Wednesday, December 29, 2010
Weak current College】 【cable interference problems and countermeasures】.
<br> Cable are electromagnetic compatibility design key link, most sensitive issues, EMI electromagnetic interference emission problems, signal crosstalk is cable. Cable is easy to produce various electromagnetic interference problem, mainly has following several causes. <br> (1) the cable .itself is a highly effective receiving antenna that can receive the space of electromagnetic interference that will interfere with the energy pass into device circuit, causing interference; <br> (2) cable is a highly effective radiation antennas to circuit interference radiation into space, causing .radiation emission; <br> (3) cable wire parallel, and are very close, conductors have a very big parasitic capacitance and mutual inductance, capacitance and mutual inductance is the root cause of the cross; <br> (4) cable to connect .the different grounding potential, the introduction of ground cable shield. <br> The first three conditions is very intuitive, the fourth, as shown in Figure 1. <br> In Figure 1, the two devices in ground potential, creating a potential difference ., driven by the potential difference, the cable shield in current. Because of the shield and inner conductors between the capacitor and the inductor, thus shielding layer can, on the current line on the induction voltage noise out. If both the signal cable is balanced ., noise voltage is the same, no noise on the input circuit voltage. But the circuit is usually not be balanced in two-wire, produce different voltage, resulting in a noise voltage. This is a great system of ground disturbance common phenomenon, the .workaround is to try to make the same ground potential, or the end of the cable shielding and grounding wire disconnected, eliminating the cable shield of current. Crosstalk between conductors and countermeasures <br> Crosstalk between conductors, a wire signal coupling to another signal line ., and the root of the signal cable connection circuit cause interference. This phenomenon often occurs in parallel conductors. In the design of the cable, pay special attention to this phenomenon. Especially when the transmission on the wire is low-level analog signals, close .to the wire on the crosstalk is system performance degradation. Therefore, the design of the system, the signal line group is to be carried out in order to design the project, grouped by the signal line, so that possible crosstalk. <br> When .there is interference in the discovery circuit, the first thing to check is the source of interference, cross-see the interfering signal is from a nearby circuits crosstalk, judgment method has the following. <br> (1) if possible, will be the .source of interference on the adjacent wire transmission signals temporarily removed, if the noise disappears, you can determine the interference signal crosstalk is imminent; <br> (2) reduce the potential interference signal frequency, for example, you can potentially interfere with the signal .outputs of a capacitor in parallel, if interference disappear or decrease, can determine interference from this road signal crosstalk from; <br> (3) with double trace oscilloscope while observing interference signals and signal potential sources of interference, if the interfering signal approximation is .interference source differential signal, you can determine the source of interference; <br> (4) a change in uninterrupted wire and interference sources the distance between the wires, if the interfering signal along with the distance between the two, you can increase and decrease .the identified two wire crosstalk between. Estimation of crosstalk voltage <br> For two parallel wires, cable length compared with the wavelength is very short (less than 1 / 20 wavelength), it is possible to use lumped-parameter models to describe capacitively coupled ., as shown in Figure 2. This can be used to circuit analysis method to calculate the cross-voltage. The results of the calculation is as follows. <br> Frequency is low, R far less than the C12 and composition of impedance C2G, .ie R <<1 / [j ω (C12 + C2G)], in this condition, crosstalk voltage: <br> VN = jωRC12V1 <br> As can be seen from the formula, the capacitive coupling of strength directly with frequency ., tampering with conductor to ground resistance, capacitance between the two conductors. <br> Frequency is high, R is much larger than C12 and composition of impedance C2G, namely: R>> 1 / [j ω (C12 + C2G) .], in this condition, crosstalk voltage: <br> VN = V1 [C12 / (C12 + C2G)] <br> As can be seen from the formula, the capacitive coupling of intensity and the frequency and circuit impedance is irrelevant, but .only with two conductor capacitance and receive between the conductor and the capacitor between, these parameters and conductor structure directly related. <br> Wire between mutual perception of crosstalk, as shown in Figure 3, when there is current in the circuit 1 I through, .not only in the circuit in flux Φ 1 and 2 in the circuit in flux Φ 12, loop 2 flux Φ between 12 and currents I1 M by a coefficient to determine, the M is mutual inductance. <br> M = Φ 12 / I .