Hi Group,
I am going to share the analysis and design of device to separate Differential Mode, DM, and Common Mode, CM, conducted emissions from two LISNs.
If you Google this subject this device is often called a 'LISN Mate' I believe the term was invented by M.J. Nave.
These devices are available commercially, one device is the Tekbox TBLM1.
Not all of the information on the internet and the designs of some of the commercial designs are correct. After I have analyzed some of the solutions that can be found on the web, I will present the design and construction of a device that works properly.
We will start with an analysis of the issue, so that we understand the requirements.
AnalysisI am going to start with the analysis of the LISN.
The name LISN, stands for Line Impedance Stabilization Network. It presents a defined impedance to the device under test.
Common LISNs are 5uH and 50uH.
The impedance is defined by the parallel combination of the inductor and the resistor.
LISN ModelThis model can be used to measure the impedance of the LISN.
This is the impedance it is 50
decreasing at low frequencies.
2 LISNs and CM and DM ImpedancesTwo LISNs are used in most EMC measurements to capture CM and DM emissions.
DM ImpedanceWhen two LISNs are used:
The result is:
It should not be a surprise, but the impedance for DM signals is 100
CM ImpedanceSimilarly for CM:
The resulting impedance:
The answer 25
should not be a surprise.
Target SpecificationWhen the LISN Mate is introduced to separate DM and CM emissions these Impedances should not change.The DM impedance should be 100
The CM impedance should be 25
To be continued …
Jay_Diddy_B