Controlled Impedance An introduction No electronics background required

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Controlled Impedance An introduction (No
electronics background required)
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Controlled impedance PCBs

• Are used in high frequency applications
• A controlled impedance trace simulates coaxial
  cable on a PCB
• by constructing the board in a special method
• (Differential traces simulate twisted pair)

3
Controlled impedance PCBs

• Are used in high frequency applications
• A controlled impedance trace simulates coaxial
  cable on a PCB
• by constructing the board in a special method
• (Differential traces simulate twisted pair)

4
Controlled impedance PCBs

• Are used in high frequency applications
• A controlled impedance trace simulates coaxial
  cable on a PCB
• by constructing the board in a special method
• (Differential traces simulate twisted pair)

5
Controlled impedance PCBs

• Are used in high frequency applications
• A controlled impedance trace simulates coaxial
  cable on a PCB
• by constructing the board in a special method
• (Differential traces simulate twisted pair)

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Controlled impedance PCBs

• Are used in high frequency applications
• A controlled impedance trace simulates coaxial
  cable on a PCB
• by constructing the board in a special method
• (Differential traces simulate twisted pair)

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Controlled impedance PCBs

• Are used in high frequency applications
• A controlled impedance trace simulates coaxial
  cable on a PCB
• by constructing the board in a special method
• (Differential traces simulate twisted pair)

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Why?

• At high frequencies PCB traces do not behave
  like simple connections
• We need to ensure that signals are not degraded
  as they route around the PCB

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What happens if you dont use Controlled
impedance tracks?

• Try replacing the 75Ohm coax that connects your
  TV aerial with a length of meter wire!
• See what effect it has on the picture

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What happens if you dont use Controlled
impedance tracks?

• Try replacing the 75Ohm coax that connects your
  TV aerial with a length of meter wire!

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When should you consider impedance control?

• In PCBs used for fast digital applications
• telecommunications
• computing 100MHz and above
• high quality analog video
• signal processing
• RF communication

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Why does a PCB behave in a special way at high
frequency?

• When signals are very fast, effects which can
  be ignored at slow speeds become important
• Electrical charge flows along a PCBtrace at
  around 1/2 the speed of light
• Think of the electrical charge as water flowing
  through a channel

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Analogy

• In a narrow channel water flows at a steady rate,
  a change in the channel width will however cause
  a reflected wave to return to the source,
• Should the channel narrow the wave will travel
  back to the source at an increased height, a
  wider channel will reflect as a reduced water
  height.

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At extreme cases

• If water flows into a closed channel, the
  reflected wave will travel back to the source at
  double its incoming height.
• If the channel ends (i.e... becomes infinitely
  wide), the wave traveling back to the source will
  empty the channel.

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Electrically

• These effects are taking place at half the speed
  of light...
• At high speed designers of digital systems need
  to consider these effects as a poor match of
  impedance will result in lower noise immunity, in
  the worst case a 0 becomes 1 and we have an error
  condition.

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Summary

• Consider controlled impedance on boards operating
  faster than 100MHz
• Remember that at high speeds PCB traces no longer
  behave as simple interconnects
• Controlled impedance boards provide repeatable
  high frequency performance
• Testing need not be complex or time consuming...

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For more information

• Contact your localPolar distributor or

Polar InstrumentsGarenne ParkGuernsey UKTel
44 1481 253081 Fax [phone]
www.polarinstruments.com
? Polar Instruments 2001. All trademarks
recognized