There is a lot of knowledge that everyone needs to master when learning high-speed PCB design, and transmission lines are one of them. However, whether they are beginners or experienced engineers, everyone often has only a half-understanding of transmission lines. In this article, let’s learn about transmission lines together!
What is a transmission line?
The definition of a transmission line is a signal line with signal return (composed of two wires of a certain length, one is the signal propagation path, and the other is the signal return path.). The most common transmission line is the trace on our PCB board.
The two most common types of transmission lines used in digital designs are microstrip and stripline.
Microstrip usually refers to the trace on the outer layer of the PCB and has only one reference plane. There are two types of microstrip lines: buried or non-buried. Buried (sometimes called submerged) microstrip is a transmission line simply embedded in the dielectric, but it still has only one reference plane.
Stripline refers to an inner layer of traces between two reference planes.
In high-speed PCB design, if it is a PCB with more than 4 layers, try to use strip lines, which means that the high-speed signal lines should be routed to the inner layer. This can not only reduce EMI and EMC, but also enhance the anti-interference ability.
Regarding transmission lines, you also need to know these knowledge points:
1. When analyzing transmission lines, be sure to contact the return path. A single conductor cannot become a transmission line;
2. Like resistors, capacitors, and inductors, transmission lines are also ideal circuit components, but their characteristics are quite different. They are better for simulation, but the circuit concept is more complicated;
3. Transmission lines have two very important characteristics: characteristic impedance and time delay.
Transmission line impedance
Let’s first clarify a few concepts. We often see impedance, characteristic impedance, and instantaneous impedance. Strictly speaking, they are different, but they remain the same. They are still the basic definitions of impedance:
a) The input impedance at the beginning of the transmission line is referred to as impedance;
b) The timely impedance encountered by the signal at any time is called instantaneous impedance;
c) If the transmission line has a constant instantaneous impedance, it is called the characteristic impedance of the transmission line.
Characteristic impedance describes the transient impedance experienced by a signal as it propagates along a transmission line, which is a major factor affecting signal integrity in transmission line circuits.
If there is no special explanation, characteristic impedance is generally used to collectively refer to transmission line impedance.
PS: For high-speed PCB design, our goal is to keep the impedance of the signal as stable as possible during the transmission process, and this must maintain the stability of the characteristic impedance of the transmission line.
Transmission delay
Time delay, also called time delay (TD), usually refers to the time it takes for an electromagnetic signal or optical signal to pass through the entire transmission medium. The delay on the transmission line refers to the time it takes for the signal to pass through the entire transmission line.
Propagation delay, also called propagation delay (PD), usually refers to the time delay of electromagnetic signal or optical signal transmission in the transmission medium of unit length. It is inversely proportional (reciprocal) to the “propagation speed”, and the unit is “Ps/inch” or “s/m”.
It can be seen from the definition that delay = propagation delay * transmission length (L).
Knowledge expansion:
How to eliminate transmission line interference in PCB design?
1. Avoid impedance discontinuities in transmission lines. The points where the impedance is discontinuous are the points where the transmission line suddenly changes, such as straight corners, vias, etc., which should be avoided as much as possible. The methods are: avoid straight corners of the traces, try to take 45° angles or arcs, large corners are also acceptable; use as few vias as possible, because each via is an impedance discontinuity point, and the outer layer signal avoids passing through inner layer and vice versa.
2. Do not use stake lines. Because any pile line is a source of noise. If the pile line is short, it can be terminated at the end of the transmission line; if the pile line is long, it will use the main transmission line as the source, causing great reflection and complicating the problem. It is recommended not to use it.