-1 fixed rate under Electromagnetic induction, when a closed circuit of the magnetic field changes will produce induction voltage circuit. Therefore, Φ 12-in-the-loop 2 in induction voltage, the voltage values are: <br> VN = dΦ12 / .dt = M.dI1/dt <br> This shows the loop 1 energy coupling into the loop interference voltage 2 formation and this voltage is loop 1 current differential. Elimination of crosstalk method <br> From the above crosstalk voltage can be seen in the .formula which parameters have an impact on crosstalk voltage, in practical projects can take the following specific measures to control these parameters. <br> <br> Decrease the capacitive crosstalk follows. <br> (1) uninterrupted wire shield, and shield received uninterrupted .circuit of public land, you can decrease the capacitive crosstalk, shield should be as complete (so that the wire's exposed parts as far as possible short), the frequency of when interference voltage is high (λ <20L), the need to shield .multi-point grounding; <br> (2) increase the distance between the wires (reduce the C12); <br> (3) will be subject to interference and close to the reference traverse (C2G); <br> (4) .may, in interference circuit input capacitance on the installed; <br> <b> </ b> R> decrease inductive crosstalk method as follows. <br> (1) an increase in both the distance between the circuit, the second circuit's flux .will decrease; <br> (2) make the loop 1 signal cable and transmission lines as close as possible to (seen from Figure 3, loop 1 signal cable and transmission line in loop 2 magnetic flux generated in the opposite direction, the mutual cancellation .); <br> (3) reduce the area of the loop 2; <br> (4) adjust the two loops of the relationship between the relative position and angle. Cable radiation and outside interference countermeasures <br> The cable is on the .device causes the device to produce excess radiation of one of the main factors in terms of EMC test done in realistic environments, often find that when a device does not have cable, electromagnetic interference problems is much better, because cable is a high efficiency of the .antenna. For purposes of receiving antenna characteristics and radiation characteristics is symmetric, a receive efficient antenna radiation efficiency is high. Therefore the following only discuss how to reduce the radiation of the cable, the cable of these measures, the cable reception outside interference problems are .solved at the same time. <br> Cable generation radiation mechanism as shown in Figure 4, there are three main mechanism. <br> (1) the signal line and loop current loop consisting of radiation (radiation called differential mode); <br .> (2) signal cable stream to load all of the current does not flow through the ground back in part through a different path (eg the Earth) stream back into an unforeseen current loops to generate radiation (called common mode radiation); <br .> (3) due to the signal cable and signal wire with the Earth or other potential difference between the reference object, forming a radiation, produce current circuit (common mode radiation); <br> (4) the box other circuit radiation of electromagnetic .energy induction into the cable, produces a voltage and current, lead cable radiation (common mode radiation). <br> Generally, the cable signal line and loop the loop current in between is not the main reason for producing radiation, signal lines and signal .lines between very close, as they flow through the above current direction on the contrary, they are in space radiation neutralize each other. After three common mode radiation is the main cause of cable radiation. Reduce cable radiation method <br> (1) control .the length of the cable, in meeting the requirements of use as a short cable, but when the cable length cannot be reduced to a maximum of half the wavelength of the radiation frequency below, reducing the length of the cable with no apparent effect. < .br> (2) in the cable using the appropriate common mode choke coils, the easiest way is to set a ferrite magnetic ring; <br> (3) cloth circuit boards, making periodic signal away from the I / O interface circuit, and .I / O interface circuit parts of the Earth and the rest of the circuit ground isolation, only in that connection; <br> (4) I / O interface circuits section of the ground wire and metal chassis radio frequency overlap between doing; <br .> (5) on-the-box i / o cable (from the circuit board to the connector part) shielding; <br> (6) the box i / o cable (from the circuit board to the connector part) length short as .possible; <br> (7) the use of common low-pass filter, it is best to install the panels on the form (eg filter connectors); <br> (8) the use of shielded cables, but pay attention to the .cable shield termination and termination, termination is not likely to increase the cable of radiation; <br> (9) the use of balance circuit..