Hello again!
The previous post demonstrated how an insulated DCDC converter can generate unwanted emissions at frequencies below 80 MHz.
The next step is to analyze two more mechanisms in more detail.
There are many similarities between these mechanisms, but before I begin to tell you about them, it is necessary to understand some of the basic elements that make up an antenna.
Exactly what is an antenna?
A transmitting antenna is made up of three elements
The 3 magic elements that create an antenna
a piece of metal which represents our antenna (it could be a cable)
a feeder
a ground plane. (it could be the PCB GND, or even the chamber earth)
See the diagram below for an example.
In RF, a simple antenna is called a monopole antenna and has a maximum emissivity at a frequency range of lampda/4.
You can use this calculator to check the peak emission frequency of an antenna: https://3roam.com/monopole-antenna-length-calculator/
What happens if we move the antenna 45 degrees?
Can you tell me what happened?
Diagram emission changes, but the frequency where we have the maximum emissivity remains the same and it depends only on the length of the cable.
What happens if the GND plane is not infinite, but finite? or even smaller than the antenna? We are still transmitting, the efficiency is poor, but we still have a transmitting antenna!
Now that we know that to make an antenna we need 3 elements (a wire, a feeder, and a ground plane) let's see if we can see these 3 elements in the diagram below.
Do you see it? Under some unfavorable circumstances, a cable, such as a power cable, a digital IO cable, an analog cable, etc., may behave just like a monopoly antenna.
The question may be asked. Where is the ground plane? Depending on your configuration, it is generally the return plane of your feeder, so in general it could be the return plane of your PCB.
Last but not least, where is the feeder? The RF signal is normally transmitted from your board over a parasitic path to your cable, and then your cable amplifies it and transmits it out into the free space.
At frequencies below 250 MHz, cables can behave like antennae, in my experience. If you go above that frequency, you will run into other problems.
So far, we have learned that to emit RF above 80 MHz, we need an antenna, and that any cable connected to our PCBA works, potentially, as a monopole antenna.
We have also learned that the antenna requires 3 "magic" elements in order to transmit.
the cable (antenna)
the feeder
the ground plane, as shown above.
So how do we stop emissions?
We break this structure!
As part of the EMC testing in the test lab, during the test, assuming you are having issues, you might want to consider shortening the GND plane of your board to the cable during the EMC testing. If it is not possible to do this in DC, then use a capacitor or an inductor instead. Check the RE result every time you do this. It is sometimes a long process, so you need to be patient.
Identifying the GND path and connecting the cable to it is the challenging part of this exercise. Usually, the answer is simple if you have an insulated DCDC converter.
When you connect the PCB GND to the cable, 99 times out of 100, the capacitor value is not important!
When you connect your board's GND plane and the cable with a capacitor, what you re actually doing is damaging the antenna structure...
The ground plane, the feeder, and the wire are no longer connected because the wire has been grounded. Therefore, it cannot work!
To return to the Insulated DC DC converter. If the cable works as an antenna, you now know what to do.
Don't forget to check the safety standards as well. Choose the right capacitor (usually a Y capacitor, which is usually large).
The nice thing is that the value of the capacitor, 99% out of 100, isn't important! The important thing is to break the RF antenna structure.
With this knowledge, you now know how to design unshielded electronic cables with low emissions!
I hope you have enoied it! don't forget to follow me and if you like it, please let me know.
Next time we will see the last common case of emission path when using a DCDC converter