Controlled Impedance An introduction (No electronics background required)

1. Controlled Impedance An introduction(No electronics background required)

2. 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)

3. 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)

4. 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)

5. 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)

6. 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)

7. 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)

8. 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

9. 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

10. 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!

11. 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

12. 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

13. 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.

14. 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.

15. 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.

16. 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...

17. For more information: Contact your localPolar distributor or: Polar InstrumentsGarenne ParkGuernsey UKTel: +44 1481 253081 Fax: +44 1481 252476 www.polarinstruments.com  Polar Instruments 2001. All trademarks recognized