[SI-LIST] Re: Radiated Emissions continued

  • From: "Curt McNamara" <CurtM@xxxxxxxxxxx>
  • To: <Americomrh@xxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 17 Aug 2005 14:59:03 -0500

Here is an excellent summary article on attaching circuit boards to
chassis for control of EMI. These techniques may have an impact on your
signal integrity as well.

http://www.compliance-club.com/archive/article.asp?artid=3D90

The source of your radiated energy may be the frequency content in your
signal edge rate (rise and fall time), rather than the frequency of your
clock. The source of your problem may well be a trace with poor signal
integrity.=20

                                Curt

Curt McNamara. P.E.=20
Senior Electrical Engineer=20
Logic Product Development=20
411 Washington Ave. N Suite 101=20
Minneapolis, MN 55401=20
Tel 612-436-5178=20
Fax 612-672-0443=20
curtm@xxxxxxxxxxx=20
www.logicpd.com=20
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Americomrh@xxxxxxx
Sent: Wednesday, August 17, 2005 2:27 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Radiated Emissions continued


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Thanks  for the informative replies with regard to the radiation
emissions =20
question.  Here is more info detailing the problem.

The  chassis, backplane, and front panel are all grounded.  The PCB has
frame=20
 ground floods on both the upper and lower layers.  The front to back =20
distance of the cavity is about 5.5 inches.  The left to right width of
the cavity=20
is about 15 inches, but there is some intervening metal for card
guides.  It=20
is mostly open.  The top to bottom size is 1.3125  inches.=20
The other anomoly is where it comes from.  I  believe the frequency is=20
actually 687.5MHz.  We have 125MHz on the  board.  At one time, I
divided it by 2 to=20
get 62.5MHz to drive some logic  so I assumed it was the 11th harmonic.
I=20
changed the 62.5MHz to  41.67MHz, but the 687.5MHz did not move.
Several other=20
lower level  spikes did move, but they are not a concern.  The 687.5MHz=20
appears to be  the 5 1/2 harmonic.  Fourier says that can't be.  Perhaps
a chip =20
does a divide by 2 internally which I don't know about.  We tried bypass
caps on=20
the power supplies as small as 100pf without effect.
=20
Robert Hanson








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Date: Wed, 17 Aug 2005 09:12:40 -0400
From: "Gary Miller" <Gary.Miller@xxxxxxxxx>
To: <Americomrh@xxxxxxx>
Subject: Re: Response to EMI Questions
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Robert,
    The resonant cavity is an interesting thought.  What is the Q of a
reson=3D ant cavity?
   The chassis, backplane, and front panel are all grounded.  The PCB
has fr=3D ame ground floods on both the upper and lower layers.  The =
front
to back dis=3D tance of the cavity is about 5.5 inches.  The left to =
right
width of the cav=3D ity is about 15 inches, but there is some =
intervening
metal for card guides.=3D
  It is mostly open.  The top to bottom size is 1.3125 inches.=3D20
    The other anomoly is where it comes from.  I believe the frequency
is ac=3D tually 687.5MHz.  We have 125MHz on the board.  At one time, I
divided it by=3D  2 to get 62.5MHz to drive some logic so I assumed it =
was
the 11th harmonic.=3D
  I changed the 62.5MHz to 41.67MHz, but the 687.5MHz did not move.
Several=3D  other lower level spikes did move, but they are not a =
concern.
The 687.5MH=3D z appears to be the 5 1/2 harmonic.  Fourier says that
can't be.  Perhaps a=3D20=3D chip does a divide by 2 internally which I
don't know about.  We tried bypas=3D s caps on the power supplies as =
small
as 100pf without effect.
    Perhaps we can discuss this when you arrive.  For now, we are moving
tow=3D ard a plastic front panel as a solution.

regards
Gary Miller
Pulsecom

>>> <Americomrh@xxxxxxx> 08/16/05 07:42PM >>>
=3D20
Gary,
=3D20
My first thought is that a resonance frequency(s) may be inherent  in
your=3D20 chassis.  A quarter wave length at 688 MHz is about 4.36 in.
which  can exis=3D t=3D20 within your structure.  Any harmonics of 688 =
MHz
(assuming they  exist) woul=3D d=3D20 have wave lengths of decreasing
distance.  Your front panel is  grounded,=3D20 how-wire, Cu, braid, =
other
and how long?  I assume that the  enclosure is al=3D so=3D20 metal, but =
it
is floating.  Therefore can  I assume it is isolated from the=3D20=3D
front=3D20 panel?  Also, what is  "ground," i.e. chassis, earth, green
wire, etc.? =3D20 When the front panel is removed the cavity is =
eliminated
which  stops the=3D20 build up of resonant modes.  Now this is all
conjecture on my  part in that=3D20=3D I=3D20 have not experienced any =
of your
development knowledge;  therefore, it is ju=3D st an=3D20 educated =
guess.
=3D20 Robert =3D20 message dated 8/16/2005 8:01:22 A.M. Pacific Daylight
Time, =3D20 Gary.Miller@xxxxxxxxx writes:

Robert,
We have just experienced an unexpected  EMI phenomena, and I would like
to=3D20 add a question to our list of  questions.

One of our units had an emission problem at  688MHz.  The device is
roughly=3D20 5" x 5" x 1.5" with a grounded, metal  front panel.  When =
we
removed the fro=3D nt=3D20 panel (leaving a gaping hole) the  overall
emission reduced by 20dB.  How is=3D =3D20 this possible?  We are  =
looking
at isolating the panel from ground as our=3D20 current  solution.

Gary Miller
Pulsecom





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