Si4010 keyfob antenna design and tuning tips

by <a href="http://community.silabs.com/t5/Welcome-and-Announcements/Community-Ranking-System-and-Recognition-Program/m-p/140490#U140490"><font color="#000000"><font size="2">Hero Employee</font></font> </a> zovida on ‎03-12-2014 09:26 AM - edited on ‎09-19-2017 06:04 PM by Administrator Nari

Question

How can I achieve the best RF performance with a keyfob using Si4010/11/12? What are the design and tuning tricks?

Answer

The Si4010 RFICs can be used with small loop antennas without external matching components. A native small loop antenna has a quite high input impedance (around 10k), but for the optimum operation the Si4010 transmitter's PA requires about 400-500 ohms antenna impedance. This impedance transformation is provided by the ratio of the top antenna capacitance (can be realized as a printed interdigital capacitor, C1) and the internal PA capacitance bank of the Si4010 (inside the chip, C2). C2>C1. 

 

Capture1.PNG

 

All designs are pretty sensitive for the top antenna capacitance (shown as C1): slightly smaller capacitance results lower radiated power at the fundamental, meanwhile slightly larger capacitance results higher radiated harmonics. The below table summarizes the effects of the top antenna capacitance on the RF performance (in this comparison the form factor size - loop antenna area - and the frequency are assumed to be fixed). 

The internal PA capacitance (C2) is automatically tuned by the RFIC itself and can be monitored ("wPA_CAP" register with Silicon Labs IDE).

 

Capture2.PNG

 

Here are some dimension parameters for reference:

If the loop antenna area is around 25x25mm and the operating frequency is 434MHz, the typical top antenna capacitance is about 2pF. 

  • The same loop antenna area at higher operating frequency needs smaller top antenna capacitance
  • Smaller loop antenna area at the same frequency needs larger top antenna capacitance  

More suggested design tricks for the small loop keyfob antenna:

  • Maximize the loop area on your given form factor for the best antenna efficiency
  • Keep at least 0.5mm free space between the antenna trace and the edge of the PCB
  • Keep at least 3mm free space between the antenna trace and the ground/circuit area
  • Try to avoid creating extra loops with your signal traces (route the signal traces as close to each other as possible and quite far [at least 2-3mm] from the antenna trace)
  • Keep the antenna symmetry

Inductive tapping: it is realized as a smaller loop on our reference keyfob designs and responsible for only the Si4010's PA biasing without the need of use any external choke inductor (since the Si4010 PA outputs have to be connected to the supply voltage).