Popular science on same-mode impedance – ChangZhou Sun-Rise Electronic Co.,Ltd.

When measuring the same wire under test, its impedance will be divided into single-ended impedance, same-mode (common-mode) impedance and differential impedance due to the different signal types transmitted on the wire under test. The difference is mainly reflected in the tested signal. On condition. Therefore, each line can measure the above three impedances. However, due to different wire applications, the influence of various impedances is also different. Today we are talking about Common mode (common/same mode); common mode impedance is also called common mode. Test the impedance between a pair of wires and the ground wire. Basically, same-mode transmission is used to simulate noise or to measure the balance characteristics of the wire. It is not used for general signal transmission. It is actually transmitted in the wire. Because the phase or amplitude of the signal cannot be maintained in an ideal state, the final output signal has a common-mode component. These common-mode signals do not have the advantage of the original differential signal in suppressing radiation, so they will have an impact on high-frequency signal transmission. ;Same mode impedance NG, how to deal with it?

Differential to Common Mode Impedance Association Specification Requirements for USB4 

Types and definitions of impedance

Characteristic impedance: It is the average value calculated based on the input impedance. (What is the characteristic impedance of a cable?)

Characteristic impedance is for AC signals (or high-frequency signals); characteristic impedance is a concept in long-line transmission. During the signal transmission process in the transmission line, at a point where the signal arrives, there will be a gap between the transmission line and the reference plane. An electric field is formed. Due to the existence of the electric field, an instantaneous small current will be generated. This small current exists at every point in the transmission line. At the same time, the signal also has a certain voltage, so during the signal transmission process, each point of the transmission line will be equivalent to a resistance. This resistance is the characteristic impedance of the transmission line we mentioned. 

Input impedance: It is the actual measured impedance value of the cable.

Differential impedance: The transmitted signal can alternate between positive and negative, also known as balanced impedance.

Due to signal integrity issues, more and more cables use differential lines to transmit signals:

Differential structure: better immunity to interference due to common mode rejection

Differential structure: reduces radiated noise (EMI) due to canceling fields

Differential structure: achieves more precise timing control

Differential structure: reduces crosstalk due to anti-interference capability and reduced radiated energy

Differential architecture: reduces power supply noise due to current transients

Differential transmission lines have two unique modes of propagation, each with its own characteristic impedance. Differential impedance refers to the impedance measured in two transmission lines when driven differentially. The differential impedance is twice the odd mode impedance. Even mode impedance is defined by monitoring one line while the other line passes first

Impedance measured when driven by the same signal. The following is a diagram of differential drive and common mode drive, which shows the corresponding electric field and magnetic field distribution.

Common mode impedance: Common mode impedance refers to the impedance of lines connected in parallel, which is one-half of even mode impedance. The signal that the conductor is braided in the positive direction or the ground wire is in the negative direction is currently used in coaxial lines or CABLEs with ground wires, also known as unbalanced impedance.

What should I do if the same mode impedance is NG?

Example analysis: wire structure 28#/2P+8C +AL+B, 2P/28# conductor 7/0.127, core wire HDPE, OD: 0.72mm, twisted twist distance 25mm; the test result is attenuation, differential impedance OK, the same Mode impedance NG.

As can be understood from the above introduction to physical concepts, differential mode (differential impedance) includes a pair of traces (or wires) between the driver and receiver. We generally think that one of the traces transmits a positive signal and the other transmits a negative signal, and the magnitude is equal and the polarity is opposite. There is no return signal through the ground; the signal goes along one trace and returns from the other. Same-mode signals are often more difficult to understand. This can include either a single-ended trace or two (possibly more) differential traces. The same signal flows along the trace as well as the return path (ground) or along both traces in a differential pair. Most of us are often unfamiliar with common-mode signals because we never intentionally generate them ourselves. They are usually caused by noise coupling into the circuit from other (nearby or external) sources. Generally speaking, the results are neutral at best and destructive at worst. Common-mode signals can generate noise that interferes with proper circuit operation and are a common source of EMI problems.

The same-mode impedance in the above figure is NG. The common-mode impedance is greatly affected by the differential impedance. If the differential impedance is small, the common-mode impedance will also be small. As shown in the figure above, first improve the differential impedance. In order to facilitate everyone's understanding of differential impedance, let's take a person living in China. examples to illustrate. If a road is very smooth, then we say that its flatness (characteristic impedance) is very high (that is, the impedance continuity is good). If there are pits and unevenness, we say that its flatness is poor (impedance discontinuity). If the road ahead is cut off (a dead end road), vehicles and pedestrians can only turn around and come back (a dead end is equivalent to an open circuit in the cable, and the impedance suddenly changes to infinity). If the road is very potholed and bumpy, the car with a low chassis will give up and turn around (impedance sudden change causes signal echo). This can help to understand: the more continuous (smooth) the impedance is, the less signal echoes there are. On the contrary, the larger the echo, the more discontinuous the impedance (the road surface is uneven). Therefore, by measuring the echo, we can understand the quality of the road surface (impedance continuity of the cable), and improving the impedance means improving the structure of the cable.

First, improve the cable structure and increase the differential impedance of the cable to 90 ohms. Common mode impedance is greatly affected by differential impedance. If the differential impedance is too small, the common mode impedance will also be too small.

Second, the same-mode impedance test is the anti-interference ability. To improve the anti-interference ability, you need to add shielding to each pair of cables or use double-sided aluminum foil.

Third, for a pair of transmission lines carrying current in the same direction, the magnetic lines of force generated by the two wires cancel each other out. The magnetic field lines "surrounding the two wires" generated by the two wires have the same direction, and the magnetic fields increase. This is an active situation in which a current generates a magnetic field. This condition is similar to the passive situation in which an external interference source generates an induced current. The external magnetic field surrounds two wires and generates currents in the same direction, resulting in common mode interference. Generally speaking, the same-mode impedance test mainly focuses on interference, and the degree of interference is related to the spacing of the wires. If the spacing is large, the uneven magnetic field will produce differential mode. So as long as we design the differential impedance and skew, the common mode impedance will not be a problem! How to control good SKEW requires reducing the stranding outer diameter of the semi-finished product and reducing the stranding pitch.