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Using chip ferrite beads

G

galapogos

Jan 1, 1970
0
Hi,

I have a design that I've found to be radiating EMI with a spectrum
analyzer and a few near field probes. My design currently doesn't have
any EMI filtering. I'm thinking of using ferrite beads to reduce EMI.
Other than the power and ground lines, I'm wondering where else would
they help? Here are a couple of places I thought may help

- My design has a membrane keypad that connects via an FPC connector.
The keypad acts like an antenna, and radiation is vastly reduced when
I place it on a metal surface. Since right now I'm unable to change
the mechanical design/material of the casing, I'm thinking of of
putting ferrite beads on all of the keypad signals. These are very low
frequency signals so I think should be fine?

- My design has a daughterboard that is connected with a 2x22 IDE type
connector that transfers IDE signals. I also find that this connector
leaks quite badly. However, since IDE signals are pretty high speed,
I'm wondering if using regular ferrite beads will cause any signal
problems?

Btw board boards have separate power and ground planes. The ground
plane of the daughter board is connected via the ground pins of the
IDE signal, and the daughter board actually helps to shield radiation
coming from the main board. I've tested this with the probes with the
daughter board over the main board(and not connected)...radiation is a
lot less than if the main board was naked.

While I didn't design the board with EMC in mind, many of the SI
design guides were followed, which help EMC too, from what I've read.
These include a separate power/gnd plane, correct impedance matching
with trace width/length/stackup configurations, trace length matching
for the high speed signals, trace parallelism for the differential
signals, decoupling caps as near to the chip as possible, etc. These
are all according to the chip/IDE/USB design guidelines...wondering
what else I can do to reduce the EMI radiation? I've tried pasting
aluminum tape(helps), and EMI absorber sheets(doesn't help at all).

Thanks!
 
J

John Popelish

Jan 1, 1970
0
galapogos said:
Hi,

I have a design that I've found to be radiating EMI with a spectrum
analyzer and a few near field probes. My design currently doesn't have
any EMI filtering. I'm thinking of using ferrite beads to reduce EMI.
Other than the power and ground lines, I'm wondering where else would
they help? Here are a couple of places I thought may help

- My design has a membrane keypad that connects via an FPC connector.
The keypad acts like an antenna, and radiation is vastly reduced when
I place it on a metal surface. Since right now I'm unable to change
the mechanical design/material of the casing, I'm thinking of of
putting ferrite beads on all of the keypad signals. These are very low
frequency signals so I think should be fine?

Since any current heading towards the keypad should be
canceled by a current returning from the keypad (except for
the generation of RFI), you might start out with a single
ferrite bead around the FPC cable.

Here is a picture of one of these:
http://rocky.digikey.com/WebLib/Steward/New photos/28R0898-200.jpg
- My design has a daughterboard that is connected with a 2x22 IDE type
connector that transfers IDE signals. I also find that this connector
leaks quite badly. However, since IDE signals are pretty high speed,
I'm wondering if using regular ferrite beads will cause any signal
problems?
(snip)

Not if it surrounds the entire cable and no other grounds or
signal paths exist between these two modules.
 
R

Robert Baer

Jan 1, 1970
0
galapogos said:
Hi,

I have a design that I've found to be radiating EMI with a spectrum
analyzer and a few near field probes. My design currently doesn't have
any EMI filtering. I'm thinking of using ferrite beads to reduce EMI.
Other than the power and ground lines, I'm wondering where else would
they help? Here are a couple of places I thought may help

- My design has a membrane keypad that connects via an FPC connector.
The keypad acts like an antenna, and radiation is vastly reduced when
I place it on a metal surface. Since right now I'm unable to change
the mechanical design/material of the casing, I'm thinking of of
putting ferrite beads on all of the keypad signals. These are very low
frequency signals so I think should be fine?

- My design has a daughterboard that is connected with a 2x22 IDE type
connector that transfers IDE signals. I also find that this connector
leaks quite badly. However, since IDE signals are pretty high speed,
I'm wondering if using regular ferrite beads will cause any signal
problems?

Btw board boards have separate power and ground planes. The ground
plane of the daughter board is connected via the ground pins of the
IDE signal, and the daughter board actually helps to shield radiation
coming from the main board. I've tested this with the probes with the
daughter board over the main board(and not connected)...radiation is a
lot less than if the main board was naked.

While I didn't design the board with EMC in mind, many of the SI
design guides were followed, which help EMC too, from what I've read.
These include a separate power/gnd plane, correct impedance matching
with trace width/length/stackup configurations, trace length matching
for the high speed signals, trace parallelism for the differential
signals, decoupling caps as near to the chip as possible, etc. These
are all according to the chip/IDE/USB design guidelines...wondering
what else I can do to reduce the EMI radiation? I've tried pasting
aluminum tape(helps), and EMI absorber sheets(doesn't help at all).

Thanks!
Without changing the mechanical design of the keyboard, one could
have the inside of the case coated with Acheson nickel-iron equivalent
of Aquadag.
This gives the shielding you saw, and absorbs RFI as well.
 
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