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Technical characteristics of high-speed PCB design impedance problem
Date:2014-3-26 17:16:38

In the high-speed design , the characteristics of controlled impedance boards and line impedance problem plaguing many Chinese engineers . Through a simple and intuitive way to introduce the basic nature of the characteristic impedance calculation and measurement methods .
 
In high-speed design, controlled impedance characteristic impedance circuit board and is one of the most important and widespread problem. First, look at the definition of the transmission line : transmission line consists of two conductors composed of a certain length , a conductor used to send a signal , and the other for receiving signals ( remember " loop " instead of "ground " concept ) . In a multilayer board , the lines are each part of a transmission line , as a reference plane adjacent to the second line or loop. A line a " good" is the key to make the transmission line characteristic impedance is maintained constant throughout the line .
 
Circuit boards become critical " controlled impedance board" is to make all the characteristic impedance of the line to meet a specified value , usually between 25 ohms and 70 ohms . In the multi-layer circuit boards , the good performance of the transmission line characteristic impedance of the key is that it remains constant in the entire line .
 
However , what is the characteristic impedance ? Understand the characteristic impedance of the easiest ways is to look at the signal transmission encountered something. When the microwave along a transmission having the same cross-section as shown in a transmission line is moved, which is similar to FIG . Assumed that the distal end of the stepped wave and a voltage V to this transmission line , such as the one volt battery is connected to the transmission line ( which is located between the transmission line and the loop ) , once connected , the signal voltage wave along the line speed of light spread , its speed is usually about 6 inches / nanosecond . Of course, this is really the signal transmission line and the voltage difference between the loops , which can be any point from the adjacent points of the transmission line and the circuit to be measured. Figure 2 is a schematic diagram of the voltage signal transmission .
 
Zen method is to " generating a signal" , then 6 inches / nanosecond propagation velocity along the transmission lines . The first 0.01 ns 0.06 inches forward , when the transmission line has a positive excess charge and the loop has excess negative charge, the charge difference between the two is maintained at 1 volt difference between the two conductors , and two conductors which form a capacitor .
 
0.01 in the next nanosecond , but also the period of 0.06 inches voltage transmission line from 0 to 1 volt adjustment , which must be added a number of positive charges to the transmission line , and add some negative charge to the receiving line . Every move 0.06 inches , must put more charge moves to the transmission line , and the more negative charge added to the circuit . Every 0.01 nanoseconds , the transmission line must be charged another paragraph , and then propagate the signal to start along this section . Charge from the front end of the battery cable , when moving along this line , give a continuous portion of the transmission line charging , thus forming the loop between the transmission line and the voltage difference of one volt . Advancing 0.01 per nanosecond , you get some of the charges (± Q) from the battery in a constant time interval (± t) flowing from the battery within a constant power (± Q) is a constant current. Negative current actually flowing into the circuit and a positive current flowing out of the same, but just in front of the wave alternating current signal through the upper and lower lines form a capacitor , the end of the cycle . Illustrated in Figure 3 .
 
Impedance of the line
 
The battery , when the signal propagation along the transmission line , and the intervals of 0.01 ns 0.06 inch continuous transmission line to be charged. Obtained when a constant current from the power transmission line that looks like a resistor , and its resistance value is constant, the transmission line which may be called " surge " Impedance (surge impedance).
 
Similarly , when the signal propagates along the line , prior to the next step , 0.01 nanoseconds , which this current can be increased to a step voltage V ? This relates to the concept of instantaneous impedance .
 
From the perspective of the battery , if the speed of signal propagation in a stable along the transmission line and the transmission lines have the same cross section , then a further 0.01 nanoseconds before each require the same amount of charge , to produce the same signal voltage. When advancing along the line , will have the same instantaneous impedance, which is seen as a characteristic of the transmission line characteristic impedance is known . If the signal at each step of the same characteristic impedance of the transmission process , then the transmission line is considered to be a controlled impedance transmission line.
 
The instantaneous impedance or the characteristic impedance of the signal transmission quality is very important. In the transfer process, if the next step impedance and the impedance equal work can be carried out smoothly, but if a change in impedance occurs , then there will be some problems.
 
To achieve the best signal quality , the design goal is internally connected in the signal transmission process keep a stable impedance , the first transmission line characteristic impedance must be kept stable , and therefore , controlled impedance board production has become increasingly important . In addition, other methods such as the shortest length of I , the end of the removal and use of the entire line is also used to keep the instantaneous signal transfer impedance stability.
 
Characteristic impedance is calculated
 
Simple model of the characteristic impedance : Z = V / I, Z represents the signal transmission impedance of each step of the process , V represents the signal into the transmission line when the voltage, I representative current . I = ± Q / ± t, Q represents incremental , t represents the time step .
 
Power ( from battery ): ± Q = ± C × V, C represents the capacitance , V represents the voltage . Capacitance per unit length of the transmission line can be used and the signal transmission capacity CL velocity v to derive . Value per unit length as the speed of the pin , and then multiplied by the time required for each step t, then get the formula :. ± C = CL × v × (±) t Taking the above , we can draw the characteristic impedance : Z = V / I = V / (± Q / ± t) = V / (± C × V / ± t) = V / (CL × v × (±) t × V / ± t) = 1 / (CL × v )
 
As can be seen , the characteristic impedance of the transmission line with the capacity per unit length and the speed of the signal transmission . To distinguish between the actual impedance and the characteristic impedance Z, we add back the transmission line characteristic impedance Z 0 is :. Z0 = 1 / (CL × v)
 
If the capacity per unit length of the transmission line and the signal transmission speed is kept constant , the characteristic impedance of the transmission line remains unchanged . This simple explanation can capacitance knowledge and new discoveries linked to the characteristic impedance of the theory . If you increase the capacity per unit length of the transmission line , such as bold transmission line , the characteristic impedance of the transmission line can be reduced .
 
Measurement of the characteristic impedance
 
When the battery is connected to the transmission line ( if time impedance of 50 ohms ) , the ohmmeter connected to three feet long RG58 cable, then how infinite impedance measuring it ? Any transmission line impedances are related to time . If you are in the cable shorter than the reflection time measurement of the cable impedance , to you measure the " surge " impedance or characteristic impedance. However, if sufficient length of time to wait until the energy is reflected back and received , can be found by measuring the impedance change . Generally speaking , it will reach a stable value of the upper and lower limit impedance rebound .
 
For three feet long cable , the impedance measurement must be completed within 3 nanoseconds. TDR ( Time Domain Reflectometry ) can do it , it can measure the dynamic impedance of the transmission line . If the cable impedance measuring 3 feet in one second , the signal will be reflected back and forth millions of times , it will be different " surge " impedance.

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