1 difference divides signal to transmit

 

What interface of CAN, RS-485 uses is transmission means of the signal that need branch. Differring minute of signal to transmit is one kind uses the method that two complementary and telegraphic date have information is delivered. It is with high speed CAN exemple, different logistic position passes CANH, CANL two line undertake transmission, receive circuit to be opposite only the signal of two line differs a value to undertake discriminating. Below good position, the weaveform of CAN bus line is shown 1 times like the graph.

 

Graph 1

 

Interference signal is common with in all modular form exists, when bus line is disturbed, two bus line can suffer an effect at the same time, but its need minute of voltage and won’t suffer an effect, if pursue,2 are shown. Transmit means to carrying signal at sheet for, the signal that need branch transmits means to have better interference rejection capacity.

 

Graph 2

 

Of course, it is not OK also to used poor cent to transmit means have nothing worry about. Bus line of CAN, RS-485 often is used at the telecommunication, the addition of wire cable length, all sorts of interference arrive through wire cable coupling on bus line, increased the probability that the outside disturbs to bus line communication greatly, if wire cable is chosen,reach use undeserved, cause communication extremely likely unusual. Wait for bus line application to CAN, RS-485, general we can be recommended use double skeining thread.

 

The coupling mechanism of 2 noise

 

Want to understand the good point of double skeining thread, how do need understands interference first affect useful signal. Interference (noise) the kind that carries coupling commonly has an effect to the system, common coupling mechanism has 4 kinds, it is to conduct coupling, capacitance coupling, inductance respectively (induction) coupling and radiation coupling.

 

Interference source and suffer interference circuit to have electric link, be like in all, the voltaic flow that obstructs a source makes common share forms electric current to produce interference tension, cause an effect to getting the signal that disturbs circuit thereby, this kind of means is conduct coupling. Graph 3 to conduct coupling sketch map, es is signal source, zs is signal source internal resistance, zc is common share impedance, zl is laden impedance, en is interference source, vl is laden voltage. The electric current that disturbs source En generation has shed Zc, on Zc generation pressing falls, cause Vl voltage change, affect the signal of laden side from this.

 

Graph 3

 

Capacitance coupling appears when the electric field that existence of two adjacent conductor changes, interference electric current is flowed into through the coupling capacitance between conductor suffer interference circuit. Because coupling capacitance is general very small, its impedance is very big, reason interference source is constant to sufferring interference circuit to be able to regard current source, when the impedance of signal circuit is bigger, the effect is particularly apparent. Graph 4 for capacitance coupling sketch map, es is signal source, zs is signal source internal resistance, cm is coupling capacitance, zl is laden impedance, en is interference source, vl is laden voltage. Interference electric current flows into Zl through Cm, cause an effect to Vl.

 

Graph 4

 

Inductance (induction) when coupling appears in the magnetic field that existence changes between two parallel conductor. Interference source electric current has shed conductor to produce magnetic flux, magnetic flux forms inductive electromotive force in sufferring interference circuit conductor, affect thereby get interference signal. Below this kind of circumstance, noise can regard constant voltage source, because this is in low impedance circuit,noise influence greatens. Graph the 5 sketch map that are inductive coupling, es is signal source, zs is signal source internal resistance, lm is mutual inductance, zl is laden impedance, en is interference source, vl is laden voltage. Electric current of interference source En has shed mutual inductance Lm, in suffer interference circuit to form voltage, cause an effect to Vl.

 

 

Graph 5

 

Radiation coupling appears in interference source and suffer interference parts of an apparatus to be apart from further circumstance, interference source and suffer interference parts of an apparatus to all serve as wireless antenna, interference source sends an interference electromagnetic wave, and parts of an apparatus of interference be sufferringed is received.

 

The advantage of 3 pairs of skeining thread

 

Double skeining thread is twined each other by the lead of two mutual insulation and become, suit to differ minute of signal to transmit a circumstance particularly, compare with parallel photograph, can restrain interference effectively.

 

1, eliminate capacitance coupling

 

Opposite Yu Ping is opposite all right line, every single track is opposite double skeining thread the coupling capacitance value that violates source or ground is more adjacent, impedance more evenly, if pursue,6 are shown.

 

Graph 6

 

Because double skeining thread is twined together cheek by jowl, agree as the coupling capacitance between two lines and noise source, basically as the impedance between the earth. The interference electric current that noise headstream joins two line is basic and identical, the poor cost of two line is fixed, the voltaic translate into of coupling capacitance in all modular interference. If pursue 7, coupling capacitance C1=C2, z1=Z2, interference headstream enters C1, the electric current of C2 is equal, namely 1, the tension that line of two 2 roots produces is equal, vn=0. Because differ minute of signal to transmit means to have good in all the model controls capacity, because this can remove the effect of capacitance coupling.

 

Graph 7

 

2, eliminate inductance (induction) coupling

 

If use parallel, two line can form a very narrow annulus road, the magnetic field in meeting collect environment violates this annulus way. The structure of double skeining thread is two conductor that reverse transmission line with fixed span, make the electromotive force direction that causes by magnetic field is in the place of ” of “ annulet road of every photograph adjacent to invert, because this is OK,offset orderly. Look from circuit, the mutual inductance that ” of road of annulet of “ of every photograph adjacent is in is being lost to noise source, lead whole mutual inductance turns into 0. If the graph is shown 8 times, when parallel is disturbed by outside magnetic field, of two lead faradic cannot offset, can produce bigger inductive tension, influence signal is transmitted. And the structure of double skeining thread makes of lead faradic mutual and quits, won’t produce inductive tension.

 

Graph 8
 

3, decrease to be disturbed external

 

Use at poor cent signal is transmitted, double skeining thread the voltaic size of two lines is equal, way is opposite. If pursue 9, below good position, the magnetic field way that the place of ” of “ annulet road of the adjacent of every two photographs that two group become double skeining thread forms is opposite, size is equal, can offset each other, reason double skeining thread external electromagnetism interference wants than parallel cable small.

 

Graph 9

 

4 summary

 

In needing minute of transmission application, double skeining thread can reduce the oneself interference to the outside not only, can eliminate the capacitance coupling that violates a source with the outside and inductive coupling at the same time, have one Shi Erdiao’s effect, got be used extensively in the application that because this is twinning,the line assigns signal to transmit in equal difference of such as CAN, RS-485.

 

Above paragraphs all is to be based on ideal double skeining thread to undertake an analysis, but because actual double skeining thread is made when wring add up to the degree, parasitism parameter difference that wrings itself of cable of the deviation that close, wire to wait, not ideal, in applying actually so, double skeining thread controls capacity to noise the meeting is abate.

 

Because the structure of double skeining thread can not remove the interference that conducts coupling and radiation coupling, in the situation with a few serious interference, still need to cooperate to keep apart technology and screen technology in order to improve interference rejection performance of the system. Segregation technique can restrain those who conduct coupling to form effectively in all modular interference, and screen technique can restrain radiation effectively to disturb.

 

Disturbing serious situation, choose segregation to send and receive implement, cooperate screen double skeining thread to use, can provide good interference rejection performance for CAN, RS-485 application, ensure the dependability of communication.