[SI-LIST] Re: Radiated Emissions continued
- From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>
- To: <Americomrh@xxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 17 Aug 2005 15:28:43 -0600
Robert -
I remember something that Larry Smith said once. He was failing EMI at some
frequency, and called in the EMC guy. The EMC guy looked for lambda/4
distances on the PC board, and found some bypass caps at that distance from
one of the chips. The transfer impedance at lambda/4 is infinite, so the
bypass caps weren't bypassing. You're lambda/4 (air) is 4.292 inches. Trap
cap value is ~ 56pF.
Larry - feel free to correct anything that I missinterpreted.
Ken
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Subject: [SI-LIST] Radiated Emissions continued
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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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