xpensive trial and error random com-
ponent change and retest cycle.
The LISN UP is connected to the output ports of either
a dual line LISN or two identical single line LISNs.
Care must be taken to make sure the interconnecting
cables are identical in length and construction and have
good RF shielding effectiveness. The output of the
LISN UP can be switched between the common mode
component and the differential mode component by a
manual two position switch without changing any ex-
ternal RF connections.
LISN UP
Application Note
Common mode currents flow on each lead and return on
the ground plane or green wire safety ground. The ampli-
tude and phase of these currents are the same on both con-
ductors. A diagram of this is shown in Figure 2.
What is the LISN UP?
How Does It Work ?
First, we need to review some of the theory so we can
start from a common understanding of noise voltage
modes. In a typical circuit, conventional current flows
from the positive voltage terminal to the load and it
eventually returns to the negative terminal of the volt-
age source as shown in Figure 1. This is differential
mode current. The current flowing into the load is
equal to the current flowing out of the load and back to
the source. The only difference is the phase between
these two currents. They differ by 180
o
and ideally they
would cancel out.
Putting these two concepts together and examining the
current flows we find that the net current on any of the
conductors is algebraic sum of the differential mode
current and the common mode current. The current re-
turning via the green wire ground is the sum of the
common mode currents or twice the common mode cur-
rent of any one of the conductors. This is illustrated in
Figure 3. Knowledge of these characteristics will be
used to our advantage when we need to determine
whether the voltages are common mode or differential
mode.
2

The LISN UP is designed to be used with either a dual LISN or
two individual LISNS. The unit under test is connected to the
power supply and LISN the same way it has always connected.
The differences are the connections between the 50 ohm meas-
urement ports and the EMI Receiver or spectrum analyzer. Each
RF measurement port on the LISN is attached to the LISN UP
input ports. The LISN UP Output port is connected to the meas-
urement receiver. The RF output is switched between common
mode and differential mode. Typical connections are shown in
Figure 6. The coaxial cables between the LISN RF output ports
and the LISN UP Input ports must be identical to maintain the
performance of the LISN UP. The cables must be of identical
length, shielding quality and characteristic impedance. If adapt-
ers are used they too must be identical.
How Do I Set It Up ?
Since we know the equations to describe the current in
each conductor we can perform mathematical opera-
tions to separate the differential mode and common
mode components. The equations are listed below:
I
a
= I
cm
+ I
dm

I
b
= I
cm
- I
dm
These currents flow across the impedance of the LISN which
give us a corresponding noise voltage to measure. The noise
voltage for each conductor can be described as follows:
V
a
= (I
cm
+ I
dm) *
Z
lisn

V
b
= (I
cm
- I
dm) *
Z
lisn
If we sum V
a
and V
b
together, ideally we will obtain twice the
common mode voltage component and the differential mode
component will cancel. This means that if we had equal com-
mon voltage magnitudes on both conductors and they had the
exact same phase we would get twice the voltage or 6 dB higher
than the magnitude of either line. This is illustrated in Figure 4.
the differential mode component is added. We ideally get twice
the differential mode component. This means that if we had
equal differential mode voltage magnitudes on both conductors
and they had the exact same phase we would get twice the volt-
age or 6 dB higher than the magnitude of either line. This is
To separate out the differential mode voltage an additional op-
eration must be performed. One of the inputs to the summing
junction must have the phase inverted 180
o
prior to summing the
two voltage signals together. When the two signals are summed
together the common mode component is ideally canceled and
How Do I Use It ?
The LISN UP is relatively simple to use if you have performed
conducted voltage emissions tests, and have an understanding of
the fundamentals of common mode, differential mode, and fil-
tering. Typically, in the past, conducted voltage emissions were
performed and the equipment under test passed or failed. If it
failed you ended up in a try a fix and repeat the measurement
scenario. Depending on your experience and luck you may have
fixed the problem in a few hours or weeks. The LISN UP will
help you focus on the dominant noise mode for those out of
specification conditions rather than blindly changing compo-
nents in the hope it fixes the problem. You may have a common 3

