called "at Austin") we discovered that the class bells on campus were indeed 
rung by a high frequency signal transmittted over the AC power lines!  (This 
after several denials by the power generation staff that there was anything 
except pure 60 Hz on the power lines. It took us about a year to realize the 
transient occurred ten minuites to the hour and on the hour, and then some bull-
headed confrontation with the "powers that be" to find out the secret of who 
rang the class bells, and how!!!)

Our solution was to design and install low pass filters on each AC power input 
to our electronic chassis.  We designed for a 3 db down point at about 100 Hz 
to allow no attenuation at 60 Hz with a steep enough attenuation slope that the 
high frequency signal we didn't want was suppressed by as much as possible.  
(The figure of 120 db comes to mind.)

If this is not your problem, make sure your chassis is well grounded, and your 
electronics are completely enclosed in a grounded metal chassis (which should 
block RF transmissin into your electronics).  Then cross check all signal wires 
that penetrate this chasis.

We followed the old trouble shooting principle that says first check to see the 
instrument is plugged into the wall outlet!  Start with the basics.  We went back 
to the AC power purity issue several times while we cycled through attempts to 
find where the spurious signal was coming from, including redesign of our 
equipment.  We would complete construction using a new design, only
to have the transient reappear.  When we finally realized the transient occurred 
when the class bell rang...We realized we had been asking the wrong people 
about the "purity" of the 60 Hz power on campus.  Even then we got on answer 
that there wasn't anything on the AC power that wasn't supposed to be there, 
60 Hz and the bell ringing pulse...


> Hello Radsafers:
> 
> While trying to trace the source of noise in our gamma spectroscopy system,
> we observed a transient sinusoidal signal on our oscilloscope. We estimate
> the frequency to be 0.9-1.0 MHz.
> It appears to be a repeatable but not periodic radiofrequency signal.
> 
> Have you observed a similar phenomenon? Any ideas on the source of this
> noise (we are in the basement of UCLA medical center)?      Any RF
> shielding solutions?
> 
> Any help would be appreciated.
> 
> Sincerely,
> 
> Tim Paul + Olga Fernandez-Flygare
> UCLA Radiation Safety Division
> (310) 794-7365
> (310) 206-5550
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> The RADSAFE Frequently Asked Questions list, archives and subscription
> information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html


Clarence W. Dittman, Health Physicist
Radiation Control Program Manager
Texas Department of Health
Public Health Region 7
2408 South 37th Street
Temple, Texas 76504-7168
(254) 778-6744
Fax (254) 778-4066
Clarence.Dittman@tdh.state.tx.us
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