L3Harris NARDA PMM ER8000 User Manual page 35

4.1 Measuring the
EMI Voltage
4.1.1 Measuring Principle
with a LISN
4.1.2 Coupling Networks
Document ER8000EN-01101-1.00 - © NARDA 2020
4 – Applications
ElectroMagnetic Interference (EMI) voltage measurements on power supply
lines or on signal lines are carried out by means of "Coupling Networks"
(e.g. LISNs) or other transducers (e.g. voltage probes). The frequency
range is dictated by the applicable standard, however it is generally limited
from 9 kHz to 30 MHz in commercial applications, while for measurements
on other equipment/accessories - e.g. military, automotive, information
technology and communication equipment (ITE), ISDN devices, etc. - the
frequency range extends up to 200 MHz, depending the relevant standards.
Nevertheless, the most common tests in the frequency range covered by
PMM ER8000 are certainly the conducted measurements, done with the
help of a LISN, and the ratiated ones, for which various types of antennas
can be used.
In the case of a system with two floating conductors, the EMI voltages of
the two conductors relative to each other and with reference to the ground
form a vector system where three kind of RFI voltages are present
(sometime EMI is also referred as RFI: Radio Frequency Interference).
They are:
• Symmetrical (or differential mode) voltages. These RFI components are
measured between the two conductors. They behave like the wanted
signal on the forward and return lines.
• Asymmetrical (or common mode) voltages. These EMI components are
measured between the electrical midpoint of the two conductor voltages
and reference ground. Usually these are the components most likely to
cause interference effects.
• Unsymmetrical voltages. These RFI components are measured from
each line conductor and the reference ground. They consist of
symmetrical and asymmetrical components. The measurement of these
RFI voltages is the easiest to do and the most commonly performed.
Indeed, for practical reasons, Standards mostly specify the measurements
of unsymmetrical voltages instead of the theoretically more meaningful
common mode EMI, but the standard doesn't necessarily reflect the real life
exactly as it is: it shall be a common practice reference aiming to create a
database of fully comparable results.
See figure 4-1 for a graphic explanations of these differences between the
EMI voltages.
Some Regulations require both measurement of symmetrical and
asymmetrical RFI voltages, also defining separate and different limits for
them.
Coupling Networks are electrical interfaces which allow to "transfer" the
quantity to be measured (e.g. EMI voltages or currents) from the lines
under test to the receiver.
Some types of Coupling Networks are: AMN (Artificial Mains Network), also
known as "LISN" (Line Impedance Stabilization Network), Current Probe
and Voltage Probe. Again, Fig. 4-1 also shows the AMN principle.
The PMM ER8000 is already equipped with a built-in LISN.
Applications 4-1