PCB design how to do crystal layout?

We often compare the crystal oscillator to the heart of the digital circuit, this is because all the work of the digital circuit is inseparable from the clock signal, the crystal oscillator directly controls the entire system, if the crystal oscillator does not operate then the entire system is paralyzed, so the crystal oscillator is a prerequisite for the digital circuit to start working. In this article, we will discuss how to do crystal vibration layout in PCB design.

The crystal oscillator we often say is a quartz crystal oscillator and a quartz crystal resonator, both of which are made of the piezoelectric effect of quartz crystals. Applying an electric field to the two electrodes of a quartz crystal causes a mechanical deformation of the crystal, whereas applying mechanical pressure to both sides of the crystal creates an electric field on the crystal. And both phenomena are reversible. Using this property, alternating voltages are applied to both sides of the crystal, causing the wafer to vibrate mechanically and at the same time generate alternating electric fields. This vibration and electric field are generally small, but at a certain frequency, the amplitude will be significantly increased, which is piezoelectric resonance, similar to the LC loop resonance we commonly see.

As the heart of the digital circuit, how does the crystal oscillator play a role in smart products? Smart home such as air conditioning, curtains, security, monitoring and other products, all need wireless transmission modules, they through Bluetooth, WIFI or ZIGBEE and other protocols, the module from one end to the other end, or directly through the mobile phone control, and the crystal oscillator is the core component of the wireless module, affecting the stability of the whole system, so choose the crystal oscillator used in the system. Determines the success or failure of digital circuits.

Due to the importance of crystal vibration in digital circuits, we need to be careful when using and designing:

1, there is quartz crystal inside the crystal oscillator, by external impact or fall easily cause quartz crystal breakage, resulting in crystal vibration is not vibration, so in the design of the circuit to consider the reliable installation of crystal vibration, its position as far as possible not close to the edge, equipment housing and so on.

2, in manual welding or machine welding, pay attention to the welding temperature. Crystal vibration is more sensitive to temperature, the temperature can not be too high when welding, and the heating time is as short as possible.

One,Problem description

The product is a field camera, which is composed of five parts: core control board, sensor board, camera, SD memory card and battery. The shell is a plastic shell, and the small board has only two interfaces: DC5V external power interface and USB interface for data transmission. After the radiation test, it is found that there is a harmonic noise radiation problem of about 33MHz.

Second, analyze the problem

The product shell structure plastic shell, non-shielding material, the whole test only power cord and USB cable out of the shell, is the interference frequency is radiated by the power cord and USB cable? Therefore, the following steps were taken to test:

(1) Only add magnetic ring on the power cord, test results: the improvement is not obvious; 

(2) Only add magnetic ring on USB wire, test results: the improvement is still not obvious;

(3) In the USB line and the power line are added magnetic ring, test results: the improvement is more obvious, the overall interference frequency has decreased.

From the above, the interference frequency point is brought out from the two interfaces, not the problem of the power interface or USB interface, but the internal interference frequency point coupled to the two interfaces caused by shielding only a certain interface can not solve the problem.

After near field measurement, it was found that the interference frequency came from a 32.768KHz crystal oscillator of the core control board, which generated strong spatial radiation, making the surrounding cables and GND coupled 32.768KHz harmonic noise, and then radiated through the interface USB cable and power cord coupling. The crystal oscillator's problem is caused by the following two problems:

(1) The crystal oscillator is too close to the edge of the plate, which is easy to cause the crystal oscillator radiation noise.

(2) There is a signal line under the crystal oscillator, which easily leads to harmonic noise of the signal line coupling the crystal oscillator.

(3) The filter component is placed under the crystal oscillator, and the filter capacitance and matching resistance are not arranged according to the signal flow direction, so that the filtering effect of the filter component is worse.

Third, solutionsAccording to the analysis, the following countermeasures are obtained: 

(1) The filter capacitance and matching resistance of the crystal are preferentially placed near the CPU chip, away from the edge of the board; (2) Remember not to lay ground in the crystal placement area and the lower projection area; 

(3) The filter capacitance and matching resistance of the crystal are arranged according to the signal flow direction, and are placed neatly and compact near the crystal; 

(4) The crystal is placed near the chip, and the line between the two is as short and straight as possible.Iv. Conclusion

Nowadays many systems crystal oscillator clock frequency is high, interference harmonic energy is strong; Interference harmonics in addition to the input and output of the two lines conducted out, but also from the space radiation, if the layout is unreasonable, easy to cause a strong noise radiation problem, and difficult to solve through other methods, so in the PCB board layout of the crystal oscillator and CLK signal line layout is very important.

(1) The coupling capacitor should be as close as possible to the power supply pin of the crystal oscillator, and the position should be placed in order: according to the direction of the power supply inflow, according to the capacity value from large to small, and the capacitor with the smallest capacity value is closest to the power supply pin.

(2) The shell of the crystal oscillator must be grounded, which can radiate the crystal oscillator outwards, and can also shield the interference of external signals on the crystal oscillator.

(3) Do not wire under the crystal oscillator to ensure that the floor is completely paved, and do not wire within the 300mil range of the crystal oscillator, which can prevent the crystal oscillator from interfering with the performance of other wiring, devices and layers. 

(4) The route of the clock signal should be as short as possible, the line should be wider, and the balance should be found in the length of the wiring and away from the heat source.

(5) The crystal oscillator should not be placed on the edge of the PCB board, and pay special attention to the point when the board is designed.

Electronic is a professional engaged in electronic products circuit board design (layout layout design) PCB design company, mainly undertake multi-layer, high-density PCB design drawing board and circuit board design proofing business. With an average of more than 10 years of work experience in PCB design team, can skillfully use the market mainstream PCB design software, professional and efficient communication to ensure PCB design progress, to help you seize the market opportunity one step earlier!