PCB Tech - How does PCB design deal with LVDS signals?

The LVDS signal is not only a differential signal, but also a high-speed digital signal. Therefore, regardless of whether the LVDS transmission medium uses a PCB line or a cable, measures must be taken to prevent the signal from being reflected at the media terminal, and electromagnetic interference should be reduced to ensure the integrity of the signal. As long as we consider the above elements in PCB layout design, it is not very difficult to design high-speed differential circuit boards.

The following briefly introduces the design points of PCB design to deal with LVDS signals:

1. Lay as a multi-layer board. Circuit boards with LVDS signals are generally arranged as a multi-layer board. Since the LVDS signal is a high-speed signal, the adjacent layer should be a ground layer to shield the LVDS signal to prevent interference. For boards with a low density, it is best to put LVDS signals on separate layers from other signals when the physical space conditions permit. For example, in a four-layer board, the layers can usually be laid out as follows: LVDS signal layer, ground layer, power layer, and other signal layers.

2. LVDS signal impedance calculation and control. The voltage swing of LVDS signal is only 350mV, which is suitable for current-driven differential signal operation. In order to ensure that the signal is not affected by the reflected signal when it propagates in the transmission line, the LVDS signal requires the transmission line impedance to be controlled, and the differential impedance is usually 100 +/- 10Ω. The quality of impedance control directly affects signal integrity and delay.

Simulation Analysis of Serial LVDS Signal

The above analyzes the matters that must be paid attention to when designing LVDS signals. Although the above rules are generally followed during PCB design, in order to improve the correctness and accuracy of the design, the complete signal simulation of the PCB must be performed, and the signal can be obtained through simulation. The crosstalk, delay, reflection, and eye pattern waveforms of the system can achieve the goal that the design is correct. The simulation process of signal integrity problem is to first establish the simulation model of the components, then perform the pre-simulation to determine the parameters and constraints of the wiring process, design according to the constraints in the physical realization stage, and finally perform post-simulation to verify whether the design meets the design requirements. The accuracy of the model in the entire process directly affects the simulation results, and the simulation analysis methods used in the pre-simulation and post-simulation stages are also critical to the simulation results, and a highly accurate spice model is used in this design . The following is a combination of actual projects to illustrate the implementation process of simulation in this design.

1. PCB stack setup

From the above analysis, it is known that the stacking setting of the PCB board is closely related to the coupling of the signal and the impedance calculation. Therefore, the stacking design must be carried out before the PCB design, and then the impedance calculation of the signal. The laminated design in this design is shown in the figure below:

Due to the high PCB density, this design uses a 10-layer laminate structure. After a reasonable arrangement of the laminate thickness, through allegro calculations, the differential line width of the surface microstrip and the inner strip line is 6㏕ line spacing At 6㏕, the theoretical calculated impedance values are 100.1 and 98.8Ω respectively.

2. Set the DC voltage value

This step is mainly to specify the DC voltage value for certain specific networks (usually power ground, etc.), to determine the DC voltage to be applied to the network, and to perform EMI simulation, one or more voltage source pins need to be determined. These voltage values include The reference voltage information used by the model in the simulation process is described.

3. Device Settings

During allegro simulation, allegro divides the devices into three categories: IC, connectors and discrete devices (resistance capacitors, etc.), allegro will assign simulation attributes to the pins of the device according to the device type, discrete devices and connector pins The attribute is UPSPEC, and the pin attribute of IC can be IN, OUT, BI, etc.

4. Model allocation

The main models used in the board-level high-speed PCB simulation process are device models and transmission line models. The device model is generally provided by the device manufacturer. In the high-speed serial signal, we use a higher-precision SPICE model for simulation analysis. The transmission line model is formed through simulation software modeling. When the signal is transmitted, the transmission line will make the signal integrity problem prominent, so the ability of the simulation software to accurately model the transmission line directly affects the simulation result.

Differential pair line model b: stripline c: microstrip line and the transmission line where the signal path and return path are located cannot be an ideal conductor, so they all have finite resistance, and the size of the resistance is determined by the length and cross-sectional area of the transmission line .

5. SI inspection

The SI Audit function is used to check whether a particular network or a group of networks can be extracted for analysis. Generally, it is to set the high-speed network that we need to pay attention to. This design mainly focuses on LVDS serial signals.

6. Extract the network topology

Extract the topological structure of the signal of interest from the PCB, which generally includes the driving end and the receiving end, as well as the transmission line and the related matching resistance and capacitance. It can be seen from the topology that the network passes through those paths, which will affect the signal transmission .

7. View the waveform

After the above related steps are set up, simulation can be carried out. Allegro can carry out signal reflection simulation and crosstalk simulation, and the differential line also needs to carry out eye diagram analysis. Of course, the simulation is also divided into pre-simulation and post-simulation. When using allegro for PCB design, it is necessary to modify the design in real time with the simulation results to meet the requirements.

There are two points to pay attention to in the wiring of the differential pair. One is that the length of the two lines should be as long as possible. The equal length is to ensure that the two differential signals maintain opposite polarities at all times and reduce the common mode component. The other is that the distance between the two wires (this distance is determined by the differential impedance) must be kept constant, that is, it must be kept parallel. There are two parallel ways, one is that the two wires run on the same side-by-side, and the other is that the two wires run on two adjacent layers above and below (over-under). Generally, the former has more side-by-side implementations. The equidistance is mainly to ensure the same differential impedance between the two and reduce reflection.

From the analysis of this article, we can see that in the design of high-speed serial signals, not only the circuit design is considered, the board diagram design and simulation analysis are also equally important, and as the frequency of the signal becomes larger and larger, the delay and crosstalk of the signal are affected. Factors such as signal integrity and signal integrity are becoming more and more complex. At the same time, it is becoming more and more difficult to control the influence of these factors. Engineers must thoroughly analyze the wiring design, rely on accurate models, effective simulation and scientific analysis methods to give correct guidance to complex high-speed designs, reduce correction cycles, and ensure PCB board The design is successful.