For RF signal you must draw a 50 ohm net between chip and antenna to get the best performance.This post is the result of my web research on this topic …
There are two ways to design your emitter to antenna solution:
- A microstrip line : basically you have a net on the top driving the signal and a ground plane on the PCB bottom
- A coplanar wave guide : your signal is drive by a PCB net on top with two ground plane area on its left & right. If you also have a ground plane on the bottom it’s a grounded coplanar wave guide
The way to get a 50 ohm communication way is different depending on your choice.
The impedance of the line is mainly depending on your PCB characteristics. The most important are:
- PCB Material
The PCB material are standardized with different norms. One of the most common is FR4. It defines range for most of the parameter we need to know for the wire size calculation. The problem is range are large and the calculation will vary depends on where your PCB is in the given range.
- The Thickness of dielectric = the PCB height.
This is why we found RF pcb : they are particularly thin as this is allowing to make smaller RF wires. This can be a variable parameter depending on your PCB provider sourcing and these variation may impact you radio quality.
- Relative Dielectric Constant (εr)
It determines how much electrostatic energy can be stored per unit of volume when unit voltage is applied and basically how the PCB act as a capacitor (as this definition is really simplified you can learn more here)
This constant depends on the material and the frequency of the communication. For a FR4 dielectric.
For a 433MHz line it will be between 4.2 and 4.5 (assume 4,35) ; for 868MHz 4.15 and 4.4 (assume 4,30). (I did not found more precise information on Eurocircuit website for non RF pool circuit)
The impedance depends on the net width, the net height, the board type and so one. There are multiple tools on Internet to calculate the net width to use. Two of them is the following one :
Its gives you the impedance based on different parameters
- Height (H)
- Width (W)
- Thickness (T)
- Substrate Dielectric (Er)
These elements refer to the following diagram :
- For Eurocircuit these information are given in the price calculator interface. The standard Height is 1.55mm ; the Thickness once finished is 35um.
- For Seeed you can choose from 0,6 to 3mm. Thickness is from 1oz (0,035mm) to 3oz (0,1 mm) – See this calculator for oz to mm conversion.
Here is a table of W depending on the other standard parameters (I choose the one acceptable in term of price)
|@868MHz – Er 4,3||H(mm)||T(mm)||W (mm)|
|RF Pool – Er 3,45||0,5||0,035||1,10|
The main difficulty is to be able to create a trace up to 3 mm large when you have as a source and destination pads around 1mm and in the middle you have to integrate capacitors & inductor CMS with width about 0.8mm. We also have to take into account that a CMS will make the Thickness to change.
As a complement you can read this paper with larger theory.
Coplanar wave guide
Grounded CoPlanar Waveguide have an advantage to not take the wire Thickness as a parameter. The other parameter are like defined previously.
|@ 868 Mhz – Er 4,3||H(mm)||S(mm)||W(mm)|
|RF Pool – Er 3,45||0,5||0,254||0,94|
As we can see with Grounded coplanar waveguide the width of the track is really thinner than with Microstrip and the gap between min and max have a lower range. As a consequence the tolerance to small variation is better.
You can calculate your wire size with this calculator :
As a complement you can read this website with larger theory