4608X-102-102 Datasheet, PDF (45/62 Pages) Bourns Electronic Solutions – Resistor Networks Product Selection Guide

EMI/RFI Filters 601 Series
Reducing EMI/RFI
The radiation of electromagnetic interference and radio fre-
quency interference (EMI/RFI) to the environment is a pressing
concern for many manufacturers of electronic equipment.
According to FCC regulations (Parts 15 and18), emissions must
not exceed certain maximum levels depending on whether the
equipment is for strictly industrial use or also for residential use.
A graphical representation of these limits is shown in Figure 1.
Similar restrictions apply to equipment sold in Europe (VDE
0871, a West German standard), Japan (VCCI), and to the mili-
tary (MIL-STD-461/462.)
F.C.C. LIMIT FOR CLASS A (INDUSTRIAL) EQUIPMENT AT 30 METERS.
225
F.C.C. LIMIT FOR CLASS B (MASS MARKET) EQUIPMENT AT 3 METERS.
200
175
150
125
100
75
50
25
25 54 88 108 136 174 216 300 400 470 1000
FREQUENCY IN MEGAHERTZ
Figure 1.
F.C.C. radiation limits for class A
and class B computing devices
RS
Vin
R
R
C
RL _Vout
Figure 2.
basic T-Filter configuration
Under steady state conditions, the capacitor C offers an infi-
nite impedance to the DC component of the input waveform
(which will be assumed for the moment to be entering from the
left side). Thus, the DC component of the signal voltage is
passed to the load, but reduced in value by the voltage drop
across the two resistors.
The impedance of C becomes lower at higher (noise) frequen-
cies. Thus, the noise component of the signal faces a voltage
divider consisting of the first resistor (R) and C. At the high fre-
quencies of the noise component, R will be much greater than
the impedance of C, therefore, most of the noise voltage will be
dropped across the resistor. Almost no noise current flows
through the load and, therefore, will hardly affect the DC volt-
ages (i.e., the signal) across the load.
Since the filter is symmetric, its principle of operation is the
same for waveforms traveling in the opposite direction, in which
case the voltage divider is formed by the second resistor and
the capacitor. Such a symmetrical design is useful for filtering
signals on a bidirectional bus.
Assuming purely resistive source and load impedances, the
transfer function is given by:
Vout ₌
RL
Vin
jωC(R + RS)(R + RL) + (RS + RL + 2R)
Several approaches are available today to control EMI/RFI
emissions, including grounded metal enclosures, shielded
cables, judicious component placement and interconnect
designs, power-supply decoupling, and low-pass filtering of sig-
nal lines.
Low-pass filtering can be effective for EMI/RFI filtering when
the noise components to be rejected occur at frequencies high-
er than the signal frequency (to be passed). For these situations,
Bourns has developed low-pass resistor-capacitor filter net-
works which are ideal for board-level EMI/RFI filtering.
A typical application would be to filter signal lines between
RS-232 drivers and their corresponding connectors. In such low
to medium frequency applications, these networks represent a
more useful (and economical) solution than inductive type filters
such as ferrite beads. In fact, ferrite beads become mostly inef-
fective below 10MHz.
The basic “T” configuration (Figure 2) is a standard R-C net-
work available in versions for 7 or 8 input lines. The 8 input-line
version is available in both through-hole DIP and surface-mount
models.
Specifications are subject to change without notice.
FOR PRODUCT SPECIFICATIONS, SEE PAGES 305 AND 306.
321