Impedance control

Impedance relates to all system parameters including delay, noise and noise tolerance.

Characteristic impedance Z0 is the single most important interconnection system parameter. The physical geometry of the circuit board and the system signal distribution properties are directly related to impedance. Some important relationships are:

- Line width (w). As line width increases, Z0 decreases.

- Dielectric spacing (h). As a line gets further from the ground plane, Z0 increases.

The impedance is largely determined by the track geometry, the structure of the layers and the dielectric constant (εr) of the materials used.

There are 4 parameters that generally affect signal losses, εr, Dielectric Thickness, Line Width and Loss Tangent. There is also a fifth issue that can cause significant losses at some frequencies, Skin Effect. To gain true control of high speed and high frequency signals, all of these must be considered.

Dielectric Thickness and Trace Width play a key role in transmission line impedance. Control of each is necessary during fabrication of the board, with the greatest degree of control needed for high frequency analog circuits.

Er (εr) – Relative permittivity is a measure of the effect an insulating material has on the capacitance of a conductor embedded in the material or surrounded by it. It is also a measure of the degree to which an electromagnetic wave is slowed down as it travels through the insulating material. The higher the relative permittivity, the slower a signal travels on a trace, the lower theimpedance of a given trace geometry and the larger the stray capacitance along a transmission line. Given a choice, lower dielectric constant is nearly always better.

Loss Tangent (tan (δ)) – Loss tangent is a measure of how much of the signal pulse (electromagnetic wave) propagating down the PCB transmission line will be lost in the dielectric region (insulating material between copper layers). Loss tangent is a function of the material’s resin type and molecular structure (molecular orientation).

Matching impedances and minimizing interconnecting lengths improves system performance. Reflections from mismatches can add propagation delay. These mismatches can also be caused by large variations in gate loading. If the mismatch is large enough, the reflection produced can cause false switching in previous gates on the line. Generally, impedance varies on PCB by 15 to 20%, due to manufacturing tolerances and the non-ideal plane effects. Critical application may require 5 or 10 % impedance control.

Differential impedance control is also performed for highly complex circuit designs.




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