Tom
11-18-2002, 03:27 PM
IPC Advanced Study Guide Page Reference: Page 250, Section 4.11
A transmission line is any pair of conductors used to move electrical energy from point A to point B. A transmission line always consists of a signal conductor and a return conductor. The moving energy is primarily contained between the signal and return conductors in the form of an electric field and a magnetic field. In the case of a printed board, the return conductor will likely be a power or ground plane in the board.
Impedance of the transmission line is basically that property which resists propagation of the energy through the circuit. To prevent energy reflections, that can cause signal integrity and EMI, it is important to maintain consistent impedance throughout all conductors and elements of a given circuit. In some circuits, it’s important to match transmission line impedance to that of some of the circuit elements (i.e.- IC Output Impedance, etc.). In other circuits, matching is not necessary. In all circuits, consistent impedance, throughout all branches of a circuit, is important.
Impedance consists of 3 elements, resistance, capacitance, and inductance. In most circuits, resistance of the transmission line is negligible. Hence impedance is primarily a function of the transmission line inductance and capacitance. In other words, to control impedance we need to tightly control inductance and capacitance. Inductance and capacitance of a transmission line are primarily a function of several variables.
Variables Affecting Impedance (Inductance and Capacitance)
A. Dielectric Constant of Substrate (Er) relative permittivity
1. Materials - Every base material has it's own Er, based in large part on the ratio of reinforcement glass to epoxy resin. Even the same material from different manufacturers may have a different Er. Reference chart (IPC-2222, Tables 4-2 through 4-6 and/or IPC-CC-110)
2. Frequency – The Er of most materials is determined at a specific frequency. If your particular circuit is operating at a different frequency, the Er may be different from that listed in the spec sheet. The laminate vendors can supply detailed data for each material.
3. Temperature – Er of FR-4 material will vary approximately 20% over a temperature range of 0 - 70 degrees centigrade. This should be considered for very stringent designs.
A transmission line is any pair of conductors used to move electrical energy from point A to point B. A transmission line always consists of a signal conductor and a return conductor. The moving energy is primarily contained between the signal and return conductors in the form of an electric field and a magnetic field. In the case of a printed board, the return conductor will likely be a power or ground plane in the board.
Impedance of the transmission line is basically that property which resists propagation of the energy through the circuit. To prevent energy reflections, that can cause signal integrity and EMI, it is important to maintain consistent impedance throughout all conductors and elements of a given circuit. In some circuits, it’s important to match transmission line impedance to that of some of the circuit elements (i.e.- IC Output Impedance, etc.). In other circuits, matching is not necessary. In all circuits, consistent impedance, throughout all branches of a circuit, is important.
Impedance consists of 3 elements, resistance, capacitance, and inductance. In most circuits, resistance of the transmission line is negligible. Hence impedance is primarily a function of the transmission line inductance and capacitance. In other words, to control impedance we need to tightly control inductance and capacitance. Inductance and capacitance of a transmission line are primarily a function of several variables.
Variables Affecting Impedance (Inductance and Capacitance)
A. Dielectric Constant of Substrate (Er) relative permittivity
1. Materials - Every base material has it's own Er, based in large part on the ratio of reinforcement glass to epoxy resin. Even the same material from different manufacturers may have a different Er. Reference chart (IPC-2222, Tables 4-2 through 4-6 and/or IPC-CC-110)
2. Frequency – The Er of most materials is determined at a specific frequency. If your particular circuit is operating at a different frequency, the Er may be different from that listed in the spec sheet. The laminate vendors can supply detailed data for each material.
3. Temperature – Er of FR-4 material will vary approximately 20% over a temperature range of 0 - 70 degrees centigrade. This should be considered for very stringent designs.