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

Shielding Enclosures & Cables

16. December 2020 15:52 by Christian in EMC/EMI, Shielding
Shielded enclosure example: Shielded enclosure connector types: Shielded cable types: Foil provi

Shielded enclosure example:

Shielded enclosure connector types:

See Troubleshooting RF Noise and Fixing Ground Loops

Shielded cable types:

  • Foil provides high-frequency shielding.
  • Drain wire carries most of the low- frequency current.

 

  • Foil provides high-frequency shielding.
  • Braid carries high-currents.

Why Cable Shielding?

  • Serves different purposes in different applications.
  • Sometimes carries intentional signal currents. This is an example of self-shielding.
  • May prevent coupling of external electric or magnetic fields to signals carried by wires in the cable.
  • Beware of transfer impedance data. It should only be used to compare similar cables for a similar application measured with the same test set-up.

Summary of Main Points:

  • Electric field shielding, magnetic field shielding, cable shielding and shielded enclosures are very different concepts requiring different materials and
    approaches.
  • It is important to be understand the coupling mechanism you are attempting to attenuated before coming up with a shielding strategy.
  • Electric field shields terminate or redirect electric fields. Where they are “grounded” is often critical.
  • Magnetic field shields redirect the magnetic field. Magnetic fields cannot be terminated. 
  • Low frequency (<kHz) magnetic fields must be redirected with high permeability materials. 
  • High frequency magnetic fields can be redirected with good conductors of sufficient thickness.
  • Shields and imperfect shielded enclosures can significantly increase radiated emissions. 
  • Shields reduce radiated emissions by disrupting the coupling from near-field sources and the “antennas” in a system. 
  • In the near field, shields are either electric or magnetic field shields that redirect high-frequency current flow.

 

 

VSWR (Voltage Standing Wave Ratio)

16. December 2020 09:06 by Christian in EMC/EMI, Test Equipment, Calibrations
For a radio (transmitter or receiver) to deliver power to an&amp;nbsp;antenna, the impedance of the radi

For a radio (transmitter or receiver) to deliver power to an antenna, the impedance of the radio and transmission line must be well matched to the antenna's impedance.

The parameter VSWR is a measure that numerically describes how well the antenna impedance is matched to the radio or transmission line it is connected to.

The voltage component of a standing wave in a uniform transmission line consists of the

  • forward wave (with complex amplitude V_{f}) superimposed on the
  • reflected wave (with complex amplitude V_{r}).

A wave is partly reflected when a transmission line is terminated with other than an impedance equal to its characteristic impedance.

The reflection coefficient \Gamma  can be defined as:

\Gamma  = Vr / Vf

\Gamma  is a complex number that describes both the magnitude and the phase shift of the reflection. The simplest cases with \Gamma  measured at the load are:

  •    {\displaystyle \Gamma =-1}   complete negative reflection, when the line is short-circuited,
  •    \Gamma =0       no reflection,                              when the line is perfectly matched,
  •    {\displaystyle \Gamma =+1}   complete positive reflection,  when the line is open-circuited.

The voltage standing wave ratio is then:

See Tutorials

ANTENNA TYPE

16. December 2020 06:35 by Christian in EMC/EMI, Troubleshooting, Test Equipment
ANTENNA TYPE FREQUENCY RANGE USAGE NOTES LOOP 1 KHz TO 30 MHz Magnetic Field 20 dB dynamic range f
ANTENNAFREQ. RANGEUSAGENOTES
LOOP1 KHz TO 30 MHzMagnetic Field20 dB dynamic range for 1 KHz
ROD1 KHz TO 30 MHzRadiated Emissions41 inches long, uses ground plane and active amplifier
BICONICAL20 MHz to 200 MHzRadiated EmissionsRequired by automotive standards
DIPOLE100 MHz to 1 GHzShielding Effectivenessmore efficient above 400 MHz 
LOG PERIODIC200 MHz to 1 GHzRadiated EmissionsCISPR 25
BICONLOG20 MHz to 1 GHzRadiated EmissionsEuropeean requirements
LOG SPIRAL200 MHz to 10 GHz Shielding EffectivenessCone-shaped. Can't distiguish between horizontal and vertical polarization
HORNabove 1 GHzRE, RI ALSEHighly efficient, directional, can be harmful causing blindness, glaucoma
RIDGED HORN1 GHz to 10 GHzRI ALSEBroadband
HOOD RI ALSEHorn antenna with a metallic hood around it for safety.
HAND-HELD Shielding Effectiveness 
DISCONE  Shielding Effectivenessnot directional
YAGIbelow 100 MHz Shielding Effectiveness 

Wavelength

16. December 2020 06:17 by Christian in Troubleshooting
Wavelength at 1 GHz = 1 foot

Rule of thumb: Wavelength at 1 GHz = 1 foot

(3 * 10meter/second)/(109 cycles/second) = 3 * 10-1 meters = 0.3 meters = 30cm

30 cm / 2.54 cm/inch = 11.81 inches

FrequencyWavelength
1 GHz1 ft
100 MHz10 ft
200 MHz5 ft
400 MHz2 1/2 ft
10 GHz1/10th feet = 1 1/4 inch
18 GHz1/20th feeth = 5/8 inch

Common Impedance Coupling, Common Power Supply

15. December 2020 17:01 by Christian in Grounding, Noise Coupling, Troubleshooting
When two circuits share a common ground, the ground voltage of each one is affected by the ground cu

See Ground Return & Common Impedance Coupling

When two circuits share a common ground, the ground voltage of each one is affected by the ground current of the other circuit.

When two circuits share a common power supply, current drawn by one circuit affects the voltage at the other circuit.