PCB Tech - Characteristic impedance, the role and position of characteristic impedance in signal integrity

When the signal propagates on the transmission line, the transient impedance felt by the signal is related to the capacitance per unit length and the dielectric constant of the material, which can be expressed as:

If the thickness and width of the lines on the PCB do not change, and the distance between the trace and the return plane does not change, the transient impedance felt by the signal does not change, and the transmission line is uniform. For a uniform transmission line, the constant transient impedance describes the characteristics of the transmission line, which is called the characteristic impedance.

If the thickness or width of the lines on the PCB increases, the capacitance per unit length increases, and the characteristic impedance becomes smaller. Similarly, the distance between the trace and the return plane decreases, the capacitance increases, and the characteristic impedance decreases.

A very important characteristic impedance is the characteristic impedance of free space, also called the wave impedance of free space, which is very important in EMC. The free space characteristic impedance is

For the common FR4 board PCB board, the typical structure of 50Ω characteristic impedance is shown in the figure. For the microstrip line, the line width W is twice the dielectric thickness h. For a strip line, the total thickness b of the medium on both sides of the line is twice the line width W.

figure 1

Another characteristic of the characteristic impedance transmission line on the FR4 board PCB board is:

Capacitance per unit length = 3.3pF/in

Capacitance per unit length = 8.3nH/in

figure 2

Understanding these special characteristic impedances has certain reference significance for the design of circuit board s, and allows us to have an intuitive understanding before making the circuit.