mode problem yet you decide to change a differential mode fil-
ter component. You repeat the measurement on both lines and
you find there is no difference in the performance. Had you
known that the common mode component was dominant you
would have chosen to add a common mode component or mod-
ify the existing common mode component. This would have
saved you valuable time!
A generic procedure for troubleshooting your input EMI filter
out of compliance conditions is outlined below.
GENERIC PROCEDURE
The RF conducted voltage emissions for each power line is gen-
erally measured first to determine compliance to the applicable
regulatory specification. If there are non-compliances, the LISN
UP is used to determine the dominant modes of conducted noise
voltage. The dominant mode at each frequency of non-
compliance needs to be characterized to efficiently troubleshoot
and modify the EMI filter. This is accomplished using the test
procedure below:
Step 1. Measure the conducted voltage emissions for each

line under test using the required LISNs.
Step 2. Note any non-compliant voltage emissions conditions

Record each non-compliant frequency, amplitude and

the offending line.
Step 3. Connect the LISN UP to the dual LISN or (2) single

LISNs. This is accomplished by connecting the LISN

RF Output Ports to the RF Input Ports of the LISN UP.

Connect the measurement receiver to the RF output of

the LISN UP. See Figure 6.
Step 4. Power up the equipment under test.
Step 5. With the LISN-UP RF Output Switch in the Differen-

tial Mode position and measure the conducted voltage

emissions at the first non-compliant frequency.
Step 6. Record the frequency and amplitude for the Differential
Mode
measurement.
Step 7. Position the LISN-UP RF Output Switch to the Com

mon mode position and measure the conducted voltage

emission at this frequency.
Step 8. Compare the relative amplitudes of the Common Mode

component and the Differential Mode component of the

conducted noise voltage. If either component is signif-

cantly larger than the other, that is the dominant mode.

There will be cases where the amplitudes are of similar

magnitudes for both components. In this case, there is

both common mode and differential mode noise present

and no one mode dominates.
Step 9
. Repeat this for each non-compliant frequency until all

have been identified.
Step 10. Depending on which mode(s) are dominant the filter

components that need to be changed can easily be
identified.



Step 11. After the EMI filter has been modified repeat steps 1
and
2.
Step 12. If the equipment under test is still out of compliance

repeat steps 3 to 11 until the EMI Filter meets the
applicable
specification.
Which Filter Components Are
Differential Mode or
Common Mode?
Differential mode filter components are usually the components
that are either: line to line capacitors, series inductors and
sometimes leakage inductance from a common mode choke.
Common mode components are line to chassis capacitors and
common mode chokes. A generic EMI filter diagram is shown
in Figure 7 and the components are labeled differential mode or
common mode. The differential mode components are labeled
with a dm subscript and the common mode components are la-
beled with the cm subscript.
Example Problem
A simple conducted noise source and EMI Filter were designed
and constructed to demonstrate how the LISN UP is used. This
will help the user better understand how the LISN UP works,
what to expect and how to interpret the data. A diagram of the
test setup is shown in Figure 8. We will use the generic proce-
dure as a guide.
Step 1. We measured the conducted voltage emissions of the

+5Vdc line and the power Return line. This is shown
in
Figure
9.
Step 2. We find that the conducted voltage emission levels

are higher than 40 dBuVrms on both lines over much

of the frequency range of 10 kHz to 30 MHz.
Step 3. Connect up the LISN UP as shown in Figure 10.
Step 4. Equipment under test is powered ON.
Step 5. Position the RF output switch to the differential mode

position and measure output.
Step 6. Record the differential mode voltage data. The differ-

ential mode data is shown in Figure 11.
Step 7. Position the RF output switch to the common Mode

position and measure the output.
Step 8. Record the common mode data. The common mode

data is shown in Figure 12. Compare t