Signal integrity (SI) power integrity (PI) learning notes (XXV) differential pair and differential impedance (V)

Differential pair and differential impedance （ 5、 ... and ）

1.（1） Minimize asymmetry between differential pairs and misalignment between drives , Thus, the conversion from differential signal to common mode signal is minimized ;
（2） Use shielded twisted pair , Use the shielding layer as the impedance of the common mode current path ;
（3） The impedance of common mode current path is increased by adding common mode choke .
There are two types of common touch chokes
① A cylinder of ferritic material surrounds the outside of the cable ;
② The second type of common mode signal choke is mainly used in twisted pair cable . The twisted pair is usually wound into a coil , Sometimes a ferrite core is inserted .
Choke is an effective element to reduce radiation .

2. If a single ended transmission line is placed close to a differential pair , Then, due to the coupling with the dynamic single ended line , The signal voltage appears on both lines of the differential pair , The polarity of the coupling noise on each line in the differential pair is the same , But the range is different .
Differential alignment , There will be more noise in the line close to the dynamic line , The deeper the coupling of difference pairs , The noise generated on the two lines tends to be more and more equal , The smaller the difference noise .

measures ： Make the attack line far away from the differential line pair , And make the difference pair tightly coupled .

Two coupling levels ：
① The line spacing in tight coupling is equal to the line width , The line spacing of weak coupling is equal to the line width 2 times , In this difference , The line farther from the attack line has less noise , Differential noise is the difference between the noise levels of two lines .
② The common mode noise of the victim differential pair is the average value of the noise voltage on the two lines , When the coupling degree of the difference pair changes , Common mode noise will not be greatly affected . Tight coupling can reduce the difference noise , But it will increase the common touch noise . Crosstalk is a typical way to generate common mode noise in differential pairs .

The single ended attack line will cause differential noise in the differential pair , The above analysis reveals a general rule for reducing differential noise , Make the coupling in the differential pair as close as possible . Of course, in order to minimize the coupling noise , It is also necessary to make the distance between the attack line and the victim differential pair as far as possible , Tight coupling does not eliminate crosstalk , But in some cases, it will be reduced .

3. The differential noise coupled from another differential pair is slightly less than that coupled from a single end routing . The degree of coupling between two lines in a differential alignment , Especially the stripline , There is little effect on differential crosstalk introduced from other routes . Only when the adjacent return plane does not exist , for example , In a connector or package pin , Tight coupling can play a strong role in reducing crosstalk .

4. The gap in the return path is usually used to isolate an area on the circuit board . When the return reference layer of the signal is selected as the power plane and the power plane is divided , There will also be gaps . If a single ended signal encounters a wide gap , Then it will feel a disruptive mutation . This is a big inductive mutation .

5. In order to enable the transmitted signal to cross the gap in the return path and maintain acceptable performance , One alternative is to use differential pairs . The use of tightly coupled differential pairs is a way to transmit broadband signals in regions with very poor return planes .
When a single ended signal passes through the gap in the return path, it will produce a ground bounce , Because its return current will see the common inductance of the gap . However , The return current of the differential signal will overlap in the common inductance , And most of it will be offset . Since there is little net return current across the gap , The ground bomb of differential signal will be much smaller than that of single ended signal .

6.（1） The biggest advantage of using close coupled difference pairs is that the interconnection density is higher when close coupled . This means using fewer layers or possibly smaller boards , Both help reduce costs ;
（2） Usually , Above 10Gbps And loss is regarded as an important performance index , Weakly coupled differential pairs should be preferred , It makes the line the widest , The loss is the lowest .
The biggest advantage of using weak coupling is that you can use a larger linewidth , When cost is the main reason , Tight coupled difference pairs can be used , When loss becomes the main cause , Weakly coupled differential pairs should be used .

7. The equivalent circuit model of the transmission line of two coupled lines , use SPICE Capacitance matrix , The inductance element is defined by four inductance matrix .

（1） When the differential pair is driven in odd mode , The impedance of a single line is the singular impedance . At this time, the equivalent capacitance of a single line is ：

In the odd mode state , The current flows into the signal line 1, And then flow out of the return path . meanwhile , Current slave 2 Flow out , In the inflow return path . When the pair is driven in odd mode , Line 1 The equivalent loop inductance of is ：

From the line pair 1 Look in at the beginning , With the increase of coupling between lines , You can see that the capacitance will get bigger , The loop inductance will decrease , From these two quantities, the singular mode characteristic impedance and time delay can be calculated ：

（2） When the pair drives the even mode state , Driving voltage and routing of adjacent lines 1 In the same . Due to the shielding effect of this adjacent line at the same potential , Line 1 The capacitance between the signal path and the return path will decrease. At this time, the equivalent capacitance per unit length is ：

In the even mode state , The current flows into the signal line 1, And then flow out of the return path , meanwhile , The current flows into the signal line 2 And then flow out of the return path , When driven in even mode , Line 1 The equivalent inductance of is ：

When the pair is driven in even mode , According to the slave line 1 Unit length capacitance and loop inductance seen at the beginning , Even mode impedance and time delay can be calculated ：

8. All field solvers use the above relations based on the elements of capacitance matrix and inductance matrix , Any degree of coupling is thus calculated , The odd mode characteristic impedance of various transmission lines of any laminated structure , Even mode characteristic impedance time delay . In this sense , Capacitance matrix , And inductance matrix elements completely define the electrical characteristics of a pair of coupled transmission lines . As the coupling degree increases , Off diagonal elements will increase accordingly , The odd mode impedance decreases , Even mode impedance increases .

9. Another alternative to describing two or more transmission lines is to use an impedance matrix . The diagonal element of the impedance matrix is the impedance of one line when there is no current flowing into the other line .
The definition of odd mode state is that the current in two lines is equal in size and opposite in direction , Or say I1=-I2 The state of . According to this definition , Voltage is ：

The definition of even mode state is that the current in two lines is exactly the same , Or say I1=I2 The state of . According to this definition , Line 1 The voltage in the even mode state of is ：

The odd mode impedance of a line is the difference between the diagonal and non diagonal elements of the impedance matrix . The greater the coupling , The larger the non diagonal elements . The smaller the odd mode impedance , Even mode impedance is the diagonal element of the impedance matrix . Sum of and non diagonal elements . The greater the coupling , The larger the off diagonal elements , The greater the even mode impedance .
From a signal point of view , The only important ones are differential impedance and common mode impedance , Available below 3 Describe in equivalent form ：
（1） Odd mode impedance and even mode impedance ;
（2） Capacitance matrix and inductance matrix elements ;
（3） Impedance matrix .