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Ferrites
What they are
What they do
How to apply
What Ferrites are
5750nH
1520nH
10700nH
12340nH
Ferrites are made of magnetic material
to create an inductance in a current path
708nH
Ferrites: What can they do?
Keep noise at bay.
Noise from:
•
•
•
•
Switched mode power supplies
Variable speed drives
Stepper motors drivers
Ground currents
They can be applied to keep noise out of your signals: signals you monitor
And they can be applied to keep noise in your device: driving signals
How do we get ground currents?
Generator
Receiver
ground
currents
magnetic
field
• By any changing magnetic field in a loop (nearby
currents)
• By capacitive coupling (nearby changing voltages)
How do ground currents generate voltage in
cables?
Generator
Receiver
resistance
ground
currents
• Resistance in the shield/return lead
• Inductive coupling one lead to another (coax
is much better but not perfect)
Ferrites: What they do
How to keep noise out.
RF goes through without
any obstruction.
Ground currents see the
ferrite.
Ferrites:
Test on network analyzer
Ferrites:
Test on network analyzer
Ferrites:
Test on network analyzer
Ferrites:
Test on network analyzer
Ferrites:
Test on network analyzer
Ferrites: What they do
How to keep noise in
Variable speed
drive
Motor
Ferrite
How to apply ferrites
Ferrites inside or outside the shield?
Ferrites how to apply them
Difference between in and outside shield
You are outside of the shield
small device
Computer
Ferrite
You are here
Ferrites how to apply them
Difference between in and outside shield
You are inside of the shield
BIG
DEVICE
Variable
speed
drive
Ferrite
You are here
Ferrites how to apply them
Difference between in and outside shield
Ferrites: how to apply them
How many turns?
Be creative with ferrites!
How to determine noise sources
A simple coil of a few turns, will do the job.
Check the noise level on a VSD
Check the noise level on a VSD
Check the noise level on a VSD
Ferrites: some examples
Ferrites: applied before an
opto coupler
It sounds unlikely but
in some cases putting
a ferrite before an
opto-coupler can
keep noise out.
The first part of the
graph is without a
ferrite on the input.
The second part is
with.
Ferrites: typical noise
Coax cable connected to a sensor, but what we see here is typical noise from a switched
mode power supply. Because there is ground current through the coax shield through the
protective earth of the scope.
A few turns of the coax through a ferrite did significantly reduce the noise.
Ferrites: applied to kill 50Hz
Ferrites: how to interrupt common mode currents
Example: a patch panel at the Australian synchrotron
coax cables
patch
panel
booster
vacuum
chamber
Booster ring
common
mode
current
Ferrites: how to apply them.
Patch panel
metal patch panel
N conn
SMA conn
Equip
ment
noise
plastic patch panel
Equip
ment
What ferrites cannot do.
Kind of noise the ferrite cannot eliminate.
• A ferrite is meant to isolate currents, it does not filter!
• Once the common mode current created a differential mode voltage, the ferrite
can do nothing about this.
•In the case below there will be common mode current through the cable shield
regardless the ferrites.
• The main reason why ferrites work in a network of cables, is because you work in
a low impedance network, and you apply a higher impedance in one cable.
Currents through
the shield, due to
capacitive and/or
inductive coupling
Noisy cable