Tom
11-18-2002, 03:54 PM
IPC Advanced Study Guide Page Reference: Page 251, Section 4.11
There are many reasons for matching the line impedance. The following are some of the main properties that help control circuit performance.
A. Emissions properly matched impedance between source, line, and load, emissions can be greatly reduced. Signal overshoot and undershoot are a major contributor of emissions, and are reduced with properly matched lines.
B. Signal integrity
1. Reflections - When a pulse travels down a signal line of impedance Zo, and reaches a load of the same impedance, all the energy is transferred to the load. If the impedance of the load, Zload, is different than that of the line, Zo, then a percentage of the pulse is reflected back toward the source. This pulse can create a false triggering of any components along the line.
2. Propagation Time - Is the time that a signal takes to travel the length of a conductor. This time will vary depending on the dielectric make up of the material into which the conductor is placed. Mixed signal timing can easily impact synchronous circuits.
3. Proper Circuit Operation - If impedance is mismatched to the point where there is excessive overshoot and undershoot, then false triggers can result. In addition, excessive undershoot could potentially degrade the performance of a semiconductor.
There are many reasons for matching the line impedance. The following are some of the main properties that help control circuit performance.
A. Emissions properly matched impedance between source, line, and load, emissions can be greatly reduced. Signal overshoot and undershoot are a major contributor of emissions, and are reduced with properly matched lines.
B. Signal integrity
1. Reflections - When a pulse travels down a signal line of impedance Zo, and reaches a load of the same impedance, all the energy is transferred to the load. If the impedance of the load, Zload, is different than that of the line, Zo, then a percentage of the pulse is reflected back toward the source. This pulse can create a false triggering of any components along the line.
2. Propagation Time - Is the time that a signal takes to travel the length of a conductor. This time will vary depending on the dielectric make up of the material into which the conductor is placed. Mixed signal timing can easily impact synchronous circuits.
3. Proper Circuit Operation - If impedance is mismatched to the point where there is excessive overshoot and undershoot, then false triggers can result. In addition, excessive undershoot could potentially degrade the performance of a semiconductor.