[SI-LIST] Radiated Emissions continued
- From: Americomrh@xxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 17 Aug 2005 15:27:17 EDT
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Thanks for the informative replies with regard to the radiation emissions
question. Here is more info detailing the problem.
The chassis, backplane, and front panel are all grounded. The PCB has frame
ground floods on both the upper and lower layers. The front to back
distance of the cavity is about 5.5 inches. The left to right width of the
cavity
is about 15 inches, but there is some intervening metal for card guides. It
is mostly open. The top to bottom size is 1.3125 inches.
The other anomoly is where it comes from. I believe the frequency is
actually 687.5MHz. We have 125MHz on the board. At one time, I divided it by
2 to
get 62.5MHz to drive some logic so I assumed it was the 11th harmonic. I
changed the 62.5MHz to 41.67MHz, but the 687.5MHz did not move. Several other
lower level spikes did move, but they are not a concern. The 687.5MHz
appears to be the 5 1/2 harmonic. Fourier says that can't be. Perhaps a chip
does a divide by 2 internally which I don't know about. We tried bypass caps
on
the power supplies as small as 100pf without effect.
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=
ant cavity?
The chassis, backplane, and front panel are all grounded. The PCB has fr=
ame ground floods on both the upper and lower layers. The front to back dis=
tance of the cavity is about 5.5 inches. The left to right width of the cav=
ity is about 15 inches, but there is some intervening metal for card guides.=
It 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 ac=
tually 687.5MHz. We have 125MHz on the board. At one time, I divided it by=
2 to get 62.5MHz to drive some logic so I assumed it was the 11th harmonic.=
I changed the 62.5MHz to 41.67MHz, but the 687.5MHz did not move. Several=
other lower level spikes did move, but they are not a concern. The 687.5MH=
z appears to be the 5 1/2 harmonic. Fourier says that can't be. Perhaps a=20=
chip does a divide by 2 internally which I don't know about. We tried bypas=
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=
ard a plastic front panel as a solution.
regards
Gary Miller
Pulsecom
>>> <Americomrh@xxxxxxx> 08/16/05 07:42PM >>>
=20
Gary,
=20
My first thought is that a resonance frequency(s) may be inherent in your=20
chassis. A quarter wave length at 688 MHz is about 4.36 in. which can exis=
t=20
within your structure. Any harmonics of 688 MHz (assuming they exist) woul=
d=20
have wave lengths of decreasing distance. Your front panel is grounded,=20
how-wire, Cu, braid, other and how long? I assume that the enclosure is al=
so=20
metal, but it is floating. Therefore can I assume it is isolated from the=20=
front=20
panel? Also, what is "ground," i.e. chassis, earth, green wire, etc.?
=20
When the front panel is removed the cavity is eliminated which stops the=20
build up of resonant modes. Now this is all conjecture on my part in that=20=
I=20
have not experienced any of your development knowledge; therefore, it is ju=
st an=20
educated guess.
=20
Robert
=20
message dated 8/16/2005 8:01:22 A.M. Pacific Daylight Time, =20
Gary.Miller@xxxxxxxxx writes:
Robert,
We have just experienced an unexpected EMI phenomena, and I would like to=20
add a question to our list of questions.
One of our units had an emission problem at 688MHz. The device is roughly=20
5" x 5" x 1.5" with a grounded, metal front panel. When we removed the fro=
nt=20
panel (leaving a gaping hole) the overall emission reduced by 20dB. How is=
=20
this possible? We are looking at isolating the panel from ground as our=20
current solution.
Gary Miller
Pulsecom
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