Why can't the crystal oscillator be placed on the edge of the PCB? Explain with actual examples!

wangerxian

Why can't the crystal oscillator be placed on the edge of the PCB? Explain with actual examples! [Copy link]

 

When laying out the crystal oscillator, it is generally not allowed to be placed at the edge of the PCB. Today we will explain this with a practical example.

A dashcam needs to be tested with an external adapter. When the machine is powered on and tested, it is found that the radiation exceeds the standard. The specific frequencies are 84MHz, 144MHz, and 168MHz. It is necessary to analyze the reasons for the excessive radiation and give corresponding countermeasures. The radiation test data is as follows:

Figure 1: Radiation test data

01 Radiation source analysis

The product has only one PCB with a 12MHz crystal on it. The frequency points that exceed the standard are all multiples of 12MHz. When analyzing the screen and camera of the machine that are prone to EMI radiation exceeding the standard, it was found that the LCD-CLK is 33MHz and the camera MCLK is 24MHz.

Through the process of elimination, it is found that after removing the camera, the over-standard point still exists, and by shielding the 12MHz crystal, the over-standard point is reduced. Therefore, it is judged that the 144MHz over-standard point is related to the crystal. The PCB layout is as follows:

Figure 2: PCB layout

02Radiation Generation Principle

From the PCB layout, we can see that the 12MHz crystal is placed at the edge of the PCB. When the product is placed in a radiation emission test environment, the high-speed device of the product under test and the reference ground in the laboratory will form a certain capacitive coupling, generating parasitic capacitance, resulting in common-mode radiation. The larger the parasitic capacitance, the stronger the common-mode radiation. The parasitic capacitance is essentially the electric field distribution between the crystal and the reference ground. When the voltage between the two is constant, the more the electric field distribution between the two, the greater the electric field strength between the two, and the greater the parasitic capacitance. The electric field distribution when the crystal is at the edge of the PCB and in the middle of the PCB is as follows:

Figure 3: Schematic diagram of the electric field distribution between the crystal oscillator and the reference ground plane at the edge of the PCB

Figure 4: Schematic diagram of the electric field distribution between the crystal oscillator and the reference ground plane in the middle of the PCB

As can be seen from the figure, when the crystal oscillator is placed in the middle of the PCB or far away from the edge of the PCB, due to the existence of the working ground (GND) plane in the PCB, most of the electric field is controlled between the crystal oscillator and the working ground, that is, inside the PCB. The electric field distributed to the reference ground plane is greatly reduced, resulting in reduced radiation emission.

03Handling measures

Move the crystal inwards, making it at least 1cm away from the edge of the PCB, and apply copper to the surface of the PCB within 1cm of the crystal, and connect the surface copper to the PCB ground plane through vias. The modified test result spectrum is as follows, from which it can be seen that the radiation emission has been significantly improved.

04Thoughts and Inspirations

Capacitive coupling between high-speed printed lines or devices and the reference ground plane will cause EMI problems, and sensitive printed lines or devices arranged at the edge of the PCB will cause immunity problems.

If the design must be arranged at the edge of the PCB due to some other reasons, then you can lay another working ground line next to the printed line and add more vias to connect this working ground line to the working ground plane.

吃饭少放盐

good

annysky2012

Thanks for sharing

木森林的年轮

The persuasiveness is very strong. Before, I just watched how others arranged things and never explored the principles behind them. This time I learned it.

freebsder

Learned a lot, very useful stuff.

se7ens

Can the reference ground plane be tested together with the PCB ground?

In addition, there is usually a designed ground loop to prevent radiation emission from components on the PCB and improve the product's immunity to interference.

damiaa

Thanks for sharing!

山野风

Thanks for sharing

happymakelife

thank!

bobde163

Learned

Gen_X

Same question as above.

We have used equipment from a famous British manufacturer and since they were purchased as bare boards, there were no problems with the inspection.

During installation, we found that the crystal oscillator was unstable. Most of them had no problem after assembly, and some were normal after "repair". However, the problem was that the crystal oscillator was arranged on the edge of the PCB, which often manifested as problems with the tightness of the surrounding screws or humid weather. After some of them were returned to the factory, foreigners suggested that the PCB needed to be padded with very thick insulation (distance) to basically eliminate this problem.

But the actual problem is PCB design problem.

wangerxian

Gen_X posted on 2021-12-10 16:01 The problem is the same as above. We used equipment from a famous British manufacturer. Because it was purchased as a bare board, there was no problem with the inspection. During installation, it was found that the crystal oscillator was not...

So there are still many things to pay attention to in PCB design!

book1

If it is an active crystal oscillator, it has been shielded. For a passive crystal oscillator, the oscillation circuit must be shielded. Whether it is an active crystal oscillator or a passive crystal oscillator, the output line must be shielded!

Gen_X

wangerxian posted on 2021-12-10 16:47 So there are still many things to pay attention to in PCB design!

that is!

Sometimes the crystal position and stability are more important than the microcontroller itself or the forward circuit.

It is the "heart" of the microcontroller system and is very important!

zuig67

This is exactly what I need knowledge in. Thanks for sharing. It’s very well explained. Thank you for your hard work!