How can I achieve the best RF performance with a keyfob using Si4010/11/12? What are the design and tuning tricks?
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.
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).
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.
More suggested design tricks for the small loop keyfob antenna:
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).