Controlled Impedance An introduction (No electronics background required)

Controlled impedance PCB’s 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)

Controlled impedance PCB’s 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)

Controlled impedance PCB’s 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)

Controlled impedance PCB’s 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)

Controlled impedance PCB’s 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)

Controlled impedance PCB’s 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)

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

What happens if you don’t 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

What happens if you don’t use Controlled impedance tracks? Try replacing the 75Ohm coax that connects your TV aerial with a length of meter wire!

When should you consider impedance control? In PCB’s used for fast digital applications telecommunications computing 100MHz and above high quality analog video signal processing RF communication

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 PCB trace at around 1/2 the speed of light Think of the electrical charge as water flowing through a channel

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.

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.

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.

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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