EMC FLEX BLOG A site dedicated to Automotive EMC Testing for Electronic Modules

LISN (Line Impedance Stabilization Network) or AN (Artificial Network)

14. September 2015 14:29 by Christian in EMC/EMI, Standards, Test Equipment
Purpose of the LISN:1. Provide well defined RF impedance to the DUT.2. The 1μF & 50μH filt

Purpose of the LISN:
1. Provide well defined RF impedance to the DUT.
2. The 1μF & 50μH filter isolates the noise that is put on the supply lines by DUT from feeding back to the power supply / battery.
3. Provide a low impedance path for the noise to be measured at the output port of the LISN coupling the interference voltage generated by DUT via 0.1μF to the analyzer or receiver.

The role of the LISN is to isolate the DM current and CM current from the power supply, and to minimize the impact of the CM current by returning it to its sources.

The wire harness inductance for large systems (aircraft) is 50μH whereas for small systems (automotive) is 5μH. However, the LISN selection criteria should be based on the frequencies of the measurements required.

     

Types of LISN

  1. V-LISN: Unsymmetrical emissions (line-to-ground)
  2. Delta-LISN: Symmetric emissions (line-to-line)
  3. T-LISN: Asymmetric emissions (mid point line-to-line)

 

There are two types of V-LISN with different impedances.

  • 5 µH inductance (CISPR 16-1-2, CISPR 25, ISO 7637, SAE J1113-41, DO160) are normally used to measure equipment for vehicles, boats and aircrafts connected to on-boards mains with DC or 400 Hz.
  • 50 µH according to CISPR 16-1-2, MIL STD 461 and ANSI C63.4 is intended to operate at mains frequencies of 50 Hz or 60 Hz.

The T-LISN measures the asymmetric disturbance voltage (common mode voltage) and provides it to an EMI Receiver. It is normally used for measuring telecommunication and data transmission equipment connected to symmetrical lines as e.g. twisted pairs.

CISPR-25 (Ed 3.0)
A network inserted in the supply lead or signal/load lead of apparatus to be tested which provides, in a given frequency range, a specified load impedance for the measurement of disturbance voltages and which may isolate the apparatus from the supply or signal sources/loads in that frequency range.
CISPR-25 (Ed 3.0) & ISO-11452-2:2004 & ISO-11452-4
The AN impedance ZPB (tolerance ± 20 %) in the measurement frequency range of 0.1 MHz to 100 MHz it is measured between the terminals P and B with a 50 Ω load on the measurement port and with terminals A and B short-circuited.

  

The 1μF capacitor is populated in CISPR-25 LISN; R=1Kohm.

 

ISO 7637-2:2011 & ISO-11452-2:2004 & ISO 7637-2:2004
The artificial network is used as a reference standard in the laboratory in place of the impedance of the vehicle wiring harness in order to determine the behavior of electrical/electronic devices.
ISO 7637-2:2011 & ISO 7637-2:2004
The resulting values of impedance ZPB, measured between the terminals P and B while terminals A and B are short-circuited, are given in figure below as a function of frequency assuming ideal electric components. In reality, the impedance of an artificial network shall not deviate more than 10 % from the given curve.

  

No 1μF populated in ISO 7637-2 LISN; R =50 ohm, C is function of voltage.

 

Sample setup: CISPR-25 require separate LISN for B+ and GND lines.

Christian Rosu

Electric Field Shielding

14. August 2015 05:17 by Christian in EMC/EMI, Shielding
Types of Electromagnetic Coupling:1) Conducted Coupling2) Electric Field Coupling The Electric

Types of Electromagnetic Coupling: Conducted, Radiation, Magnetic Field, Electric Filed

Electric Field Coupling

The EF lines start on positive charge and end on negative charge from higher voltage conductors to lower voltage conductors. Any two conductors at different potentials (voltages) have electric field lines between them. EF shields are connected to “ground” to maximize their effectiveness.

 

     The electric field lines are passing through ungrounded metallic planes. 

 

Grounding a copper enclosure does not increase or decrease its shielding property but it reduces the crosstalk within the product itself. The ungrounded shield allows coupling signals from circuits within the shielded enclosure. If the device is connected via external cable to another module the ungrounded shield can serve to capacitively couple signals from outside the enclosure. 

EMI Control Techniques

12. July 2015 03:52 by Christian in EMC/EMI, Shielding
EMI suppression involves grounding, shielding, and filtering.1. GroundingAn ideal ground plane is a

EMI suppression involves grounding, shielding, and filtering.

1. Grounding

An ideal ground plane is a zero-potential, zero impedance body that can be used as a reference for all signals in associated circuitry, and to which any undesired current can be transferred for the elimination of its effects.

The multiple-point grounding minimizes ground lead lengths. The ground plane might be a ground wire that is carried throughout the system or a large conductive body.

Bonding

The physical implementation for grounding is done through bonding of a low-impedance path between two metal surfaces to make a structure homogeneous with respect to the flow of electrical currents, thus avoiding the development of potentials between the metallic parts, since such potentials may result in EMI.

  • provide protection from electrical shock
  • power circuit current return paths
  • antenna ground plane connections
  • minimize the potential difference between the devices
  • can carry large fault current
  • direct bond is a metal-tometal contact between the elements connected
  • indirect bond is a contact through the use of conductive jumpers

Bond Quality

The dc resistance Rdc = length of the bond / (conductivity * cross-sectional area)

The ac resistance Rac =  length of the bond / (conductivity * width of the bond * skin depth)

Bonding effectiveness can be expressed as the difference (in dB) between the induced voltages on an equipment case with and without the bond straps.

2. Shielding

The purpose of shielding is to confine radiated energy to a specific region or to prevent radiated energy from entering a specific region. Shields may be in the form of partitions and boxes as well as in the form of cable and connector shields.

Shield types:

  • solid
  • nonsolid (e.g., screen)
  • braid, as is used on cables.

Shielding Effectiveness SE = 10*log(10) * (incident power density / transmitted power density)

  • incident power density is the power density at a measuring point before a shield is installed and the
  • transmitted power is the power density at the same point after the shield is in place
  • electric field strength SE = 20*log(10) * (Ei / Et)
  • magnetic field strength SE = 20*log(10) * (Hi / Ht)

3. Filtering
An electrical filter is a network of lumped or distributed constant resistors, inductors, and capacitors that offers comparatively little opposition to certain frequencies, while blocking the passage of other frequencies. Filters are used to substantially reduce the levels of conducted interference.

Insertion Loss IL = 20*log(10) * (V1 / V2)

  • V1 is the output voltage of a signal source with the filter in the circuit
  • V2 is the output voltage of the signal source without the use of the filter

Low-pass filters IL = 10*log(10) * (1 + F^2) dB

  • F = PI*f*R*C for capacitive filter (f = frequency)
  • F = PI*f*L/R for inductive filter (f = frequency)

Lumped System is an electrical circuit with passive elements (e.g. R, L, C) that are constant.
For example, the current at a capacitor with capacity C is i(t) = C * (dv(t) / dt)

A lumped element size is much smaller than the wavelength of the applied voltages and currents. In this case wave propagation effects may be neglected.

Distributed System is an electrical circuit with passive elements (e.g. R, L, C) where the  inductance, capacity and resistance are not constant but functions of time and space length. This leads to partial derivatives of i(t,x) and v(t,x) in t (time) and x (position).