Impedance matching circuit for esp8266

The datasheet of ESP8266 says that the impedance matching circuit should of 50 ohms. The ESP-01 uses a L-C-L filter. The value of capacitor is 5.6pF. What should be the value of incutors? How do I calculate it? The inductors are named “NC”, what does that mean?

Please help me. Thanks in advance :slight_smile:

impedance matching.png

Why do you want to do this?

I want to build my own esp8266.

I had thought about this in april but both of us couldn't succeed :(. Check out the link.

It is remarkable that even the datasheet of the ESP8266 does not say anything detailed about the output impedance of this device.
It is important to match the output of the ESP to 50 ohm if you want to connect a coaxial cable to the device.
Also is it important to match the ESP-output to the antenna, PCB , ceramic chip or whatever antenna you use,
The reason for this is to transfer maximum power to the antenna so the device will give the best results and at the same time make the receiver's sensitivity optimal.

To me it looks like a lot of the ESP module's have no matching circuit at all and therefor will not produce optimal.
For instance look at the ESP02 where the output is connectod to an U-FL connector with only a small capacitor, I doubt this connector sees 50 ohms.
If you look at pictures of what is inside the metal can you find that sometimes there is this L-C-L matching circuit but often there is no matching circuit at all, only a small series capacitor.

I want to believe that some manufacturers of these modules have optimized there circuit by trial and error while using the right (expensive) equipment but some manufacturers will just do it as cheap as possible and will not use any matching at all just to save a few cents on these components and on design cost.

The datasheet says the input impedance is 50ohm. Most probably the output impedance is the same. Therefore you do not need a matching circuit when connecting a 50ohm antenna (thus they use only a decoupling capacitor).
Look at this picture - Espressif ESP8266 WiFi-serial interface : weekend die-shot : ZeptoBars - you may see a lot of inductors (and capacitors) on the chip which create the matching..

Noticed that the datasheet v4.3 says about pin 2:

2 LNA I/O RF Antenna Interface. Chip Output Impedance=50 Ω
No matching required but we recommend that the π-type matching network is retained.

If the output is a real 50 ohms output than there would not be a recommandation to retain the π-type matching network.
Using that would transform the 50 ohms output to something else.

So at least the datasheet is omnivalent, it is not clear about the real and imaginary parts of the output impedance.
Ofcourse when using a onboard pcb or ceramic chip antenna it can work fine but when connecting a coaxial cable to the U-FL connector the result can be unpredictable and dependant on the length of cable.

but when connecting a coaxial cable to the U-FL connector the result can be unpredictable and dependant on the length of cable.

When you get a 50ohm coaxial cable, and both ends of it are connected to 50ohm impedance (50+0j), then the length of the cable plays no role..