Transmission line has two very important characteristics: Characteristic impedance and delay. Both of these features can be used to predict the behavior of each signal and to describe many of the transmission line .
Characteristic impedance describes the signals along the transmission line when the propagation by transient impedance , which is an inherent property of the transmission line only and inductance per unit length of the transmission line on the L, the distributed capacitance C, the material properties and the dielectric constant related to the transmission line length irrelevant . The width of the wire without changing the characteristic impedance of the fixed , only the geometry and material characteristics of the wire remains the same , then the characteristic impedance of the transmission line is constant.
The characteristic impedance of the transmission line
In the transmission line theory books , more complete expression for the characteristic impedance
Wherein , R, G impedance and admittance respectively ; ω is the angular frequency of the signal . Because the R and G is much smaller than the other items normally approximate characteristic impedance of the formula ( 3-2 ) can, only in the VHF or line of great loss , the impedance of the R and G components before admittance becomes important.
In addition , the line can also be detrimental to export more complex characteristic impedance, which contains the imaginary component . However, in engineering design, only consider the magnitude of the characteristic impedance . Approximate formulas given directly in the PCB and common symmetric stripline microstrip line , as shown .
Figure A is a microstrip line , B is a symmetric stripline
The concept is relatively easy to understand the delay , the amount of time that the signal transmission through the entire length of the line used in the timing analysis in the detailed knowledge of the propagation velocity and time-related , here given directly calculated delay :
Wherein , td represents the delay ; ι said transmission line length ; υ said signal transmission speed .
