PCB News - Interpretation and testing of characteristic impedance

I. Introduction
Abstract and complex digital high-speed logic principles, and how to transmit square wave signals in transmission lines, and how to ensure its signal integrity (Signal Integrity), reduce its noise (Noise), reduce the misoperation and other professional expressions, if it can be simple To illustrate the life examples of the people, if you bring in a bunch of mathematical formulas and difficult physical language instead of moving, then the enlightenment and benefit to the novice or the intervener will be more effective if it is easier to use.
However, many undergraduate professionals, even doctoral professors who are teachers in Xingtan, don’t know if they haven’t really entered the situation yet. Or they deliberately show off what they know in order to frighten the educated, but they don’t know, or they have both mentalities! There are a large number of books and journal articles in the market, and most of them are inexplicable. There are few examples. It really makes people look at the flowers in the fog. It is strange to understand it!

The author recently obtained a briefing about impedance control, which was provided by Nissho HIOKI, a professional electrical testing company. The content can be said to be comprehensible at a glance, which makes people love it. It is the realm that the author has been pursuing for a long time. Under the overwhelming joy, I obtained the consent of the original "Wen Kong Construction" company, with the strong assistance of Hong Kong Construction Company Vice President Liao Fengying, as well as the original author Hiroshi Yamazaki and his superiors. Thanks to Toshihiko Kanai and others for being able to complete this article. And welcome all seniors and advanced students to give a lot of similar information to benefit the student readers, and you will be very good in the industry.

B. Treat the signal transmission as a hose for watering flowers
2.1 In the multi-layer PCB board signal line of the digital system, when the square wave signal is transmitted, it can be imagined as a hose to send water and flowers. One end is pressurized in the hand grip to make it shoot out the water column, and the other end is connected to the faucet. When the pressure exerted by the grip tube is just right, and the range of the water column is correctly sprayed on the target area, both the giving and receiving will be happy and the mission will be completed successfully. Isn't it a handy little achievement?

2.2 However, once the water injection process is too far, it will not only vacate the target and waste water resources, but may even have nowhere to vent due to the strong water pressure, so that it may rebound from the source and cause the hose to break free from the faucet! Not only is the mission failed, it is also a big frustration. It's so prickly and full of bean curd!

2.3 Conversely, when the grip is not squeezed enough to make the range too close, the desired result will still not be obtained. Too much is not what you want. Only when it is just right can everyone be happy.

2.4 The above simple details of life can be used to illustrate that square wave signals (Signal) are carried out in multi-layer board transmission lines (Transmission Line, which is composed of signal lines, dielectric layers, and ground layers). Fast delivery. At this time, the transmission line (commonly known as Coaxial Cable, Microstrip Line or Strip Line, etc.) can be regarded as a hose, and the pressure applied by the holding tube is like the "receiving end" on the board. (Receiver) The resistor connected in parallel to Gnd is general (it is one of the five terminal technologies, please refer to the article "Development of Embedded Resistors" in the 13th issue of TPCA Proceedings for detailed description), which can be used to adjust its end point Characteristic Impedance (Characteristic Impedance) to match the internal requirements of the receiving end components.

3. Terminal control technology of transmission line (Termination)
3.1 It can be seen from the above that when the "signal" travels in the transmission line and arrives at the end point, and wants to enter the receiving element (such as CPU or Meomery and other ICs of different sizes) to work, the "characteristic impedance" of the signal line itself must be It must be matched with the internal electronic impedance of the terminal element, so that the task will not fail in vain. In terminology, it means to execute instructions correctly, reduce noise interference, and avoid wrong actions." Once they fail to match each other, there will be a little energy bounce back towards the "transmitting end", which will cause the trouble of reflection noise (Noise). .

3.2 When the characteristic impedance (Z0) of the transmission line itself is set as 28 ohm by the designer, the grounding resistor (Zt) of the terminal control must also be 28 ohm, so as to assist the transmission line to maintain Z0 and stabilize the whole Design value of 28 ohm. Only in this matching situation of Z0=Zt, the signal transmission will be the most efficient, and its "signal integrity" (signal integrity, a special term for signal quality) is also the best.

4. Characteristic Impedance (Characteristic Impedance)
4.1 When a square wave of a signal moves forward with a high-level positive pressure signal in the signal line of the transmission line assembly, the reference layer (such as the ground layer) closest to it is theoretically necessary. The negative pressure signal induced by the electric field accompanies it (equal to the return path of the positive pressure signal), so that the overall loop system can be completed. If the "signal" travels forward and freezes its flight time for a short time, you can imagine the instantaneous impedance (Instantanious Impedance) that the signal line, dielectric layer, and reference layer will experience together. This is the so-called "Characteristic impedance".

Therefore, the "characteristic impedance" should be related to the line width (w), line thickness (t), dielectric thickness (h) and dielectric constant (Dk) of the signal line.
4.2 Consequences of poor impedance matching

Because the original term "characteristic impedance" (Z0) of high-frequency signals is very long, it is generally referred to as "impedance". Readers must be careful, this is not exactly the same as the impedance value (Z) that appears in the low-frequency AC (60Hz) wire (not the transmission line). In digital systems, when the Z0 of the entire transmission line can be managed properly, and if it is controlled within a certain range (±10% or ±5%), this high-quality transmission line will reduce noise and prevent misoperation. However, when any one of the four variables (w, t, h, r) of Z0 in the above microstrip line is abnormal, such as a gap in the signal line, the original Z0 will rise suddenly (see Z0 and Z0 in the above formula). W is inversely proportional to the fact), and can not continue to maintain the due stability and uniformity (Continuous), the energy of the signal will inevitably occur part of the advancement, but part of the lack of rebound reflection. In this way, noise and malfunction cannot be avoided. The hose in the picture below was suddenly stepped on by Yamazaki's son, causing abnormalities at both ends of the hose, which just illustrates the above-mentioned problem of poor characteristic impedance matching.

4.3 Poor impedance matching causes noise

The rebound of some of the signal energy mentioned above will cause the original good-quality square wave signal to be abnormally deformed immediately (that is, the Overshoot of the high level upward, the Undershoot of the low level downward, and the subsequent Ringing of the two; details See also TPCA Proceedings Issue 13 "Embedded Capacitors"). When such high-frequency noise is severe, it will cause malfunctions, and the faster the pulse speed, the more noise and the easier it will be to make mistakes.

The above is an introduction to the interpretation and testing of characteristic impedance. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.