PCB Design Principles for EMC

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					 PCB Design Principles for EMC

  Tony Maddocks and Martin Ganley
          ERA Technology
EMC and Safety Engineering Department
                         Basic principles


Semi-conductor device                           Receiver/transmitter
                                    Antenna




                                              Transmission line
                            Equivalent to:



  Track, wire or cable
Concept of EMC Control Levels


              Option B



                         Primary


   Option A
                              External cable

     Circuit board


     Enclosure
      Common & Differential Mode radiation

                                  Radiated Emission




   I/O Cable                                                    PWB
                     1cm

PWB                                                 Signal

                      GND Plane
                       or Grid                  I
                                                GND
       GND Wire

Common mode radiation             Differential mode radiation
   (monopole type)                        (loop type)
              Estimating Radiated Fields

 •   Short dipole                               E = 0.I.l.f./2r
      – high impedance
 •   Small loop                                 E = 0.I.A. π.f2 / 2r.c
      – low impedance


E = field strength (V/m) , 0 = permeability of free space (4..10-7
H/m), I = current (A), I = Length (m), A = area (m2), f = frequency
(Hz), r = distance (m), c = velocity of light (m/s)

        Example:         Loop 1cmx1cm, I=100 mA, f=50 MHz, r=10m

                              E=30.3 dB(V/m)
                 Prediction of emissions



       Source Frequency Spectrum                     Spectrum of Radiated Field

80                                            60                                       0.1s
                                                                                       0.3s
60                                 0.1        40


40                         0.3                20
           Frequency MHz                                   Frequency MHz

 0.1              1.0                    10    0.1               1.0              10
          1/             1/tr
       Common & Differential Mode Radiation

                  ID        IC         I1




                         Area = A                      IC = (I1 + I2)/2
       ~                                                ID = (I1 - I2)/2



                  ID        IC          I2



                                 
•   A smaller common-mode current can produce higher emissions than a
    larger differential mode current
PCB Track Returns


Clock line      Ideal return path

                    Poor OV return layout
                       Ground Planes
    OV Track                                 Signal Track




                                         Insulation Material
               Circuit Component
                                                                                   Signal Track

Double sided Board




                                   Copper Ground
                                    Plane (OV)
                                                                                                  Insulation
                                                                       Circuit Component
                                                                                                   Material


                                              Copper Ground Plane Board : 20-30 dB better
Preferred Component Arrangement:
No high frequency signals off the board
                       Cable       (Low frequency interface)
                      Connector
        Connector




                                              Connector
                    High-speed
Cable               components                            Cable



                    Medium-speed
                     components
                     Slow-speed
                     components
                      Connector

                       Cable
PCB Grounding Scheme




                        Noisy
     Analog




              Digital
       Summary

• Design and layout of PCBs can control emissions
  and immunity of products
• Important in the range 300-2000 MHz
• Track layout is the most important consideration
• Multi-layer configurations give up to 30 dB
  improvement over single sided boards
• Routing and length of bus tracks
• Careful interface design is required to prevent
  energy transfer to external cables
• Reciprocity is generally applicable
     Software tools for EMC Analysis
EMC Design Rule Checking                               Emission Prediction
                                   Near fields




                           Field Strength [dB(uV/m)]                Far Fields
      Design Rule Checking Tools

• Effective implementation of rules for very
  complex boards
• Give a “goodness” factor not a field strength
• Only as good as the rules used
• May need to tailor the rules for type of
  application
        Emission Prediction Tools

•   Field strength/signal integrity calculations
•   Computer intensive – analyse less tracks
•   Various methods (MM/BEM, FDTD etc)
•   Speed up simulation with approximations
    e.g. Spice-type circuit simulation followed by
    emissions
Validation – 6 track PCB
Validation – improved measurements

  Monopole
                        Ground plane
  antenna

                        PCB




             Network
             analyser
                            Validation – ideal sources
Field Strength [dB(uV/m)]
Validation – Using ICs
      Summary of EMC tools

• Design Rule Checking and Emission
  Prediction tools are complimentary
• The tools are being used more widely and
  are developing all the time
• Advances in computer speeds and memory
  as well as improvements to methods is
  increasing usability