Some illustrated RF PCB layout tips
1. No RF layout can be perfect
It is not your fault if your antenna does not perform like it was expected to perform in your simulations. It is perfectly normal because the antenna impedance is influenced objects surrounding it and may shift in a way that cannot be predicted in software simulations.
The best you can do is add a matching network that lets you tune the antenna in your final product.
Matching is not just required for antennas, but also for proper interfacing between different RF components or subsections on board.
2. Make everything 50 ohms
If you have never tried out RF PCB layout before and you do not have access to proper tools for simulating your design in 3D, the best you can do is pick components that have a characteristic impedance of 50 ohms on RF ports.
The trace impedance on your PCB that carries RF signals can also be made 50 ohms by adjusting its width appropriately. You can calculate the trace width using online trace impedance calculators or microstrip impedance calculators.
It is also easy to find parts (such as filters, amplifiers, antennas, etc) with 50 ohms characteristic input/output impedance.
3. Lay out RF first – ALWAYS
Remember, RF traces are EXTREMELY high frequency signal carrying structures. If you lay them out last, or if you try to lay them out when the board has already gotten rather clumsy, you will make compromises with the trace layout that can make your design fail. For example, here is the RF stripline (marked in yellow) with a sharp bend (90 degrees) just because there was no other way to route it. This is NOT how RF traces should be laid out!
Always leave adequate space around the signal trace for smooth bends and isolation of the RF signal.
4. Isolation is important
Isolating an RF trace is important. An RF trace must be properly isolated from other high speed signals (such as clock traces for crystals, USB differential pairs, HDMI, ethernet, etc).
This is often done by “via stitching”, i.e. stitching vias around the RF trace to prevent it from interfering with other components on board.
Note that improper isolation will not render your design defunct, but it deteriorates the receiver performance and average data throughtput in most cases.
5. Keep inductance low
The ground inductance really makes a huge difference in RF design. If your RF chipset is grounded through a single via or narrow ground trace – you have a massive ground inductance. And as you know, high frequency does not like inductance. Therefore, always ground the RF chipset adequately. If it is a QFN with ground pad, use at least 9 power vias. Ensure a large, continuous ground plane under the chip and RF trace as well.
If you have free space on the top layer, you should always have a ground fill that is connected to the inner ground layer through as many vias as practically possible (your PCB manufacturer would be mad at you, don’t add a thousand vias!).
6. Don’t break the ground plane
You may sometimes find yourself designing an RF system where you also have audio or analog circuitry on the same circuit board – and close to RF system.
It may be tempting to have a different ground plane to isolate the ground for the analog section. But by doing that, you are probably affecting your RF section severely. Also, note that if you break the ground plane under the RF trace, you may have a design that does not work.
Here is an example of what you should NOT do:
