PCB Blog - Six types of module PCB board debugging technology

The components of the six types of modules are PCB board. Its design structure and manufacturing process basically determine the performance indicators of the product. When domestic counterparts design their PCB boards, they often do not fully understand the failure mechanism, resulting in product performance indicators that are not high enough or cannot meet the requirements.

1. Implementation standards and definitions of important indicators

The six-type module implementation standard is EIA/TIA 568B.2-1, and the important parameters are insertion loss, return loss, near-end crosstalk, etc. Insertion Loss: Due to the existence of the transmission channel impedance, it will increase the attenuation of the high-frequency components of the signal as the signal frequency increases. The attenuation is not only related to the frequency of the signal, but also to the transmission distance. As the length increases, so does the signal attenuation. It is measured by the number of signal losses along the transmission channel per unit length, which represents the ratio of the signal strength of the source transmitter to the receiver signal strength. Return Loss: Due to the change of impedance in the product, local oscillation will occur, resulting in signal reflection. A portion of the energy that is reflected by the transmitter will form noise, which will distort the signal and degrade transmission performance. For example, the full-duplex Gigabit network will mistake the reflected signal as the received signal, causing fluctuation of the useful signal and causing confusion. The less energy reflected, the better the impedance consistency of the line used by the channel, the more complete the transmission signal, and the less noise on the channel. The calculation formula of return loss RL: return loss = transmitted signal ÷ reflected signal. In the design, ensuring the full line consistency of the impedance and cooperating with the six types of cables with 100-ohm impedance is the way to solve the failure of the return loss parameter. For example, the uneven distance between layers of the PCB circuit, the change of the copper conductor cross-section of the transmission line, the mismatch between the conductors in the module and the six types of cable conductors, etc., will cause the return loss parameters to change. Near-End Crosstalk (NEXT): NEXT refers to the coupling of signals from one pair to the other in a pair of transmission lines, that is, when a signal is sent from one pair, it is received on another adjacent pair. signal of. This crosstalk signal is mainly due to capacitive or inductive coupling between adjacent pairs. How to reduce the signal coupled by capacitance or inductance, or to offset and weaken its interference signal by means of compensation, so that a standing wave cannot be generated, is the main method to solve the failure of this parameter.

2. Technology and failure mechanism

The following content is mainly based on the explanation of the trial production process of the super six-type module PCB board of a Korean company, which has a very important reference significance. In the trial production stage of the module, the theory is used as a guide and the computer-aided design is used as the basis to quickly achieve the expected effect. In the design of six types of module PCB boards in our country, mainly based on the theory of line diagonal compensation, a lot of trial production work is carried out, and the expected effect can also be achieved. The following theory is used as a reference.

1) Signal leakage caused by modules and plugs

Signals are on the link, and there will be signal interference with each other. In order to prevent the phenomenon of signal interference, the conductors are twisted in the balanced link to achieve the purpose of balanced transmission. Although the twisted structure will cause the phase change between signals, at the same time, it will increase the signal attenuation on the line. This structure is called Unshielded Structure (UTP). The twist distance of each pair of the 4-pair balanced twisted pair is different, which is to achieve this purpose. The end of the cable uses a modular connector, that is, an information module, to form the connection between the connector and the connector, and a balanced structure between conductors is formed in the interconnection area, that is, the link of the six types of systems. In the link, the signal interference phenomenon that occurs in the balanced line, that is, crosstalk is a technology for manufacturing connectors for high-speed communication to solve the problem of crosstalk. Contact loss occurs between the contact terminals, resulting in attenuation, reflection loss, and other phenomena. This loss is a problem that causes obstacles and failures in high-speed signal transmission. By solving these problems, it is a technology to manufacture connectors for high-speed communication.

2) Explanation of signal leakage caused by module and plug

In the connection line between the module and the plug, each pair of connection terminals in the plug is also a balanced line. Conductors in balanced lines produce signal leakage and impedance losses. The factor that hinders communication is signal leakage. The solution to the external leakage problem can be found by studying the E field and H field, or by studying the reverse attenuation method, which is the technology of manufacturing connectors for high-speed communication.

3) E field and H field

The signal interference that occurs on the balanced line, that is, the electromagnetic field interference can be described by the distribution of the E field and the H field. The main parameter of electronic communication line testing is the relative measurement made under swept frequency. Add voice or data packets to this frequency signal for transmission. The higher the transmission speed is, the faster the frequency is. Using computer simulation technology, this part is viewed with some special instruments.

4) Solution to signal leakage

The basic method to explain the signal leakage phenomenon of the socket that caused the problem is to collect the signal in the signal concentration area and return it according to the simulation diagram of the signal leakage caused by the inductance and capacitance. The following diagram is a simulation diagram of solving the external leakage signal at the IDC terminal in the reverse direction coupling mode. The amount received at the IDC terminal is returned in full, thereby solving the problem of external leakage. In the design, the design of the coupling capacitor is a key parameter, which is related to the length of the coupling line, the distance between the lines, the width, and the layout of the compensation line. Considering that the six types of systems use 4 pairs of lines to transmit signals at the same time, it will inevitably produce comprehensive remote winding and comprehensive remote winding. Considering all the influences, computer simulation is carried out to design the compensation circuit. The figure below shows the computer simulation and the circuit design process when designing the super six kinds of circuit boards.

5) The six types of module trial production process generally carried out by domestic counterparts

The six types of module processes generally carried out by domestic counterparts are mainly to design the compensation circuit after determining the main circuit, and carry out a large number of scheme designs and sample production. After the compensation circuit and the structure between the layers of the PCB are basically determined, the follow-up work is mainly to Improve performance through process improvement.

The main parameters to be adjusted are:

a Parameters of the interlayer gap; parameters of copper foil thickness; parameters of arrangement of 8 main transmission lines, width and relative distance of 8 main transmission lines;

b Adopt the diagonal compensation method to adjust the compensation of each line pair and other line pairs, including compensation line position distribution, compensation line length, and width, compensation line gap, etc.;

c For the adjustment of the process parameters of the PCB board processing plant.