Rockwell Automation System Design for the Control of Electrical Noise User Manual
Page 43
Publication GMC-RM001A-EN-P — July 2001
Shielding Wires, Cables, and Components
4-3
Figure 4.3
Ferrite sleeves increase common mode impedance
Figure 4.4
Common mode rejection in shielded cable
The following implementation guidelines apply to ferrite sleeves:
•
Always install ferrite sleeves to data cables where specified.
•
Always use ferrite sleeves when cable length is greater than 10 m
(30 ft).
•
If power frequency ground currents are expected, or measured by
current clamp, one shield/ground connection could be made via a
1uF, 50V capacitor.
Ferrite sleeve greatly
increases impedance at RF
Signal
Source
Differential noise voltage
Panel B
Panel A
Optional capacitor
V
Vn
1:1
0V
Vn
Vn
In this physical circuit, the core and
shield are effectively connected
together at the transmit end.
Vn is the noise voltage.
In this equivalent circuit, the core
and shield form two windings of a
1:1 transformer.
The ferrite sleeve (more turns are
better) forms a core increasing the
magnetic coupling. The signal is
unaffected.
This is known as a common-mode
choke.
Secondary voltage
matches the
primary voltage.
Transmit
Core
Transmit
Core
Receive
Core
Receive
Core
Ferrite Sleeve
Transmit Shield
Plane
Transmit Shield
Plane
Receive Shield
Plane
Receive Shield
Plane