Some easy-to-understand explanations of characteristic impedance

The author recently obtained a briefing on impedance control, which was provided by the professional Japanese business HIOKI. The content of the text can be said to be simple and easy to understand. It is the realm that the author has been pursuing for a long time. Under the great joy, he obtained the consent of the original "Qiangangjian" company, and was assisted by the vice president of Hong Kong Construction Company Liao Fengying, and the original author Hiroshi Yamazaki and his supervisor. Thanks to Toshihiko Kanai and other confuses, I am able to complete this article. I would like to thank you here. And welcome all the advanced people of the predecessors, and many more to give similar information to the students of the students, the merits of the industry.

I. See the transmission of the signal as a hose to water the flowers

1.1 In the multi-layer signal line of the digital system, when the transmission of the square wave signal occurs, it can be assumed to be a hose to water the water. One end is pressurized at the hand to eject the water column and the other end is connected to the faucet. When the force applied by the grip tube is just right, and the range of the water column is properly spilled in the target area, it is not a handy little achievement to perform the mission with both of them.

1.2 However, once the force is too far, the water injection process is too far away. Not only does it vacate the target to waste water resources, but it may even be vented because of strong water pressure, so that the source rebounds and the hose is free from the faucet! Not only the task fails, but also the big failure. It’s leaking with beans!

1.3 Conversely, if the grip is not squeezed so that the range is too close, the desired result will not be obtained. It’s too much to do what you want, only the right ones can be happy in the middle and the bottom.

1.4 The above simple life details can be used to illustrate the Square Wave signal (Signal) in the multi-layer transmission line (Transmission Line, which consists of the signal line, the dielectric layer, and the ground plane) Fast transfer. At this point, the transmission line (common cable Coaxial Cable, Microstrip Line or Strip Line, etc.) can be regarded as a hose, and the pressure applied to the tube is like the "receiving end" on the board. (Receiver) components are connected to Gnd's resistors in general (one of the five terminal technologies, please see also the detailed description of the development of embedded resistors in the 13th issue of TPCA), which can be used to adjust the end point. The characteristic impedance (Characteristic Impedance) is such that it matches the requirements inside the receiving end component.

2. Terminal control technology of transmission line (Termination)

2.1 It can be seen from the above that when the “signal” travels in the transmission line and reaches the end point, in order to enter the receiving component (such as CPU or Meomery and other ICs of different sizes), the “characteristic impedance” of the signal line itself must be It is necessary to match the electronic impedance inside the terminal component so that the task does not fail. In the terminology, it is the correct execution of instructions to reduce noise interference and avoid false actions. Once they fail to match each other, there will be a little energy returning to the "sending end" to bounce back, thus forming the trouble of reflecting noise.

2.2 When the characteristic impedance (Z0) of the transmission line itself is set to 28 ohm by the designer, the grounding resistor (Zt) of the terminal control tube must also be 28 ohms, so as to assist the transmission line to maintain the Z0, so that the whole is stabilized. Design value for 28 ohms. Only in the case of such Z0=Zt matching, the transmission of the signal is most efficient, and its "Signal Integrity" (the special term for signal quality) is also the best.

3. Characteristic Impedance

3.1 When a square wave of a signal is propelled forward by a high-level positive pressure signal in the signal line of the transmission line assembly, the theoretical reference layer (such as the ground plane) closest to it is theoretically required. The negative pressure signal induced by the electric field is accompanied by the forward path (the return path equal to the reverse direction of the positive pressure signal), so that the integral loop system can be completed. If the "signal" advances its flight time for a short period of time, it can be imagined that it suffers from the instantaneous impedance value (Instantanious Impedance) from the signal line, the dielectric layer and the reference layer. This is 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. The micro-belt line of one of the transmission lines is illustrated and calculated as follows: [Note to the author] The correct translation of Dk (Dielectric Constant) should be a medium constant. In the original text, r should be called "relative capacitance". "Relative Permitivity" is right. The latter is seen from the standpoint of parallel metal plate capacitors. Due to its proximity to the facts, many important specifications (such as IPC-6012, IPC-4101, IPC-2141 and IEC-326) have been renamed...r in recent years. And the E in the original picture is not correct, it should be the Greek letter (Episolon).

3.2 Consequences of poor impedance matching

Since the "characteristic impedance" (Z0) of high-frequency signals is very long, it is generally referred to as "impedance". The reader must be careful that the impedance (Z) that appears in the wires (not the transmission line) of the low-frequency AC (60 Hz) is not exactly the same. Digital system When the Z0 of the entire transmission line can be properly managed and controlled within a certain range (±10% or ±5%), this good quality transmission line will reduce the noise and prevent malfunction. However, when any 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 in the figure, the original Z0 will suddenly rise (see the above formula). The fact that Z0 is inversely proportional to W), while unable to continue to maintain the desired stability, the energy of the signal will inevitably partially advance, while the part will lack the rebound reflection. This will not avoid noise and malfunction. The hose in the picture below was suddenly slammed by the son of Yamazaki, causing an abnormality at both ends of the hose, which just explained the problem of poor impedance matching of the above characteristics.

3.3 Impedance matching causes noise

The rebound of some of the above signal energy will cause the original good quality square wave signal to immediately exhibit abnormal deformation (ie, high-level upward Overshoot, low-level down Undershoot, and subsequent Ringing; details) See also the TPCA issue of the 13th issue of "embedded capacitors". When such high-frequency noise is severe, it may cause malfunction, and the faster the pulse speed, the more the noise is more error-prone.