Electrical parameters - General Principles of Impedance control
7 important questions on Electrical parameters - General Principles of Impedance control
What is a transmission line?
What is the impedance of a transmission line basically?
It is important to maintain consistent impedance throughout all conductors and elements of a given circuit.
Which variables are affecting impedance?
- Dielectric Constant of Substrate
- Base Material
- Frequency: Dk may vary with frequency
- Temperature: for FR4 20% from 0 to 70 °C - Dielectric Thickness: impedance increases with thickness
! be carefull with thin dielectric and its tolerance - prepreg and core material will probably have a slightly different Dk
- Conductor width: impedance decreases for wider conductors.
- Conductor Thickness: As conductor thickness increases, the overall impedance will decrease but the impact isn't that big.
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- What are the two most important factors in determining impedance?
- Dielectric Thickness
- Conductor Width: A lot of times the conductor width cannot be varied much due to routing constraints.
What are the main reasons for matching the line impedance?
- Emissions: Signal overshoot and undershoot are a major contributor of emissions, and are reduced with properly matched lines.
- Signal Integrity:
- Reflections
- Propagation Time
- Proper circuit Operation
Different Conductor Impedance Configurations (transmission lines) ? #5
- Surface Microstrip
- Embedded Microstrip
- Stripline (balanced & unbalanced)
- Dual Stripline (balanced & unbalanced)
!Signals are orthagonal to signals on other plane - Discrete wiring techniques
Which formula do you use to calculate the impedance?
Also, it's very important to coordinate the printed board dielectric and
stack-up requirements with the fabricator.
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