The basic principles of high-speed PCB layout are as follows:
① Considering the distribution parameters of circuit components under high-frequency working conditions, all components should be arranged evenly, neatly and compactly on the double-sided circuit board, so as to minimize and shorten the lead length between components.
② Analog circuits should be separated from digital circuits. Eliminate the interference of digital signals to analog signals.
③ Reasonably arrange the position of the clock circuit.
The clock circuit can’t be directly connected to the signal line, so it is placed in the center of the double-sided circuit board and grounded. The layout of the optical burst module circuit can be considered from the following four aspects:
① The position of the laser MAX3656 and its connector shall be preset according to SFMSA specification, and the laser and driver shall be as close as possible.
② The position of the limiting amplifier MAX3747 is as close as possible to that of the back-end main amplifier chip MAX3748, so as to ensure the signal direction and the correct reception of the amplified signal, and minimize the interference.
③ Clock and data recovery circuit MAX3872 should be placed in the center and reliably grounded.
④ Max3654-47 ~ 870 MHz analog CATV transimpedance amplifier can be combined as a functional module area.
Firstly, the package component library is expanded to meet the needs of layout design. Then, the relevant software is used to directly call the package symbols of components, and the preliminary layout design of the circuit is completed. After the layout of most components is preliminarily determined, simulation analysis before and after wiring is usually carried out. The simulation analysis before wiring is mainly to determine the routing length and impedance matching of key signals, etc., and make repeated adjustments in combination with the simulation analysis results of delay, reflection and noise that affect signal integrity, so as to ensure signal integrity as much as possible from the system point of view, and the relative delay of input signals should not exceed 0.2ns
In the design of high-speed double-sided circuit board, EMI design must meet EMC design requirements. To measure the EMC design quality of the system, it is necessary to conduct accurate EMI test first. In the test, measuring instruments should be used on the basis of frequency domain, and the test methods should strictly abide by various standards. As a test equipment, the spectrum analyzer can conduct all-round three-dimensional tests on the components of the whole module, and can display the overall situation of electromagnetic radiation. According to the designed size, the printed circuit board is processed, and the board is pasted and soldered, and the EMI of the circuit board is debugged. Assembling the circuit board into the small package shell of the optical module; Finally, the optical module is tested.
