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Question on EMC

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I have a question on how does EMC issue arise in a PCB board.


Suppose I have a PCB board that has switching DC-DC converters and other high speed interfaces such as Ethernet, and USB.

If my board says, radiates say 75MHz, and it goes above the threshold limit, I will find out the source of the 75MHz.
While debugging, I find that I don't have a 75MHz source, but rather a 25MHz clock.

Now my question is:

1. Why does the 75MHz signal radiate and cause the issue where the 25MHz does not cause the failure in EMC?
Or in other words, how does or which factor (say PCB parasitics, layout, RC component values etc) is responsible for making the 3rd harmonic cause the problem and not the fundamental?
 
Hi,

If my board says, radiates say 75MHz, and it goes above the threshold limit, I will find out the source of the 75MHz.
While debugging, I find that I don't have a 75MHz source, but rather a 25MHz clock.
If you do a frequency scan you will surely see the fundamental 25MHz ... but of course it may be lower in amplitude.

In most cases you don´t have a pure sine wave. Any other waveform consists of fundamental plus it´s overtones.Thus it´s quite expectable that you will see 75MHz overtone of a 25MHz fundamental.

Now you ask why the 25MHz is not the problem but the 75MHz is. It´s because of the "antenna". An antenna may be a rod, a straight piece of wire. Every trace on your PCB definitely is an antenna. But length matters (and shape). Antennas work best on lambda/4 or lamda/2. "Best" means how much energy they can transmit / receive.

So it´s no surprise that one trace on your PCB works betther for 75MHz than for 25MHz.

And for sure there my be resonances that "amplify" the one frequency more than the other.

******
All in all ... it comes back to what is already said many times before: PCB layout is very important for reliable operation. This is more true now than before, because the electronic devices become faster (higher dV/dt) resulting in higher frequency overtones and the external frequencies are rathere high (referring to EMC). Many hobbyists think if they use just frequencies below 1MHz they are on the safe side. But this is not true. Stille the PCB "antennas" receive WiFi, BT, TV, mobile phone... HF signals. And they also transmit the overtones of a 1MHz square wave which you might see even above 10MHz.

My simple recommendation (prayer) is: a really solid GND plane. (No copper pour! ). With a solid GND plane you extremely reduce the "efficiency" of an antenna. In both directions: transmit and receive. (EMI and EMC).
There are two functions of a GND plane:
* shielding ... similar to an faraday cage
* it automatically generates opposite current direction in the plane ... referred to the current in the signal trace. This compensates the magnetic field.

--> If your PCB fails EMI tests .. I bet you don´t have a solid GND plane.

Klaus
 

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