EMC, also known as electromagnetic compatibility, refers to the ability of a device or system to operate in accordance with requirements in its electromagnetic environment without generating unbearable electromagnetic interference to any equipment in its environment. So how to design the emc of the active crystal?

EMC includes two requirements: on the one hand, it means that the electromagnetic interference generated by the equipment in the environment during normal operation cannot exceed a certain limit; on the other hand, it means that the appliance has a certain degree of electromagnetic interference in the environment. Immunity, ie electromagnetic sensitivity.

Overview of EMC Design Points for Active Crystal Oscillators

Key points of schematic design:

(1) Decoupling of the crystal power supply is very important. It is recommended to add magnetic beads. Choose two to three decoupling capacitors with decreasing capacitance.

(2). Match the clock output pins. The specific matching resistance value can be determined according to the test results.

(3). The reserved capacitor C1 has a small capacitance value, which constitutes a level of low-pass filtering. The selection of resistors and capacitors depends on the specific test results.

PCB design points:

(1) In the PCB design, the shell of the crystal oscillator must be grounded to prevent the year-to-year radiation of the crystal oscillator and shield it from external interference.

(2), the ground should be laid under the crystal to prevent interference with other layers. Because some people are laying multilayer boards, the top and bottom layers are not paved, but it is recommended that the crystal is on the floor.

(3) Do not route under the crystal, and do not route and other components within a range of 5mm (some books recommend a range of 300mil, everyone can refer to), mainly to prevent the crystal from interfering with other wiring and devices.

(4) Do not place the crystal on the edge of the board, because for safety reasons, the ground of the board and the metal shell or mechanical structure are often connected together. This place is temporarily referred to as the reference ground plate. If the crystal is placed on the board, At the edge, the crystal oscillator and the reference ground plate will form an electric field distribution, and the edge of the card often has a lot of cables. When the cable passes through the crystal and the reference ground plate, the electric field is disturbed. Whereas the crystal is placed far from the edge, the electric field distribution of the crystal and the reference ground plate is divided by the GND of the PCB board, and the electric field distribution to the reference ground plate is greatly reduced.

(5) Of course, the clock line should be as short as possible. If you don't want the clock line to go all the way to interfere with it, then make it short. Another point, regarding the choice of crystal, if your system can work at 25M, try not to choose a 50M crystal. The clock frequency is high and it is a high-speed circuit. The rising edge of the clock is also a high-speed circuit. Signal integrity needs to be considered.