I. Introduction
A dash cam is an instrument that records the image and sound of the vehicle while driving.
After installing the dash cam, it can record the video images and sounds of the whole process of the car, which can provide evidence for traffic accidents. Those who like to travel by car can also use it to record the process of conquering difficulties and obstacles. When driving, you can record the video while walking, and at the same time record the time, speed, and location in the video, which is quite a "black box". It can also be used at home as a DV to capture the joy of life, or as a home monitor. Usually you can also do parking monitoring, install a dash cam, video data can not be cropped, if cropped, after the accident can not help. It is also to prevent the inevitable porcelain touching behavior in today's society.
The approximate modules of most dash cams mainly include: camera (module), screen, storage (TF card, etc.), power supply, main control chip, decoding chip, BT, GPS, etc. Therefore, when we generally solve the problem of dash cam radiation EMC, we can trace the source of the problem from the above points.
2. Cases
What I share today is a radiation rectification case of a certain brand of tachograph:
From the test results, we can see that the 50MHz~200MHz frequency band may have excessive power supply and data envelope, and there are many single clocks in 100MHz~400MHz.
In the case of excessive power supply and data envelope, we can check from the power module, camera module, storage module and other places that may cause the noise. You can verify this by disconnecting a power module, disconnecting the camera, or unplugging the memory card. We can troubleshoot the problem of a single clock from the camera, main control, storage, screen and other modules that carry the clock, or we can troubleshoot it by disconnecting the clock in a certain place.
3. Rectification methods
Through troubleshooting, I found the following issues:
1. The radiation of the machine itself can be radiated out through the wiring harness;
2. The main control part of the MCU will have the radiation problem of excessive clock;
3. There will be power supply noise problem in the DCDC power supply part.
[The rectification plan is as follows]:
1. Add a common-mode filter (TLDCM4745-2-102TF-TA) at the input end of the vehicle charging source line to prevent the noise of the machine itself from radiating out through the harness, add a signal line common-mode filter (TF2012A2X900MT) at the signal line of the terminal of the rear camera harness, and add capacitive filtering for AHD signals, and also add capacitive filtering at the power supply foot of the camera.
2. If the MCU clock part exceeds the standard, you can add a spread spectrum IC (SSDCI1108AF-08-CT) to effectively reduce the frequency doubling of the clock.
3. For the DCDC power supply part, we can add RC absorption circuit or capacitor filter at the input and output of the DC chip.
[Special attention]:
When it is found that the filtering at the harness port is useless and it has been clearly radiated through the harness, it can be tested by replacing the better shielding harness (double braided mesh shielding) scheme.
This is the corrected test data.
Fourth, summary
Most dash cams will basically have some clock over-standard problems, and the spread spectrum IC is a very good means for clock problems. When you find that filtering, grounding and other means cannot pass the test, you can check whether it is because the wiring harness shielding effect is too poor, instead of blindly dealing with it on the host side.
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